REPEATER BUILDING TOPIC SUMMARY - Still under construction...... Version: 6/25/03 By Mark D.Lowell, N1LO. First posted in December 1999 Check http://www.qsl.net/n1lo for the latest update. The existence, accuracy, content and organization of any section may change at any time as new discoveries, understandings, and concepts arise. I add new sections whenever appropriate. This document is a series of notes that I have made concerning repeaters and installation that started after reading and digesting the message archives of the Repeater Owner's remailer (reflector) forum sponsored by the folks at www.onelist.com. The archive is located at: http://www.onelist.com/arcindex.cgi?listname=Repeater I have also combined ideas from other readings and personal experiences as well. I have paraphrased some subjects after reading the general consensus of many messages. In other cases, the originators of these messages have already addressed the topic in the most eloquent form, and I have simply copied their messages here. REPEATER BUILDING TOPIC SUMMARY - Still under construction...... 1 OBTAINING REPEATER COORDINATION 4 ESTIMATING COVERAGE 4 PLANNING AND SELECTING COMPONENTS 4 SITE SURVEY 5 CONTROLLER 5 ACQUISITION 5 MAINTENACE/TECHNICAL PERSONNEL 6 PRETEST 6 ANTENNA 6 FEEDLINE 7 DUPLEXERS 7 LEASING YOUR TOWER TO OTHER SERVICES 7 DOCUMENTATION 8 DUPLEXERS 8 SELECTING DUPLEXERS 8 CONNECTING DUPLEXERS 9 TUNING PASS/REJECT DUPLEXERS 9 USING PREAMPS 12 TROUBLESHOOTING DESENSE PROBLEMS 12 TESTING BEFORE INSTALLATION 13 ADJUSTING/TAILORING AUDIO 14 AN AUDIO EMPHASIS PRIMER 14 EXAMPLES OF COMPONENT VALUE SELECTION 17 EQUALIZATION FOR BROADCAST AND 2-WAY 18 BUILDING A LIMITER CIRCUIT FOR AUDIO 19 CHOOSING THE AUDIO PATHS BETWEEN RX, TX, AND CONTROLLER 19 DTMF FALSING 20 ANTENNAS FOR REPEATERS 21 CONVERTING COMMERCIAL ANTENNAS TO AMATEUR USE 22 DB PRODUCTS ANTENNAS 22 FOLDED DIPOLE SPACING 23 FOLDED DIPOLE ORIENTATION 23 AUTOPATCH 24 TYPE OF TELEPHONE LINE SERVICE 24 AUTOPATCH LINE WIRING 25 MULTIPLE DEVICES SHARING ONE LINE 25 EMERGENCY AUTODIALING 26 OFF THE HOOK MONITORING 26 INTERFERENCE PROBLEMS 26 INTERMOD PRODUCT CALCULATOR 26 AN INTERMOD TROUBLESHOOTING EXAMPLE 28 EXAMPLES OF UNUSUAL SOURCES OF INTERMOD AND NOISE 30 AM BROADCAST STATION INTERFERENCE 30 PAGING SYSTEM INTERFERENCE 30 HOW TO DOCUMENT THE INTERFERENCE 31 SPECIAL NOISE PROBLEMS ON THE 6 METER BAND 32 POWER SUPPLIES 33 PROTECTION FROM OVER-VOLTAGE 33 REPAIRING ASTRON POWER SUPPLIES 33 EMERGENCY POWER SYSTEMS 34 FLOAT CHARGING BATTERIES 34 MARAUDERS, PIRATES, AND JAMMERS 34 TRANSMITTER HUNTING 34 SOURCES 35 REPEATER BUILDERS FOR HIRE 35 SOURCES OF CONTROLLERS 35 SOURCES OF ANTENNAS 36 SOURCES OF DUPLEXERS 36 SOURCES OF REPEATERS 36 SOURCES OF CONVERSION KITS 37 SOURCES OF PREAMPS 37 SOURCES OF RF/INTERMOD FILTERS 37 PAR Model VHFDN152 specs: 37 SOURCES OF CRYSTALS 37 SOURCES OF CALIBRATION & REPAIR OF SERVICE MONITORS 37 SOURCES OF SURPLUS COMMERCIAL RADIOS 38 SOURCES FOR MOTOROLA MANUALS 38 SHOPPING FOR A GE MASTER II RADIO 38 OBTAINING REPEATER COORDINATION 'How long does a typical coordination take to process?', you may ask. You must realize that these coordination groups are volunteers and most will not turn the paperwork around over-night. Its more like 60 to 90 days in many areas. It really depends on two factors. 1) The density of the repeater population in your geographic area and 2) the level of activity of the local volunteer coordinator. The two do not always correspond. The key word to remember is the word "volunteer." Unless and until coordination become mandatory -- and we have to pay a fee for it -- we deal with volunteers who only have a limited amount of time and resources. Most are very responsible persons and we owe them a debt of gratitude. Without them there would be anarchy. If you apply for the recommendation of your coordinator and don't receive a reply in 90 days, the logical assumption is they are out of business or don't care. Perhaps you are cleared for "self coordination". If you receive no recommendation, what else can you do? Coordination is desirable but not absolutely required. If you received no reply, put up your station and if there is no interference from your transmitter to another repeater receiver or link receiver, you are in compliance with the FCC rules. If a frequency coordinator does not perform the function as defined in Part 97.3 ,self-coordination or no operation are the only choices. ESTIMATING COVERAGE CALCULATING RADIO HORIZON For VHF/UHF propagation, the rule of thumb is to use "four thirds earth" to determine where the radio horizon is. In other words, the radio horizon extends beyond the optical horizon by 1/3. A simple formula to approximate the radio horizon with smooth terrain is: d = 1.4 x sqrt ( h ) where d = distance in miles to the radio horizon and h = antenna height in feet above the average terrain (HAAT). As an example, with an antenna height of 400 feet, the radio horizon is 28 miles. PLANNING AND SELECTING COMPONENTS A commercial repeater solution is very expensive to say the least! $2000-3000 easily. If you do not have someone who can or will mod a older repeater you can either go the totally commercial route, or hire another amateur to build one for you. You can buy a commercial repeater and wire an outboard commercial or amateur controller for more flexibility with less RF circuitry work. SITE SURVEY First you have to do a site RF survey and determine what you need (as opposed to think you need) in the way of RF equipment. Do you have a site yet? Are there other repeaters there? If so, look at what's there and that fact alone may tell you what you need to survive there. If you have a quiet site you have different needs than if you have a dozen TV and broadcast FM transmitters nearby. Go to your potential site with an HT for the band of interest and scan around. Many HT's have preamps built into them and will be a good indicator of the local RF environment. Listen on your input frequency now and see if you can hear anything at all. If you do it will be harder to get set up. If the radio howls and moans and has intermod galore, you may want to consider another site. If you're in a commercial 2-way site there may be some owner-imposed equipment requirements that affect your design decisions.... As an example, a site may have a single community UHF antenna (406-512mhz) at 120' feeding a 32-port multicoupler which feeds all the commercial and ham repeaters. Each UHF transmitter feeds one port on a combiner. Every group of 5 transmitters feeds a separate transmit antenna. The combiners have a 125w per-TX max rating on each port. So, yu may be limited on the power you can use, and individual preamps are a waste of time (high RF level site). CONTROLLER The controller handles audio switching and control duties. There's no need to rush acquiring the controller, you won't need it until long after your RF parameters are determined. You'll first need to acquire, rack and tune the radios, and start to assemble the actual system. In general a good RF package will outlast several controllers. If you do acquire a turnkey repeater, don't plan on using the controller that comes inside the repeater except as an initial stopgap controller (until you get the real one working), or as a backup. The stock controllers are generally pretty limited in what they can do. If you set up your repeater with an external controller, with the addition of a DB-25 connector, set up a toggle switch labeled "internal/external". If the external controller has to be disconnected for any reason the switch is flipped, you use the internal controller, and the site is not off the air. ACQUISITION The repeater RF equipment is a separate consideration from the duplexer, feedline, or antenna. Each has to be considered individually. A turnkey repeater will cost a lot of money to start, and more as you add the isolator, pass cavity, controller/autopatch, any remote bases, etc. You can acquire much better repeater equipment and build your system for a lot less. Or a better system for the same money. If you are intending to use mobile radios as the UHF RF decks in your repeater, buy THREE identical radios. Modify all of them identically. Some people are partial to the GE Mastr-II or GE Century II first, then a 25w Motorola Micor. The Motorola products are more common. Whatever you get, test all the radios and pick the best receiver as your prime RX unit, the one with the second-best receiver as the prime hilltop TX, and the third as the backup. This way if something breaks and you need it on the air RIGHT NOW (like during a natural disaster) you can drive to the site and swap the radios and get it back on the air (all you should have to do is swap the two cable sets on the RX and TX chassis - they should already be crystaled and tuned). You can go back later with the spare chassis and swap it in and take the bad one back to the bench at home. MAINTENACE/TECHNICAL PERSONNEL Which leads to another facet of the "new repeater" situation: who's going to build it? Or maintain it? Is there a savvy technical person in your group? Does he know the difference between a silver-handle Mastr-II mobile and a brown-handle mobile? Does he know what an isolator is? And why it should always be paired with a pass cavity? (Does he care?) Does he know when NOT to use a preamp? If not, prepare to BMW (Bring Money in Wads) - you'll have to hire the expertise. Having a commercial 2-way tech in your group is a BIG help. PRETEST After you are sure that you have a good solid system, then hook it to an Antenna and a phone line and use it as a garage repeater for another week - or two - or even a month. ANTENNA As to antennas, your decision will be made more by the site requirements than anything else. Some sites have high winds and ice, and the only thing that will stay up is a Scala or a Storm-Master. Others are less severe, and you can use a simple fiberglass stick (use a top support and it will last a LOT longer - the wind will tend to fatigue flex it to death after some years). On the antenna, go with top quality stuff and you only have to buy it and install it once. Lesser brands, such as the "ham grade" antennas by Diamond, Comet, etc., may not survive a serious iced winter on a mountaintop. If you have a person in your group that can handle a heliarc or TIG welder, then you have a half a chance to do it yourself. Find a PD or Cellwave 4-pole (the type that has a DC grounded element) and copy it. If you have the space, use multiples - an 8-pole or even a 16. FEEDLINE As to feedlines, again pay the $$$, do it right, and do it once. Depending on the situation, you may need one run, maybe 2. Most systems use a duplexer and one feedline up the tower. You may not need one at all if you plug into a receiver multicoupler port and into the combiner port. Don't be tempted to use 1/2" heliax on 440 - the loss is too high - go with 7/8" (at least). As to sources of cable and equipment, develop connections with people in the broadcast and 2-way industry, and keep your ears open. Opportunities do pop up - but you need to have things set up to where the people who hear about it have a desire to call you. Over the years various people I know have turned up complete repeaters, Stationmasters, sites, etc. DUPLEXERS Use Wacom or other good duplexers as much as you can afford. Use the best antenna system (biggest coax and most dipoles or best antenna that you can afford). Building a repeater can be fun. It's one heck of a learning experience. But it can also tax your patience - it can take anything from a couple of weekends to a year's worth of evenings from the time the first dollar is spent until the rack goes up to the site. There always seems to be one more addition around the corner... LEASING YOUR TOWER TO OTHER SERVICES Here are some important things to consider if you own a tower and are ever approached by a business that wants to lease tower space from you: Do you rent tower space to commercial operators? Access will be required to the site 24 hours a day by people you don't know. If there's not separate access, the extra traffic can destroy the road, which you will have to repair at your cost. If you don't keep it open you can be held liable for incidental damages due to lack of access. Fact that you are a private person no longer brings any sympathy. You're a party to a commercial enterprise. When stuff doesn't work in the shed, they will knock on your door. Sometimes just to ask to use your bathroom, or place a phone call because their cell phone has lost charge. Discuss this and the prior paragraph with the wife. See if the arrangement is in her comfort zone. If your ham operation seems to cause them trouble, they will be all over you. The reciprocal does not apply. If their stuff is bothering you, you will have to pull teeth to get anything done. Your stuff will be the automatic reason for whatever goes wrong because of their poor maintenance. If someone is hurt on the tower, you can guarantee that you will be named in any lawsuit. You will have to spend your money to get out from under. Your home insurance company will drop you as soon as they find out you're renting space on your property. Your bill will go up anywhere from 3 to ten times, because of the greater risk of being named in a law suit, in the COMMERCIAL insurance you will have to carry. Check this out with your State Farm agent. If he comes up blurting roses and sure it's OK, BE SURE you get the ruling from their internal underwriter IN WRITING. Take along a video camera to record the song and dance. If your tower gets blown down you will wind up having to prove that it wasn't your faulty design and construction, probably in court. Their monetary damages are your fault. A hold harmless clause may save you, IN COURT, but you will still have to pay a lawyer from your money to do it. (Don't even THINK of representing yourself.) It does not matter who the person on the other end of this deal is, and how great a guy he is. The company can be sold and you can then be dealing with your worst nightmare. Your agreement is with the company, not the nice person. What I have told you is just the tip of the iceberg. Unless you have dealt with the public and the radio business in general, you probably won't believe some of the stories I could tell you. The only way to really deal with something like this is to subdivide your property. Form a corporation. Sell the property to the corporation, including actual access (not right of way) to the main road. Let the corporation build the tower, shelter, run power, build the road, etc. Get the small business administration to help you lay out the business and legal aspects. Put a BIG fence between your place and the corporation's property. Make sure you, personally, are a hidden partner, so they won't get this idea they can ask to use your bathroom. Lease the space, tower, and especially power. Put an ironclad radio-worthiness clause in the lease so you can LEGALLY pull the power on the service if they don't keep it up. Do NOT lease to any government agency no matter HOW much money they offer. Do it and you are possibly at the hands of idiotic appointees that know nothing about radio. Further, you can almost never beat the government in court, and they can put you in jail. BUT, before you do this, have a real lawyer and accountant cost this out to determine what you would have to charge. Then compare that with what whoever is offering you for space on your tower. See if they aren't trying to get space and elevation at far less than what it really and normally costs on the open market. Project your chances of a smooth operation from a skinflint company that's trying to maximize it's profit by putting commercial radio on ham towers, and in so doing, place their customer's interests at risk. Rent space in your backyard, and if it goes sour, the only way you may be able to get away is to move, and you may have a legal judgement against you as a lien on your sale of the house. DOCUMENTATION Document, document, document everything! Have manuals on everything. And make up a duplicate set that is kept at some other location. Try to write your reference materials for someone who may know very little about repeater systems. Don't think you will always remember everything you need to know - you will forget. Document it, no matter how trivial the detail may seem. DUPLEXERS SELECTING DUPLEXERS Saying "duplexer" is like saying "car". They come in various types, makes, models, and performance levels. Some are notch-only, others are notch-pass. There are 4-can and there are 6-can models (which give a higher level of isolation, but at a cost of insertion loss). And you may not need a duplexer - split sites are possible. A good system can be built with no duplexer at all. CONNECTING DUPLEXERS Double-shielded coax such as RG-214 is the best thing to use to connect duplexers. RG-214 has two silver-plated braids and a silver-plated inner conductor for maximum reliability and noise rejection. Avoid coax types such as Belden 9913 that use an aluminum foil shield, since it is problematic getting good connections between the foil and the connector. RG-142, a smaller version of RG-214 but sharing the same construction, can also be used. The jumpers between cans in a duplexer set such as a Wacom 641 set, are especially critical. Do not replace these with arbitrary lengths. The jumper should be just under 1/4 electrical wavelength. Just under because the tee on each can adds something like 3/4" to the circuit. Here is an example for duplexer cables at 146 MHz, using solid dielectric coax such as RG-214: 2951" / 146 MHz = 20.2" X 66% vf = 13.34" Now, subtract about .75" for the Tee = 12.59" total cable length. This is tip-to-tip on the PL259, so you can cut the cable exactly to this length and make sure the end of the center conductor comes flush to the end of the PL259. TUNING PASS/REJECT DUPLEXERS Tuning of the band pass/band reject design duplexer is easy compared to conventional notch duplexer. The cavity plunger sets the passband frequency. Here you want as much RF to pass through the duplexer as possible. The tuning capacitor C2 sets the notch frequency. Clearly mark the pass and reject (notch) frequencies on each cavity. Remember that they are opposite for transmit and receive! EXAMPLE: RECEIVER CAVITIES: Pass 144.77 MHz Reject 145.37 TRANSMITTER CAVITIES: Pass 145.37 MHz Reject 144.77 The easiest way to tune cavities is with a FM service monitor equipped with a tracking generator and a spectrum analyzer. If at all possible, contact someone with this type of test equipment to do the tuning for you. If this is not possible, do not despair. Simpler amateur methods are possible and will work just fine. You just have to take more care. You are going to need some kind of stable signal source and method to measure RF amplitude. A old fashioned tunable RF signal generator that can put out at least a volt of RF is very useful. You can use to sweep the cavities for initial passband tuning. For a stronger signal source an HT will work. With the HT your passband indicator can be a RF watt meter. For the finer job of tuning the reject (notch) frequencies, a simple RF voltmeter will work. You could also use a "S" meter equipped receiver with a step attenuator in front of it. This would be perfect for fine tuning the notches. Just be careful how much RF you inject into the duplexer while tuning, you would not want to fry your receiver front end when you hit the correct passband! If you are going to build your own RF voltmeter probe, see the ARRL Handbook for details. Build it in a connector that you can attach directly to the duplexer connecting cables. For the RF voltmeter use a old fashioned analog moving needle meter like a VTVM or a simple microamp meter. Its much easier to tune for peaks and notches with a analog then a digital meter. ONE AT A TIME Start with just one cavity connected. Turn the RF output up all the way on the generator and sweep up and down the band till you see a peak on your RF voltmeter or S meter. Remember that turning the plunger in lowers the pass frequency. Turning the plunger out raises the pass frequency. Set the output of the generator to keep the meter in the linear portion of the scale. All we are doing is a rough tuning first, so don't try to get things perfect. Connect the next cavity in the chain and repeat the process to put it on the pass frequency. Then set the second set of cavities to their pass frequency. Here is where it is easy to get confused and why it is important to have the frequencies clearly marked on the cavities. Doing that will save you a lot of trouble. With both the transmit and receiver cavity sets rough tuned to the pass frequency, connect all of the cables on the duplexer set. Connect a watt meter and a dummy load as in Figure 5 to the antenna connection. Set your HT on simplex to the pass frequency of the transmitter cavity set. In my example this is 145.37 mHz. Key the HT and tune the cavi- ties for the maximum amount of power output. Connect the HT to the receiver cavities and set the HT frequency to 144.77 MHz simplex (- 600 split from the transmit frequency in this ex- ample). Repeat the pro- cess of tuning the pass- band plungers for maximum RF on the wattmeter. Note that if by accident the reject frequency is too close to the pass band you may not be able to tune a cavity properly. If you suspect this, move the reject frequency adjust rod a short distance and try the passband tuning again. TUNING NOTCHES Tuning notches is a little more difficult. You need a vastly more sensitive detector than the watt meter used so far. If your signal generator can put enough RF into the cavities, a simple RF voltmeter probe will work. Remove the watt meter and replace it with a T connector. Connect the RF probe to the open port of the T. Your cavities will now be properly terminated with a 50 ohm load and you will be able to measure the RF voltage across that load. It is critical that your RF source be dead on frequency. A old fashioned tunable generator will not work. If your HT has a low power position, you can start with that. Warning! The HT will not be seeing a 50 ohm load when its signal is notched out. If you are uncertain if the HT can sustain high SWRs, don't use it. Key the transmitter and adjust the first cavity for the proper notch frequency. You may have to increase the meter sensitivity or RF power to clearly see the notch. It should be quite sharp. Then repeat the process on the next cavity. Other than changing frequency, the process should be the same for the other set of cavities. If everything went well you should be 99% in tune at this point. Go back and touch up the passband tuning and the notches one more time. Calculate the pass frequency loss across the receiver and transmitter cavities. It should be less than 2 dB and no more than 3dB. If the loss is excessive, check your tuning again. If that is not the problem, check the loss of each of the cables and connectors. One very nice feature of the band pass/band reject design is the ease of tuning. They also rarely need retouching when connected to the repeater. In some rare cases you may want to touch up the notch adjustment to eliminate the last trace of desense white noise from the receiver. You need a weak, unmodulated, off-the air-signal to do this and a AC voltmeter connected across the repeater monitor speaker. You can use a whip antenna on your service monitor output to supply the weak signal. Adjust the output until you have about 10 dB of quieting with the squelch open. Turn the transmitter on and off (you do have a local service switch don't you?) and observe the difference in the background noise. Any additional hiss is that is heard when the transmitter is on the air is white noise from the transmitter. If you feel the noise level is excessive touch up the notches. WHITE NOISE Many repeaters do have some audible white noise on the receiver. Perfection is hard to achieve in the real world. Worry about it only if the noise level is so high that it obscures the weakest modulated signal that you could normally expect to communicate with. For example, lets say that you can clearly hear a .25 uV signal on your receiver with the transmitter off. With the transmitter on the noise level increases 2 dB. This you could accept after final tuning. If the same signal disappears completely when the transmitter is keyed, then the desense is excessive and there are still problems in the system. ANOTHER METHOD Another method I've found to work well is to insert a Bird 43 thru-line in the duplexer's antenna leg with their 4275 Variable RF Signal Sampler attached to the Bird, set to very loose coupling. (One could probably use an iso-tee in absence of 4275, if your sig gen had enough output) Then just inject a strong receiver carrier, with 1000 Hz @ 3kHz, into the Sampler's port, while watching a Sinadder on the receiver's output speaker. Then just tweak EVERYTHING (tx, rcv, even duplexer if you're careful) for best sig to noise sinad WITH XMTR KEYED (this is why the loose sampler coupling, to keep the xmit RF out of the signal generator). The best part of this method is that all the tweaking is done with the critical rig-to-duplexer cables still in place and everything in the operational configuration. The only place you could go wrong is if you inadvertantly detune the xmtr down to lower power, which may result in a lower sinad, assuming a clean reduction. For this reason I usually hang the Sinadder near the Bird so I can watch both meters. With judicious use of the sig gen, one could always tune by ear, but I like to see what I'm hearing as a backup, especially as I get older... ;-> I've successfully used this method on mountain tops and gained weak-signal coverage we never knew we had...it will truly optimize a system. USING PREAMPS For some sites, where the local noise floor is not too high, placing a preamplifier in the receive chain can enhance performance. If you are using Band pass/Band reject cavities (BP/BR), then you should also have an additional bandpass type cavity immediately in front of the receiver to limit out-of-band signals. The best place for the preamp is in-between the band pass cavity and the receiver. One very popular type of preamp is that manufactured by Advanced Receiver Research (ARR). You can find their ads in QST. They make GasFET models with gains up to 24 dB as well as DGFET models with a gain of around 15 dB. Unless you have more than a couple of dB loss in your RX chain, the lower gain units would probably perform the best without introducing additional desense or intermod interference. ARR recommends the use of a 6 to 10 dB pad after the preamp as necessary to eliminate desensing. Receiver performance and noise floor are secondary objectives behind eliminating desense. The best way to optimize the preamp is to add attenuation on the output until you find the best sensitivity to weak signals with the least amount of desense. The overall noise figure of the receiving system with a preamp of gain Gis:NF (ratio)=NF(preamp)+NF(receiver)/gain(preamp)Everything is in ratio not dB. If the receiver has 5 dB NF and the preamp has 0.5 dB Nf then to get 0.75 dB overall requires 17 dB gain. If the preamp has 20 dB then you can add a 3 dB pad to the output and still meet your NF objective of .75dB. Leaving the pad out brings the system NF to 0.62 dB, which is not much of a change. These units are also subject to damage from voltage surges induced by lightning. Here are some guidelines to installation: 1) Ground the chassis 2) Connect a large cap across the preamp's power terminals in the range of 20,000 to 30,000 microfarads to create a voltage tank. 3) Isolate this voltage tank from the 12V supply using the largest series resistor that allows at least 10VDC to develop in the tank. 4) Install the preamp between the BP/BR cavities and the bandpass cavity 5) Again, if the local noise floor is too high, you may not see any improvement. TROUBLESHOOTING DESENSE PROBLEMS A switching type power supply can cause trouble in radio equipment. The switching ripple amplitude modulates TX output and generates a lot of unwanted sidebands. This is an often overlooked problem. Try substituting power supplies or running off battery power and retest. Some switchers are very sensitive to rf. If you must use a switching power supply, try putting a couple of ferrite snap on's over the dc output, and filter the ac input. Also, it is very important to use double-shielded coax to connect the repeater to duplexers and feedline. When arranging the repeater components, keep the exciter and PA as far away from the receiver as possible. To narrow down the source of the desense, you can also try terminating the exciter output with a 50 ohm dummy load and seeing if you still have desense. That may help you determine whether it's exciter-to-receiver desense or if maybe the PA compartment is leaking excessively. If it turns out there's no desense with the PA bypassed, you might consider just racking the PA up onto a separate mount. Think about how you are measuring your Rx performance? That is, what are you using as an indicator that you have desense? Another thing you can check, although by no means definitive, is what the limiter reading does when you key the Tx. If the limiter reading drops, it could be indicative of strong Tx carrier desensing the receiver. If the limiter reading goes up, it could be indicative of strong on-channel noise(i.e. broadband noise from the Tx). Again, neither of these tests are definitive, but they may help you in determining whether you have a transmitter noise problem or receiver overload. If you have strong RF coming down your feedline (like from a nearby TV or FM BC, cell site, paging transmitters, etc.), the RF may be borderline overloading your Rx and the symptom may not show up until your Tx carrier comes up. For plain duplexers, try putting a band pass cavity after your duplexer on the Rx leg? With the exception of true bandpass duplexers (pass/notch duplexers don't qualify), they have little out-of-band rejection. TESTING BEFORE INSTALLATION Don't put a new system at a site until it's proven itself. Set it up in a garage and key it down into a dummy load for a week - yes, 24 times 7 (or more) of continuous transmit. Do your own thermal chamber testing: mimic summer and winter conditions - take some cardboard cartons made for refrigerators and a roll of duct tape and make up a vertical coffin big enough to shroud the entire rack. Cut a porthole at the bottom to let cool air in, and a big 3-sided hole at the top but bend up the flap and use it to control the temperature. Adjust the flap so the hot TX keeps the temperature up to 90-100 degrees F (or so) and then try and break the system. As an example - disable the main fan and key the system - see if the backup fan kicks in. Run it for 20 or 30 hours - make sure that the backup fan delivers enough air. Then run it the same way at the lowest temperature you can generate - maybe you can put it in an unheated garage in the winter. You may say "our site never gets above 80 degrees, even in the summer" - but running at extreme temperatures helps find marginal components. ADJUSTING/TAILORING AUDIO AN AUDIO EMPHASIS PRIMER 1. Pre-emphasis and de-emphasis in two-way radio follows 6 dB per octave across the entire usable audio frequency (AF) range, nominally 300 Hz to 3kHz. A single-pole RC network will provide 6 dB per octave, and, in fact, you can't do any better than 6 dB per octave with a single-pole RC filter(nor can you do any worse than 6 dB per octave theoretically with loss-less capacitors). So, usually you'll see a resistor followed by a capacitor shunt to ground to provide de-emphasis, and likewise a resistor shunt to ground following a capacitor to provide pre emphasis. 2. A phase modulator responds linearly to both frequency and amplitude. That is, if you increase either the frequency or the amplitude of the audio into the modulator, you will increase the deviation proportionally. Double the frequency, double the deviation. Double the amplitude, double the deviation. In contrast, a true FM modulator responds to amplitude only; changing the frequency of the AF going to the modulator doesn't make it deviate any more or any less (intentional low-pass filtering not withstanding). Thus it is often said that PM is "self-pre-emphasizing", as it provides 6 dB/octave pre-emphasis inherently without an RC network. Because of this, the AF signal flow in an FM exciter is pre-emphasis-limiting-modulation, while in a PM exciter it's pre-emphasis-limiting-de-emphasis-modulation; the extra de-emphasis step is necessary because of the extra "automatic" pre-emphasis that happens in the PM modulator. There's actually a low-pass filter step ahead of the modulator on both FM and PM modulators to scrape off odd harmonics generated in the limiting process, but ideally it won't affect the preemphasis curve; it only serves to attenuate out-of-band energy (that above 3 kHz in two-way). 3. Other FM applications (such as broadcast) use something other than 6 dB per octave "across the board" like we do in two-way. In North America, broadcast FM uses 75 us (microsecond) pre-emphasis and equalization. 75 us is the time constant of the RC network that is used. Remember time constant? t=RC, the product of resistance times capacitance. Any combination of R and C that when multiplied together yields 75 x 10^-6 gives you 75 us pre-emphasis or de-emphasis. By the way, pre-emphasis/de-emphasis is often called "equalization", I'll get back to that word in a second. You could use a 0.01 uF cap (1 x 10^-8) and a 7.5K resistor (7.5 x 10^3) to get7.5 x 10^-6. Somebody correct my math if I screwed up, it's late. 50 us is used in Europe and other countries in broadcast instead of 75 us. The greater the time constant, the better the resulting S/N ratio at the expense of reduced high frequency response due to the limiting process. With this type of pre-emphasis/de-emphasis, only part of the audio is pre-emphasized/de-emphasized, that being the part above the cut-off frequency determined by the time constant. The cut off frequency is the inverse of the time constant times 1/(2*pi). For example, the cut off frequency of 75us is 2.122 kHz. The cut off frequency is the point in the pre-emphasis curve where the audio has been boosted by 3 dB. Above 2.122 kHz, it is pre-emphasized at the full 6 dB/octave, just like in two-way, but below the cut-off, it isn't pre-emphasized at all. In a nutshell, the RC values are chosen such that there is virtually no pre-emphasis done below the cut-off, and 6 dB/octave above the cutoff. The same is done on de-emphasis at the receiver to maintain an overall flat frequency response (ignoring limiting effects). 4. Equalization..oh yeah, remember your cassette deck? If you put it in "high bias" or "metal" tape modes, it provides 70 us of equalization on playback, and on "normal" it provides 120 us of equalization. Remember seeing those numbers on the cassette shell? Well, here's the deal. In the case of tape playback, in contrast to FM, the 70 or 120 us value isn't where de-emphasis begins, it's where it ends (why this is necessary is a whole other issue -email me if you want the full explanation). So, at 70 us EQ, the cutoff frequency where de-emphasis stops is about 2.2 kHz, while with 120 us EQ, the cutoff frequency where de-emphasis stops is about 1.3 kHz. So, when you playa high bias or metal tape on a tape deck that only does "normal" equalization it sounds tinny - the cut-off frequency is lower than it should be, thus the high frequencies are under-de-emphasized. 5. In two-way, we want 6 dB/octave across the whole 300-3000 Hz spread. So, what is done is the time constant is made relatively large, thus pushing the cut-off frequency below 300 Hz. Generally speaking, it's pushed down below60 Hz so that PL tones are also properly preemphasized.4. Pre-emphasis also has its place in broadcast AM. It was introduced in 1990 (the NRSC-1 standard). I won't bore you with those details, just suffice it to say that pre-emphasis/de-emphasis is used in places other than just FM. Pre and De Emphasis 2 The use of pre and de-emphasis in FM broadcasting (Entertainment and Communications) gives a decided improvement in received signal to noise ratio. The higher audio frequencies are boosted in level at the transmitter and are reduced in level at the receiver. Any higher frequency noise that was picked up along the way also gets reduced at the receiver. Entertainment broadcasting usually has the high frequency boost begin around 1500 Hz to 2000 Hz and continue upward at an approximate 6db per octave rate. This translates to a 75 microsecond pre-emphasis. Communications equipment seems to have pre-emphasis start at a lower frequency and is different with various manufacturers. Simple RC circuits can be used although other methods are popular. Resistance and capacity work because the AC resistance of the capacitor (impedance) becomes lower as the frequency goes higher. At the "crossover" frequency the AC resistance of the cap equals the resistor value. In pre-emphasis the 2 elements are in parallel. In de-emphasis they are effectively in series, an AC voltage divider. If you do any transceiver modifying for packet use, you'll eventually be faced with having to accommodate "emphasis." You may wonder, "What does that mean, 6db per octave?" First you need to know what an octave is. Every time a frequency doubles, it has increased by an octave. Assume you are measuring a 1200 Hz tone from an oscillator across a 600 ohm load. It measures .775 volts RMS. The tone is increased to 2400 Hz. The frequency has now increased one octave - it has doubled in frequency. The level of this 2400 Hz tone is now increased by 6db. The voltage would now measure 1.545 volts RMS - about a 1.99 increase. In 1200 baud VHF packet, we use tones of 1200 Hz and 2200 Hz. Not quite an octave but very close to it. The 2200 Hz tone should be about, but not quite, 6dB higher in level than the 1200 Hz tone. The actual db increase is closer to 5.45dB but 6 is close enough. Calculating the frequency response characteristics of the circuits is not too difficult, and requires simple match. Here are some formulas: t = time constant, microseconds R = resistance, Ohms C = capacitance, microfarads Fc= crossover frequency, Hz pi= the value of pi, 3.14159 t = R*C t = 1,000,000/(2*pi*Fc) Fc = 1,000,000/(2*pi*t) Fc = 1,000,000/(2*pi*R*C) R = t/C R = 1,000,000/(2*pi*C*Fc) C = t/R C = 1,000,000/(2*pi*R*Fc) Here is a chart showing standard resistance values versus capacity to obtain a 75 microsecond "emphasis.": RESISTANCE, CAPACITY 100 ohms, .75 uf 220 ohms, .34 uf 470 ohms, .16 uf 1,000 ohms, .075 uf 2,200 ohms, .034 uf 4,700 ohms, .016 uf 10,000 ohms, 7500 pf 22,000 ohms, 3400 pf 47,000 ohms, 1600 pf 100,000 ohms, 750 pf 220,000 ohms, 340 pf 470,000 ohms, 160 pf For pre-emphasis, the resistor and capacitor are placed in parallel and then this combination is placed in series with the audio to the transmitter at an appropriate point. For de-emphasis, the resistor is placed in series with the receive audio, at an appropriate point, and then the end of the resistor closest to the last device in the chain (TNC, speaker amplifier, etc.) is bypassed with the capacitor. You must give some thought to the resistance value selected. Input capacity of the port involved will affect your results. Watch out for that input capacity. If the input capacity is already .01 Mfd, a series resistance of 10,000 ohms would put you at greater than 6db per octave and give you too much high frequency roll-off without the use of an additional capacitor! "Purists" will object to this but my rule of the thumb allows me to use a resistor value roughly equal to the impedance it is FACING. If you were going to place a de-emphasis resistor in series with an audio signal feeding a device with a 600 ohm input, you probably wouldn't want to use a 22,000 ohm resistor - unless you had a large surplus of signal and a low input capacity. Simple math indicates that you would approach a 40 to 1 reduction in signal voltage. For a 600 ohm input you'd probably want to use a 470 ohm resistor - this would cut your audio by something less than half. The same holds true on the pre-emphasis side. I will admit that I have been known to "split the difference" on a resistor value if I am feeding a low impedance from a high impedance and have a surplus of signal voltage and a low input capacity. Try it, if you're not satisfied, try another combination. EXAMPLES OF COMPONENT VALUE SELECTION (refer to formulas above) EXAMPLE 1 You want to pre or de emphasize the audio going into a controller that has a 10,000 Ohm input impedance, using a standard 75us time constant: Choose R = 10,000 to have significant effect considering the input impedance. Fc = 1,000,000/(2*pi*R*C) Fc = 1,000,000/(2*pi*75) = 2122 Hz C = t/R C = 75/10,000 = 0.0075uF EXAMPLE 2 You don't have the parts in Example 1 to make up an 0.0075uF capacitor and you want to substitute the nearest value of 0.01uF R = 75/0.01 = 7500 Ohm EXAMPLE 3 You want to improve DTMF decoding of a device that is using discriminator audio. Since discriminator audio is already emphasized, you need a de-emphasis network to roll off the highs, including the white noise that is interfering with the decoding. The frequencies used in DTMF range from about 700Hz to 1500Hz, so you want to start rolling off the audio above them, say around 1600Hz. Fc = 1600 Choose R = 10,000 (assuming 10K input impedance) C = 1,000,000/(2*pi*10,000*1600) = 0.01uF This circuit would deliver almost 6dB reduction at 3000 Hz. Alternately, you could choose a slightly lower crossover frequency to achieve more noise reduction at 3000 Hz. EXAMPLE 4 The audio from your transmitter is way too 'tinny' or 'thin'. You want to de-emphasize the audio before being delivered to the transmitter. To help bring out the lower end of the speech passband, you want to set a crossover frequency much lower, say around 212 Hz. You're not sure of the input impedance of the transmitter. Fc = 212 Hz t = 1,000,000/(2*pi*212) = 750uS Pick standard value for C C = 0.1 uF R = 1,000,000/(2*pi*0.1*212) = 7500 Ohm Or, let C = 0.047uF, then R = 1,000,000/(2*pi*0.047*212) = 16,000 Ohm Since you don't have a 16K but you do have a 15K resistor: t = 15,000 * 0.047 = 705uS Fc = 1,000,000/(2*pi*705) = 225Hz EQUALIZATION FOR BROADCAST AND 2-WAY Here's a little further examination of this topic from another perspective. Yes, the FM broadcasting industry uses pre-emphasis and de-emphasis techniques to improve their signal-to-noise ratios. It's been correctly pointed out that audio frequencies below the breakpoint are transmitted flat, and audio frequencies above the breakpoint are transmitted pre-emphasized. (There have been other such "curves" used to tailor response, such as the RIAA curve in phonograph records, and the NAB curve in tape recording.)But that isn't the reason pre-emphasis and de-emphasis are used in narrow band radio. The early transmitters were PM (phase modulated), not FM, so they naturally had a 6 dB/octave pre-emphasis. PM became the standard modulation method. When FM transmitters came along, their audio had to be intentionally pre-emphasized to maintain compatibility with the PM transmitters already in service. In very early narrow band literature, you won't even find the terms "pre-emphasis" and "de-emphasis". Engineers simply "rolled off" the audio in the receiver with a single-pole filter because they had to defeat the PM transmitter's characteristic "roll-up". The pre-emph and de-emph terms came from the broadcast people. (I wish the narrow band radio industry had better terms for these characteristics. Unlike the broadcasters with their middle-of-the-band breakpoint, in narrow band radio the breakpoints are outside the voice bandwidth.) So, de-emphasis has little to do with signal-to-noise radio and everything to do with making the response correct. If FM had always been used, there never would have been pre-emph or de-emph in narrow band radio. We must recognize that early narrow band radio was intended for one-transmitter, one-receiver applications. This business of linking repeaters came much later. We pre-emph the audio to the FM transmitter to simulate PM, but must maintain a narrow bandwidth to be a good neighbor. So, we roll off the audio at, let's say, -3 dB at 3 KHz. If we hop through another similar system, the resulting audio is then down another 3 dB at 3 KHz, or a total of-6 dB at 3 KHz. This narrowing of the audio bandwidth is what everybody complains about in linked systems. So, the popular answer is to eliminate de-emph and pre-emph in the repeater. Just feed the user's pre-emph'd audio from the repeater receiver's discriminator to the repeater transmitter after the pre-emp stage, thus bypassing the repeater's de-emp and pre-emp circuits, resulting in a "flat" repeater, right?. (Of couse, you still have all those controller mods to make.) Everyone then assumes de-emph and pre-emph are evil!!! They must be, since the audio sounds better without them! But from an engineering perspective, there is nothing inherently evil in pre-emph or de-emph. The transmitter still rolls off at 3 KHz. By feeding pre-emph'd audio to the transmitter, you are artificially increasing the amount of high-freq audio fed to it. You are "peaking" the transmitter so that it rolls up. You are effectively widening the bandwidth. My response? First, let's at least admit that we are attempting to make narrow band systems wide-band. No bones about it. You want a nice, high-fidelity linked system? Go after the transmitters. Replace their audio filters with high-order, brick-wall audio filters that allow wider bandwidth signals(at the expense of smaller guard bands between channels). Or use wideband links on higher bands. But if you want to continue defeating de-emph and pre-emph, at least admit that it's similar to putting an audio equalizer in line. One last thing - - FM'ing crystals is really hard (they're nonlinear). FM'ing in a synthesized transmitter is easy. So, if someone tells you he FM'ed his crystal-controlled repeater transmitter with a few wiring changes and a capacitor, make him prove it. What audio signal generator did he use to sweep the transmitter? What receiver was used to produce the measured audio? Remember, the proof is in the pudding (lab grade test equipment). If it's really an FM transmitter, the received audio will be at a constant amplitude regardless of frequency. Anything else is modified PM. BUILDING A LIMITER CIRCUIT FOR AUDIO If you are worried about over deviation, build the simple limiter at http://www.repeater-builder.com/rbtip/limiter.html and use this to drive the exciter. No clipping is created with this limiter and some clipping usually is desirable, so if you want clipping you will have to build a different clipper / limiter circuit. CHOOSING THE AUDIO PATHS BETWEEN RX, TX, AND CONTROLLER Here's a little background on transmitter and receiver audio that will help you to better understand what's going on: In a transmitter, there is usually a pre-emphasis filter in the microphone input section, which enhances the high audio frequencies and 'rolls off' the lower audio frequencies. So, any audio applied to the mic input of a transmitter is going to be 'modified' in this manner. When you talk or inject audio into the microphone input of a radio, then your voice or the audio is thus 'changed'. To compensate for this pre-emphasis, the audio that is recovered from the discriminator in a receiver is 'de-emphasized' - the exact opposite of the pre-emphasis. So, the audio that you hear out of the speaker of a receiver sounds 'normal'. Now, that means that the audio that you 'see' at the output of the discriminator is NOT de-emphasized. It's still pre-emphasized, lacking much of the lower audio frequencies. Now, if you route discriminator audio back into the mic input of another transmitter, you are again pre-emphasizing the audio (in the mic input circuit of the transmitter) that is already pre-emphasized. Your audio, then, is going to be VERY 'tinny' sounding. Some repeater controllers will include jumpers to add de-emphasis in the controller audio path itself. That way, the audio that is being fed to the transmitter mike input will be 'normal' before it's pre-emphasized again in the transmitter. If the controller does not have any such de-emphasis circuit, then you can build your own circuit and run the audio from the discriminator through the de-emphasis circuit, and then into the controller. If your controller has 'flat audio', and is neither pre or de-emphasized, then insert your homebrew de-emphasis network between the output of the controller and the transmitter's mic input. Another alternative is to try to pick up your receiver audio after the de-emphasis filter in the receiver. Sometimes, you can do this and still get unsquelched audio that isn't affected by the volume control. In other cases, de-emphasis is accomplished after the squelch circuit and volume control, and so this isn't a good alternative. You could bypass the 'junk' in the mic input circuit of the transmitter, but this is typically not good practice. The mic input circuit has more than just pre-emphasis circuitry in it - it also has high-frequency cutoff filtering (also known asa 'splatter filter') to make sure that your signal doesn't get 'broad' by limiting the upper frequency response of the transmitter to around 3000 hz. Bypassing this can cause your transmitter to become very 'dirty', which could of course cause interference to others. Bypassing the splatter filter could cause you serious problems. DTMF FALSING Every now and then a person's speech pattern may trigger your controller's DTMF decoder and the controller will play a cover tone for a second or two. It may not happen very often, but it is annoying. It isn't always a problem with the controller itself, just the different peoples voices. Some people, usually women, create tones in their voices that will cause falsing on all sorts of controllers. Back in the prehistoric days when people constructed their our own decoders, falsing was a common problem. The answer then was longer validation timers. With the advent of IC's, this function is now all on one chip. There are two leading causes of falsing with the new chip decoders: 1) excessive audio input level to the decoder, and 2) overly short tone detect time. The detect time is the length of time it must hear a DTMF pair before it declares a valid digit. If there is no "DTMF trigger level" control on the controller that you can lower, you may want to lower the RX level into the controller, and boost it on the controller output. This should lower the DTMF trigger threshold if it isn't separately adjustable. If the controller uses an 8870 DTMF decoder chip, check the value of the resistor between pins 16 and 17. Falsing can occur more easily if the value is around 300k. This resistor controls the detect time, with higher values corresponding to longer detect time. Changing the value to about 510k should reduce the falsing satisfactorily. More rarely, falsing can be caused by a user over modulating, or being a little off frequency. This causes his voice peaks to bounce off the walls of the receiver's IF filters, generating a little audio splatter. Not much you can do about that, except to make sure your repeater's receiver is exactly on frequency, then bug the user to back off his deviation pot a little. The effects of falsing can be avoided by removing the "cover tone" and repeating audio during DTMF commands. You may want to avoid this, for operational/privacy reasons however. It is up to you to decide whether privacy of DTMF commands or false triggering is the bigger issue. Another thing you can do is set the dtmf mute timer to 0.8 seconds and then turn off the cover tones. ANTENNAS FOR REPEATERS The antenna of choice, enjoying the best reputation, seems to be a multiple folded dipole configuration, such as those from DB products. After that, comes the Comet GP-6 and the GP-9, which seem to work real well for the money, do not hold water, and have a vent so moisture can be released. Some other comments: Stay away from Diamond, too many noise problems and I have heard that the Hustler, while solidly built, has a small matching coax that burns out easily. Here is a comparison of types: Folded dipole pros: - excellent lightning survival - survive falling ice well - easy to service, nothing but metal and hardware - harness is replacable if it ever does get damaged - pattern can be tailored to suit your needs on many types - predictable pattern that doesn't suffer from beam steering on frequency excursions - rigid nature results in little pattern distortion due to wind-induced flexing - generally more broadbanded than series-fed sticks, both in terms of VSWR and gain/pattern Folded dipole cons: - a fair amount of mechanical parts that can work loose over time - VSWR may climb when iced up (moreso than sticks in some cases) - external harness types may require re-taping of the harness if the tape/ty-wraps break - shortened lifetime in salt-air or corrosive environments Stick pros: - radome keeps ice from detuning antenna too severely, although performance may still be affected - sometimes easier to "blend in" when aesthetics are a consideration (becoming more and more of an issue nowadays); can be painted just about any color Stick cons: - flexing eventually causes fatigue in the element joints, requiring repair or replacement (stiff-arms help this greatly, but when top-mounting, that's not an option) - flexing in the wind distorts the pattern and leads to "wind fading" - not easy to service/impossible to service while still on the tower - don't survive lightning as well as all-metal dipole arrays; a strong strike will cause the air inside the radome to heat up so fast that it will blow it apart, even if the conductors are capable of handling the current - radomes can break, after which time, you'll have water in the antenna and feedline and you have to start all over again - not as broadband as dipole arrays; pattern is steered up/down as you vary frequency CONVERTING COMMERCIAL ANTENNAS TO AMATEUR USE DB PRODUCTS ANTENNAS The model DB 450 has several versions, with a letter suffix. The A model is for 406 to 420, the B is 450 to 470, the C is 470 to 488, and the D is 488 to 512 MHz. This is all with VSWR of 1.5 to 1 or less. The factory specifies 20 MHz bandwidth, 9.2 DB Omni and 10.4 Db gain. elliptical. The power rating is 250 Watts and it has 16 folded dipoles. The DB408 covers the same frequencies in the same four models plus they offer one with a "T" suffix for down tilt. The 408 is rated 6.6 DB omni and 7.8 DB gain elliptical. If you wish solid HT coverage close to the repeater , the 408 is preferred. The 420 provides more gain and like most gain antennas achieve this by compressing the radiation pattern. This results in some deep nulls close to the antenna. The vertical beam width of the 408 is 14 degrees and the 420 is only 7 degrees. The DB 450B will go to 443.125 MHz. If it came from the factory tuned to 450.050 it will not require any further adjustments. You can typically use many 420 and 408 antennas in the 440 to 450 amateur band with no adjustments required if they were a B suffix. When comparing with one that was adjusted for the lower frequency there was less than .2 DB improvement. This is typically not worth the trouble for me. It will work OK in terms of VSWR/return loss. The gain will be slightly reduced due to the element spacing being a bit short. For that extra bit of gain, you can compensate for that by "stretching it out" a bit. Loosen up the clamps and adjust the elements such that you create more space between each pair of bays by about 1/4 to 3/8 inch. You'll probably find the harness has enough play in it to allow you to increase the spacing this slight amount. The DB 420 (450-470) works just fine on UHF amateur frequencies without modification. If there appears to be any sign of excessive wear, or the connectors are open to atmosphere, carefully remove the old tape and examine the connectors, being very careful not to twist the braids as they enter the N-connectors. If you break a connector free from its braids, you will be in a bit of a fix. You cannot cut the harness to reconnect the connector and reasonably expect the SWR and phase relationships not to go awry. FOLDED DIPOLE SPACING The correct vertical spacing for folded dipole arrays is as follows. Measure the end-to-end (vertical) distance of one of the elements. Use this distance for the tip-to-tip spacing between elements on the mast. Don't worry if the harness isn't quite long enough to make the spacing perfect, it's not critical as long as it remains in the 0.75 to 1 wavelength neighborhood, and it remains symmetrical about the center of the bays. In a 224E, four-bay unit, you'll probably notice that the distance between dipoles 1 and 2, and between dipoles 3 and 4 is less than the distance between 2 and 3. In other words, there is a little more "space" in the center of the array. That's OK, as the array is still symmetrical; what's above the midpoint is exactly the same as what is below the midpoint. What IS critical is that power is delivered to the bays in phase, which is something the harness does correctly. The fact that the spacing isn't uniform is no cause for concern here. Feeding the bays out of phase would result in uptilt/downtilt. Altering the spacing does NOT affect uptilt/downtilt for any reasonable amounts of skew, it only alters the peak gain by adding/removing power from the minor lobes, but the center of the main lobe always remains on the horizon. Ensure that the spacing between 1 and 2 matches that between 3 and 4, and call it good. One thing you definitely should NOT do is to allow the top bay to get too close to the mast, laterally, or too close to the top of the mast, vertically. First, the horizontal spacing to the mast is part of the matching to get each dipole's impedance right, so if you change this or extend the upper bay above the top of the mast, the match will be off. Second, for lightning protection, you want the bays to be as far from the top as possible/practical. The same applies to the bottom element - keep it away from the mounts. FOLDED DIPOLE ORIENTATION For mast-mounted arrays having 4 folded dipoles, such as the DB-224 series, the preferred orientation of the four bays, from top to bottom, is North, South, East, and West. They don't have to actually point in those specific directions, but it's the relative orientation that is important for the most uniform, omnidirectional pattern. This has the top two at 180 degrees apart from each other, as are the bottom two, and each set is at 90 degrees from the other. If the topmost bay were pointed North, this forms the order N-S-E-W from top to bottom. N-S-W-E would also be acceptable. The DB224-E is the elliptical version where all the elements are inline on one side of the mast...this makes the main lobe 9db gain, 6db on the side and 3db off the back (actually it is less off the back in practice). Usually it is labeled a DB224x-E where the x is A through E. Yes, you can swing the elements of ANY DB224 to the elliptical mode; all it takes is a screwdriver. The DB224E is the 138-148 MHz version for the amateur band BUT it is less gain than the standard versions A-D...this is because the 20ft mast is too SHORT for 2mtrs....the dipoles cannot be spaced properly....if you side mount the elements to a tower or longer mast you can get full gain (the elements are spaced an element length apart tip to tip but again you cannot do this on the 20ft mast with the ham band version...hence the lower gain) If you have a sidemounted antenna, and it is VHF, then the best orientation for omnidirectional coverage is to have all elements pointed back in toward the tower, according to the DB products catalog. The interaction with the tower cancels the elliptical pattern that would otherwise result as described above, for VHF only! AUTOPATCH Here's some data that might help you get a better rate for your ham radio Autopatch. In some states, there's a way to get residential rates in commercial locations IF the line will be used for emergency ham radio communication. State Phone Company Tariff Notes Links Credit FL Bell South No provision FL GTE Company Policy Ask for supervisor to approve MS Bell South PSC Order See linked documents PSC Order NC Bell South A2.3.6.C.7 See Tariff quoted in message TX SW Bell See News release CA GTE GTE Letter, maybe all GTE areas GTE Letter Any USWest US West letter, Sunny Jones USWest Letter CT SNET Contact K1TAV K1TAV Letter AR Any Rule 3.04 (B) Special Rule, Document 92-248R Any PacBell PacBell Press Release PacBell Much of this information comes thanks to the ARRL. While we're at it, here's some other ideas to keep your autopatch costs low. TYPE OF TELEPHONE LINE SERVICE Since most patches don't allow for long distance calls, ask for a long distance carrier of 'none'. For single line installations, this will lead to a charge of $0.53 per month on your local phone bill, but it will save you charges from some long distance companies that are much more. Most of the larger LD companies are now charging minimum usage fees or 'regulatory' fees. A PIC of NONE will avoid those charges. However, a PIC of none will NOT prevent long distance calls. People can still use PIC codes (e.g. 10-321) to made long distance calls. If you don't trust your controller to block long distance or there are other people with access to your equipment, you still need toll restriction. If you're in a club, check the phone bill yourself every year. You'll be surprised how extra charges can get added to a bill over the years. Many aren't needed. While it might result in more bills to pay, keep each site on it's own bill. That makes you look like a single line user and then certain surcharges are lower. For example, the fees that are supposed to go toward getting the Internet into schools go up for additional lines. If you had writing checks, get the automatic checking account withdrawal option. I haven't had to mail a check to a phone company in years. And there's another $0.32 a month you can save! Since we don't want our friends looking for us on our patch line, or to prevent hackers, we usually pay to get the patch line non-published or unlisted. This usually costs about $2.00 a month. There's no need for it. The Listed name does NOT have to match the Payer name. For example, on my repeaters, the bills come in my name, but the service is listed as 'Otto Patch' It does have to look like a real name, but there's no requirement that it be real. When other hams see 'Otto Patch' on the caller ID, they get the idea and know it's a half duplex call before they even say hello. AUTOPATCH LINE WIRING When ever possible, do your own inside wiring. In every case, I've met the phone company at their terminal in the phone room. You might need to borrow a 'fox and hound' from a friend to trace your cable pair. The fox puts a loud audio tone at the repeater end of an unused pair, then you go to the phone room and feel around with the hound until you hear the tone. The tone is picked up inductively, so no physical contact with the conductors is needed. Punch everything down nice and neat like the other lines in the building, then leave a tag on the wire for the installer. Most are so happy to see that most of the work has been done, they'll hook it up to their terminal with no questions asked. And you save a lot on the inside wiring charges. MULTIPLE DEVICES SHARING ONE LINE Do you have other devices at the site that need to answer the phone? I have 3, the controller for DTMF commands, the controller's modem for remote updates/backups and an APRS weather station computer modem. Did I order 3 lines? No, just one. But that one line has 3 numbers. This distinctive ringing service is called a lot of things (see table below). The bell rings with a different cadence depending on what number was dialed (short-short or long-short-long) There are devices available that listen to the first ring, then route the call to the right port by the second ring. Once a call is established, the same device gives the other ports a busy signal to prevent them from barging in. Most of these devices cost under $80 and are available at telecom distributors like Graybar. One such unit is the RD3 by Command Communications, Inc. If your only other option is additional lines, they will pay for themselves in no time. EMERGENCY AUTODIALING It's always a good idea to check with the communications supervisor of your local dispatch center about your patch. If it dials 911 directly, it will commonly show an address that will confuse the issue with many dispatchers. Dispatchers send officers to checkout 911 hang ups, to an address that doesn't exist or they can't get from a patch call. In addition, the 911 display the dispatcher sees might confuse the issue or even route your call to the wrong area. Many if not most Communications Centers will have a 7 digit number that they receive cellular or other mobile and special 911 calls on, and may prefer to have your patch call that number. It immediately identifies the call as a mobile call,and all the problems/challenges associated with it. If they want the call to go to 911 they will need the phone number those calls originate from, so that they can update the 911 information screen so that it provides an indication that it may be a mobile call. All this varies greatly from place to place, so check with your local officials to find out how they prefer to handle those calls. A few minutes ahead of time can save a lot of confusion and time in an emergency. OFF THE HOOK MONITORING If you have more than one device sharing a phone line, you will need a way of monitoring the line to be sure that a device does not pick up when another device is already on. You can build a monitoring circuit. Take a low current 5 volt reed relay (SPST from Radio Shack) and put it in series in one side of the phone line (it doesn't matter). Put a .1uf 200v cap across the coil. Basically, when a device (answering machine, modem, whatever,) closes the circuit on answer or call-out, the line current through the coil of the reed relay energizes the coil. You can then use the closure to feed the landline busy input on your controller (the Scom has this). INTERFERENCE PROBLEMS INTERMOD PRODUCT CALCULATOR Anyway, I found this really cool program for calculating intermod products and bunches of other stuff. Best of all, it's free for the downloading at the following site: http://www.rfspec.com/rfs.htm ISOLATORS The use of an isolator on the transmitter output can help. The isolator has to be tuned to YOUR transmit frequency. It gets installed between the transmitter and duplexer with an appropriate load on the proper port. It is possible that you may need to run a dual section isolator and possibly a bandpass can tuned to your TX or a notch tuned to the interfering transmitter's frequency (these would go on the transmit leg). RX FILTERS PAR Electronics enjoys an excellent reputation for making RF filters for your receiver. They now make a three "cavity" notch filter, similar to the VHFDN152, called the VHFDN152-158. This new model provides "two steep notches" between 152-153, and a single in the 158-159 paging range. Given the design of these filters, you have to select the model you need based on the operational frequency. With some models, if you're using it above 153 you need a different model than you would use on 2 meters, because the shape of the skirts. The difference can range from a db or two, to around 10-15 db. There are also HT models, commercial models and marine models, so shopcarefully and maybe solve many problems! Do the research and buy the one you need, and your IMD may go away. They will also custom tune one for any frequency you need. There was an article once in 73 magazine about using open stubs to get rid of VHF paging in two meter recievers. Basically, they used a open 1/4 wave stub cut to the frequency of the paging transmitter on a tee on their reciever. A stub might be a simple solution to someone's paging interference to a repeater. Don't even try the pass cavity approach......buy one of the Par Electronics pager notch filters for your repeater.........well worth the investment..........we use the triple notch version and have 2 of the notches tuned for the 157.740 frequency and the third tuned for the 158.400 frequency.........works great........installed it between the duplexer and pre-amp.......we are located about 400 yards away from a 157.740 "digital squalker" running 500 watts and this is quite effective.....check out this web site for Par Electronics....... http://www.rf-filters.com/filterham.html Ron Rogers -WB8ERB- Sawnee Mountain Amateur Group Fred, I know of two manufacturers making preamps with HR front ends, Ramsey and GLB. I use several GLB preamps that employ helical resonator filtering. These are much better than the Ramsey design. (in my opinion) The Ramsey is only available in 2 meters where the GLB is available in anything from 40 mcs. to 1 gig. GLB has been absorbed by another company, however they seem to still provide the preselector preamps as I found this page: http://www.aria-glb.com/products/reset_frames.htm?/products/preselector.htm The page lists the specifications.... Hope this helps... Kevin Custer GLB makes them. Back in the late 70's and early 80's that was the "hot setup" on repeaters. I have one of their UHF ones around here somewhere. Works OK, but I prefer cavities. Helical resonators tend to have a "window" passband while bandpass cavities are much more narrow. On 2m, a small helical resonator like in the GLB would likely have little attenuation 600 kHz away, while a pass cavity might give you 10 dB or so more attenuation of your transmitter (assuming your repeater transmitter is at the same site) which you may need if the transmitter carrier supression in the duplexer is borderline. For your particular case, whether or not you need something really steep or not can be determined by looking at what's coming down the hose with a spectrum analyzer. You may only need 10 dB or so of additional isolation from the paging Tx to take care of the problem, or you might need 30 or 40 or more. But at 2 miles and 14 MHz away, I'd be suprised if you need a whole lot of additional help; a shorted stub made out of scrap heliax (the bigger the better) may even do the job. You might look at the DCI window filters. They are VERY large helical resonators. I've used several and was quite impressed with how they swept out. They make 144-146 MHz model and a 144-148 MHz model as standard configurations; sounds like the 144-148 model would work well for your Rx on 144.770. Look at their web site for response plots. Cheap too, and built nice. Generally speaking, bandpass cavities are preferred over a notch cavity tuned to just the offending transmitter's frequency. With a bandpass cavity you get all of that extra rejection of other off-channel stuff as an added bonus (including stuff very far removed such as FM broadcast, TV, etc.). Notch cavities are useful when you have to reject something close in frequency to your desired frequency, but at 14 MHz away like you're dealing with, pass is the way to go. AN INTERMOD TROUBLESHOOTING EXAMPLE Here's an example for troubleshooting an actual repeater system on 146.61/146.01 that has two pager transmitters nearby on 158.1 and 158.7 MHz: You've got a third order product mix. Fundamental of 158.1 plus the fundamental of 146.61 minus the fundamental of 158.7 = 146.01 (smack dab on the input frequency). Not to nitpick, but there's no such thing as a "1st harmonic". It goes fundamental, second harmonic, third harmonic, etc. But the above relationship does yield a third order mix A+B-C. Anyway, the first question to answer is where the mix is occurring. It can be happening in one of five places: 1) In the 158.1 Tx; 2) In the 158.7 Tx; 3) In the 146.61 Tx; 4) In the 146.01 Rx; 5) Externally (such as a rusty tower joint). Regarding #1 and #2; You already said that both paging Tx's are running isolators. That's a good start. I'd check them anyway by looking at the output of each of the paging transmitters with a spectrum analyzer and a sampler (the three Tx's need to be connected to their respective antennas and all keyed up). Use a notch cavity in line with the spectrum analyzer (NOT the feed line!) tuned to the paging Tx's fundamental to gain as much dynamic range on the analyzer as possible. Look for signs of anything on 146.01. If both are using isolators, chances are the mix isn't happening in either of these two transmitters unless all of the antennas are very close together. If that is the case, that the antennas are all close, additional isolator stages or cavities should cure it. Re #3. Do the same test as above, but on your repeater Tx on 146.61. If you don't already have an isolator on your Tx, you should. An extra cavity on your Tx leg wouldn't hurt either (pass/reject preferably, passing 146.61and rejecting 158.4).Re: #4. Look at what's coming into your receiver with a spectrum analyzer. If any of the three transmitters are excessively strong (say over -50 dBm),there's a good chance the mix is happening in your Rx. Considering that when you changed Rx's the problem got worse, this makes me think that it's a receiver IM problem, not a transmitter mix. Re: #5. This one takes the most effort to diagnose. But I'll teach you a trick here that is a sure-fire way that will distinguish #5 from the rest, and also show you how to narrow your hunt down further. This is a trick that has helped find many a mix product. 1. Hook up a spectrum analyzer to the Rx port of the duplexer in place of your receiver. Key up your repeater Tx and measure the signal level of the repeater transmitter making its way back through the duplexer into your receiver (say, for sake of argument, it's -60 dBm). 2. Disable your repeater transmitter. Disconnect it from the duplexer. Put a 50 ohm termination on the Tx port of the duplexer just to keep everything happy. 3. Stick a lossy tee or sampler between the Rx port of your duplexer and the spectrum analyzer. Inject a signal on 146.61 from a signal generator into the lossy leg of the tee (the "through" ports of the tee connect the duplexer to the spectrum analyzer directly). Adjust the sig gen such that you see the same signal strength on the spectrum analyzer as you saw when the real repeater transmitter was hooked up to the Tx port of the duplexer(the same -60 dBm you measured initially). Ideally, if you have enough headroom, crank the sig gen up relatively high and add extra pads between the sig gen and the lossy tee to further isolate the sig gen from the feedline, but keep the level measured on the analyzer where it should be(-60 dBm or whatever). 4. Leave the sig gen on and the tee in line, but reconnect your receiver in place of the spectrum analyzer. At this point, your receiver is seeing the outside world through the duplexer as it normally would, plus the "fake" 146.61 transmitter carrier your sig gen is injecting. 5. If you still have the mix product when the two paging transmitters key up, you know that the mix product is happening in your receiver. How do you know this? Because there won't be enough signal from the sig gen leaking through the duplexer "backwards" to get out your antenna and into either of the paging transmitters under this test setup, nor will there be enough signal radiated by your antenna to catalyze the mix in a rusty join ton the tower, so if you still have a mix, it's happening in your receiver, done deal. Fix it by further attenuating any one of the three transmitter signals via cavities added to your receive leg. 6. If you no longer have the mix, at least now you know it's not in your receiver. But wait, there's more! You're not dead in the water if you got to #6 above. You can use this same trick to inject "fake" carriers into the paging transmitters using a lossy tee, again at a level that is the same as what would normally be received by the respective transmitter's own antenna. For example, measure the 146.61 and 158.1 signals getting into the 158.7transmitter, inject signals of the appropriate level (one at a time, you don't need two signal generators) with the corresponding "real" transmitter shut off, and see if you still hear the mix on your repeater Rx. If you go through these four more iterations (two on each of the paging transmitters) and you can't reproduce the mix, then it has to be happening externally (rusty tower joint or other). If it IS happening on the tower, you can try to find the culprit or move antennas around on the tower empirically until you find locations that minimize or eliminate the product. If the mix changes with the weather (particularly rain), that may be a further indicator that it's a rusty joint kind of mix. There IS one other possibility that I haven't mentioned, because it's a real longshot. The mix may be happening in your Rx but NOT because of RF coming down the feedline into the receiver, but instead getting into the receiver directly due to inadequate shielding in the receiver itself, the patch cables, audio cables, DC cables, or other means by which RF can get in without coming in through the antenna. Bypass the DC and audio going to/from the receiver chassis (preferably via feed through caps). Try relocating the receiver to somewhere else. You get the idea. A crystal bandpass filter on the input would help only if the mix was occurring in the receiver. If the mix is occurring elsewhere, you could possible hold a foxhunt type search in the immediate area on 146.01 MHz. A sharp filter passing only this frequency and an attenuator would be required to prevent overload of your ht. A possible solution may be to use isolators. There should be an isolator on each of the three transmitters, each isolator tuned to that transmitter's frequency, and installed right at the PA output. There should also be a harmonic filter after the isolator. EXAMPLES OF UNUSUAL SOURCES OF INTERMOD AND NOISE We had a stainless steel hose clamp that fell and was hanging over a tower leg. It was in the near field of one of the antennas at the site. When they keyed up, IMP was created by the dissimilar joint of the clamp and the tower leg. Removing the clamp fixed the problem. AM BROADCAST STATION INTERFERENCE Very often, when you're hearing an AM station somewhere that you didn't used to, you have a ground loop problem. The poor grounding situation may be causing some of the RF from the AM to be rectified and causing problems within the repeater's circuitry. Take a look at how you have it wired, and whether you have the ground attached to chassis ground or to system ground (the latter is the correct place). You may also need to add some AC (capacitive) coupling to the audio in/out of the board depending on how you have it hooked up. PAGING SYSTEM INTERFERENCE Paging base stations can run many kilowatts of ERP. The FCC requirements are not strict enough, in my opinion, for spurious and harmonic output. Typically this is -90dBc, which means the spurious and harmonic output only has to be -90dB less than the main carrier signal level. When you start with a 3,000 watt ERP signal, this interference can be quite high in level. The paging company can be operating legally, but still create problems for the other services in the area. And, digital FSK NRZ modulation (square wave) only adds to the problems. A repeater located in this area of high RF levels can suffer many problems. Receiver overload due to insufficient filtering and/or lack of isolators comes to mind. The ham repeater PA can be a source of intermodulation. Hams build on a budget, so many times the equipment is marginally adequate. I know some repeaters are built very, very well, so please no flames. Keep in mind who pays the rent on the site that many hams get free rent on before you throw stones. Sometimes the solution is to ask the problem station to leave the site. It may be the paging base station, or it may even be the ham repeater. Put yourself in the site owners place. Would you like to loose $250-1000 a month rent because some hams are complaining? Try to find out if the paging equipment is interfering with any commercial/public service communications. It is easier to get my company to buy a $1,000 filter to solve a problem with an ambulance or fire department system than with a ham repeater. You can deal with interference from paging transmitters, one at a time, presenting solid facts and resolve them each time, without the help of the law or FCC. Paging companies are usually responsive when presented with solid evidence, including the specific site that the problems are coming from. Once they are fixed, watch out for the next storm or temperature extreme, as these systems can and will act up again. That's USUALLY the kind of response you'll get. However, you usually need to come to them with more than "your transmitter is clobbering our receiver input." You need to be able to demonstrate to them that, in fact, they have spurs, and that it really is their transmitter causing them. HOW TO DOCUMENT THE INTERFERENCE You should document: The callsign of the offending station (they all should ID in CW) The frequency of the offending station. The location of the offending station (based both on FCC records and DF'ing or other techniques) The type and nature of the spurious emissions, such as "a wandering carrier that starts out around 147.2 MHz and drifts upwards at roughly 100 kHz every 5 seconds until it settles at 147.95", or "periodic spikes within 30 dB of carrier spaced about 1.5 MHz apart". Something that a tech will be able to readily identify and confirm when he visits the offending transmitter. When the interference was first noted (it may correlate to when they installed new hardware, replaced a PA, etc.). In the case of spurs, note whether or not the spurs come up as soon as their Transmitter keys up, or if they only start after the transmitter warms up (again, to help the tech duplicate the problem) Any other information that a) demonstrates that you know what you're talking about, b) presents definitive proof that it is indeed their transmitter responsible for the problem, and c) accurate descriptions of the nature of the problem so it can be confirmed. Make sure you're not crying wolf. Just because you hear something coming out your repeater that shouldn't be doesn't mean it's somebody else's spur. It could be a mix, it could be an image, it could be a problem in your receiver, it could be a dozen other causes aside from a transmitter spur. Any reputable company will follow up on a properly documented interference complaint like that. If they don't, they don't qualify as reputable. If you can't work your way up their internal corporate ladder and get some results, you may have to go to the FCC. The same rules apply when dealing with the FCC: complete and accurate descriptions, and document, document, document everything. I'd highly suggest corresponding by phone with the> operator only as a first pass. If you don't get any response after the first phone call, revert to paper correspondence and request that they respond by mail (or fax) as well; you may need the documentation later. Most responsible tower and building owners require 1 or 2 cavities set to no less than 1 dB per loop & an isolator, which will stop a lot of spurs, if they are being generated, and a lot of the intermod mixes, which are usually most of the interference problems. Most responsible tower and building top leases require not only that, but that the lessee's cure the interference in a very short time frame or shutdown and get off the tower. Most of the paging companies want no problems and if the paging company tech does not respond (which is rare), a call to their boss will get action quickly. SPECIAL NOISE PROBLEMS ON THE 6 METER BAND Normal background static or atmospheric noise tends to peak around 40-60 Mhz and of course you KNOW is in the middle of that! It gets worse on cold dry days that ionize the air (and cause static electricity to pop you every time you walk across carpet or slide out of a cloth car seat and then try to close the door!! OUCH!)...Without a Noise Blanker in the receiver, this noise can cover up a mobile signal easily on lowband VHF and 6. A noise blanker (NB) can reduce the noise 20db or more...making a signal that is hardly readable become almost full quieting. This is typically a problem near power lines and other sources of "static". Static noise exists across the spectrum but peaks at 6m and really tears up the band. The NB in MOST lowband commercial radios is a second receiver in a nutshell: a WIDEBAND AM receiver (on channel & non-heterodyne). It detects the noise pulses and clamps the IF to ground, taking out the noise pulses. The filters in the IF recreate the missing part of the signal without the noise and you get a cleaner signal. In a duplexed repeater setup on 6m, the NB cannot function because: 1) The transmitter will desense it if the duplexer is notch only. 2) The NB will not be able to hear anything if the duplexer is a BpBr type. The NB channel must operate 1.5 Mhz or more AWAY from the receiver channel to be effective and not degrade the main receiver channel. A Bp filter or BpBr will not pass a high enough signal that far off. Hence, you need a separate receiver site, far enough out of the near field of the transmitter for good operation of the NB. POWER SUPPLIES Models with good reputation: Astron, G.E.. Models with BAD reputation: Pyramid (similar design to Astron, but poorly made). PROTECTION FROM OVER-VOLTAGE One good way to protect your repeater equipment from over-voltage delivered by your power supply is to install a high capacity zener diode across the power feed, downstream of a fuse. For a nominal 13.8VDC supply, select a 15 VDC Zener diode. Connect the cathode to negative and the anode to positive. When the voltage regulator in your supply gives out and the voltage goes over 15, the Zener will break down and conduct, providing a direct short to ground. This blows the supply fuse and shuts down your equipment. You also get reverse polarity protection with this scheme. The Astron Linear series supply is a great supply for powering repeaters as their over-voltage protection circuit is second to none as far as tripping. The problem is in resetting it. You must go to the site and turn off the power switch for about 10 seconds then restore it to reset the supply if it has gone into over-voltage protect by a near lightning strike or power surge. Astron makes a circuit to automatically reset the supply without having to make the dreaded trip. This reset circuit is designed for their linear series like the RS-20 through RS-70 supplies. Information for building their circuit and another can be found at: http://www.repeater-builder.com/rbtip/astron.html REPAIRING ASTRON POWER SUPPLIES Here is a web addresses for schematics and other information on Astron model power supplies: http://www.repeater-builder.com/rbtip/astron.html EMERGENCY POWER SYSTEMS FLOAT CHARGING BATTERIES This is normally done at a voltage of about 2.16 to 2.2 volts per cell. 12v batteries contain 6 cells, so the optimum float charge for these is about 13.0-13.2V. Since this is normally delivered through a diode to prevent the battery from 'seeing' the power supply, the actual voltage delivered from the supply has to be somewhat larger to compensate for the voltage drop through the diode. In any case, do not exceed a charging voltage of 2.3 volts per cell (13.8 VDC for 12 V batteries). MARAUDERS, PIRATES, AND JAMMERS TRANSMITTER HUNTING There is no one solution to good DFing. A doppler DF unit is good for short or long duration transmissions, but not real sensitive (some better than others). A good yagi or quad beam mounted in the vehicle and hand rotated will work good on longer duration signals. An attenuator is needed with this setup. Best advice is to get some practice at an organized hidden transmitter hunt in your area, and see what they do. Its a good way to learn. Information on hidden transmitter hunting & equipment can be found at these sites: http://members.aol.com/homingin/index.html http://www.frontiernet.net/~n2ki/ http://home.att.net/~jleggio/projects/rdf/rdf.htm http://www.qsl.net/n6bg/thunt/ http://www.dfsystems.com/ http://members.aol.com/BmgEngInc/ http://members.aol.com/Arrow146/index.html http://www.wenet.net/~ahha/ There are lots of equipment descriptions, kits and hunting tips at these sites. If you do not want to build your own DFing equipment, you can purchase everything you need from a company called Doppler Systems Inc. They have been supplyng hams (and others) with moderately priced DXing gear since the early 1970's. Also, go to http://members.aol.com/homingin/ on the world-wide-web to see thelatest info on ll aspects of t-hunting including jammer locating. Joe Moell, K0OV, wjo sponsors the page is one of the worlds leading experts in this aspect of two-way radio. LIGHTNING DETECTORS You can build a static field detector that can be used to trigger disconnection relays for your feedlines or telephone lines. Take an NE-2 neon bulb [used to be common, bet you have some in your junk box] and tape it to a cadmium sulphide photocell or a phototransistor with black electrical tape, so the only thing the photocell can see is the NE-2. Connect one leg of the NE-2 to ground and the other leg to 10 feet or so of wire through a 220 k resistor. String the wire up as an antenna. Use a darlington pair or some other little transistor DC amplifier to couple the photocell to the trigger of your relay circuit, such as a 555-based timer circuit. The idea is that lightning discharges are generally preceded by some pretty hefty voltage gradients, on the order of kilovolts per meter. By putting the antenna up in the air a few meters, you'll get enough differential to light the NE-2. Not all gradient buildups will result in lightning strokes. SOURCES REPEATER BUILDERS FOR HIRE Matt Bush, KA9RIX, of Ft. Myers, FLA, 941-694-5911. He uses Motorola Micor and whatever controller you want. He gets the crystals, modifies the radio, will get your duplexers if you need them, tune everything up and send it to you ready to go on the air. All you need is an antenna and coax. Matt advertises in QST in the ham ads every month. SOURCES OF CONTROLLERS NHRC http://www.nhrc.net Basic controller with link capabilites for the GE MAster II's that plug right in. CAT (Computer Automation Technologies) http://www.catauto.com Full-featured controllers with many options. Link Communications S-COM http://gandalf.rmsd.com/scom/ Pacific Research Solutions http://www.directcon.net/pacres/pacres.htm HAMTRONICS SOURCES OF ANTENNAS DB PRODUCTS Marketronics Corporation (DB products reseller) 1-800-845-1230 Janine Ext. 244 http://www.marketronics.com DIAMOND COMET CUSHCRAFT SOURCES OF DUPLEXERS WACOM TX-RX CELLWAVE SOURCES OF REPEATERS Yaesu Icom Maggiore Electronics 610-436-6051 Hamtronics Bob Barnett Electronics / Lonoke AR 800.423.3858 - 501.676.5506 Source of GE Master II's & others Micro Computer Concepts 8849 Gum Tree Avenue New Port Richey, FL 34653 (727) 376-6575 Controllers & complete repeaters based on GE Master II radios http://home.tampabay.rr.com/k4lk/mcc/ SOURCES OF CONVERSION KITS There's a company called Versatel in Casper, Wyoming that offers a neat GE Master II conversion kit with all the parts, and the instructions required for the conversion. I think the docs and parts kit are $25 each, but you can get free docs off various internet sites. The $25 for their parts kit saved at least that much aggravation finding odd parts. SOURCES OF PREAMPS Advanced Receiver Research Box 1242, Burlington, CT 06013 (860)485-0310 SOURCES OF RF/INTERMOD FILTERS PAR Electronics 6869 Bayshore Drive Lantana, FL 33462 http://www.parelectronics.com/ http://www.rf-filters.com PAR Model VHFDN152 specs: Design Impedance: 50 ohms Max. Stopband Attenuation: 50db 2m VSWR: 1.25:1 max. 70cm VSWR: 1.3:1 max. (yes it works nicely on a dual bander!) Power handling 2m/70cm: 50 watts Typical in-band insertion loss: 0.3db Charts also claim a max rejection of 58.69db at 152.40 mHz. SOURCES OF CRYSTALS International Crystal xxx-xxx-xxxx NXK 1(800)237-6519 SOURCES OF CALIBRATION & REPAIR OF SERVICE MONITORS Triton Electronics, Inc. 4300 Lincoln Ave. Unit O Rolling Meadows, IL 60008 Ph. (847) 934-6426 Fax (847) 934-7195 tritonelec@aol.com HTTP://www.tritonelec.com SOURCES OF SURPLUS COMMERCIAL RADIOS PANIK'S ELECTRONICS -N- SURPLUS, INC 2885 ELECTRONICS DRIVE BAY# D-8 MELBOURNE, FLORIDA 32935-2164 E-MAIL ADDRESS: jpanik0889@aol.com 407-253-0889 http://www.surfusa.com/panikelectronics.html Carolina Radio P.O. Box 399 Sophia, NC 27350-0399 (336) 434-4388 Crobbinsnc@aol.com http://members.aol.com/crobbinsnc/carolinaradio.html SOURCES FOR MOTOROLA MANUALS Motorola Phone No.: 1-800-422-4210 SHOPPING FOR A GE MASTER II RADIO TROLLING FOR MASTER IIs from QST May, 1995 By Paul Gilbert, KE5ZW Many hams today feel that they need to have the latest and greatest Gee-Whiz XYZ-345 all-mode radio with nine zillion channels, just to work their local repeater. In the frenzy to keep up with that trend, many great buys of used commercial gear are overlooked at hamfests and auctions. Look for familiar brand names and models. When purchasing a used commercial radio, such as the General Electric Master II, see if it's for VHF or UHF and check its options, because many dealers don't know. Because I'm most familiar with the GE's, I can give some info and hints on the GE Master II. Although most of these great commercial radios are crystal-controlled, and big and clunky, they can be excellent buys at $100 or less. The Master II mobile was made in the late 1970s and the '80s as a replacement for the Master Pro. It's a trunk-mount radio with a control head and cable arrangement mounted in the driver's compartment. The radio came in frequencies from 29 MHz to 800 MHz with one, two or eight channels. To determine what model you're looking at, decode the combination number printed on the metal plate to the right of the cable connector on the Master II. The combo number is on the back of the radio on an MVP and under the handle on an Exec II. Here is a reference table: How To Tell What You Have =-) 1st digit - d = desk mate 30" cabinet m - mobile unit s = desk mate 44" cabinet p = pole mount cabinet v = floor mount cabinet 2nd digit - i - intermittant duty cycle c - continuous duty cycle 3rd digit - 4 - 8 to 20 watts 5 - 21 to 40 watts 6 - 41 to 80 watts 7 - 81 to 128 watts 4th digit - 4 - 20khz channel spacing 5 - 25khz channel spacing 6 - 30 khz channel spacing 5th digit - e - extended control j - ext local tone remote k - ext local/dc remote n - ext local repeat r - dc remote t - tone remote u - dc/tone remote v - tone remote/repeat y - repeat 6th digit - a - 1ch tx/rx b - 2ch tx 1ch rx c - 2 tx/rx channels d - 1tx and 2rx channels e - 3tx/rx channels f - 4tx/rx channels r - 3tx - 1rx channels s - 4tx / 1rx channels 7th digit - d - duplex g - channel guard and preamp l - channel guard and duplex n - noise blanker p - preamp s - standard u - channel guard (pl tone) w - channel guard and noise blanker 8th and 9th digit 12 - 25-30 mhz radio 13 - 30-36 mhz 23 - 36-42 mhz 33 - 42-50 mhz 44 - 66-78 mhz 45 - 78-88 mhz 56 - 138-150.8 mhz 66 - 150.8-174 mhz 77 - 406-420 mhz 78 - 420-450 mhz 88 - 450-470 mhz 89 - 470-494 mhz 91 - 494-512 mhz 10th digit a - +/- 5ppm b - +/- 2ppm c - pll +/-5ppm d - pll +/-2ppm taken from the ge manual Look for a 56 or 66 in the combination number for a VHF rig and an 88 or 78 for UHF. These will be the eighth and ninth digits in the combo number. The 66 model has a frequency range of 138 to 150.8 MHz. The 78 is for 420 to 450 MHz and the 88 covers 450 to 470 MHz. One quick way to tell the UHF version of the Master II is by the orange wire coming out of the receiver going to a point on the circuit board in front of the receiver cans. Other things you can learn from the combo number are the number of channels the radio will use, whether it's equipped with CTCSS, and if it has a receiver preamp built in. Channel capabilities are listed in the fifth-digit position: A for 1, C for 2 and K for 8. The CTCSS option is the seventh digit : S for no CTCSS, U for CTCSS, P for no CTCSS with receive preamp and G for CTCSS and preamp. Master II models came with various output-power levels from 40 to 110 W, depending on frequency. The power of the radio is the third digit from the left of the combo number. A 4 is 20 W, 5 is 40 W, 6 is 75 W and 7 is 110 W. There were two basic versions of the UHF power amplifiers. The early versions used a large hybrid power-control chip (usually black) and had a blue, flat power-control pot. Later versions had a multi-component power-control circuit and a red pot. The "red pot" version is more desirable for ham use and 420 MHz work because it will put out full or near-full power in the 440 to 420 range. The older "blue pot" version is about 50% efficient in the 420 range. The radios can be easily tuned to the ham bands with no modifications, with one minor exception for using the 88 or 450 to 470-MHz radio in the 420 MHz range: You must add a 5-pF capacitor to three coils (T-106, 107 and 108) in the exciter, and order receive crystals for high-side injection. Then tune the radio in a normal fashion. Otherwise, order crystals and tune per manufacturers' specs. Other Master II trivia includes the case style the radio was built in. The most common Master II radio comes in the standard case, about 2-1/2 inches thick, with only one receiver and transmitter. It's painted a medium olive gray. Another case style is the E-line case. It was made to accommodate several options, including four additional channels (for a total of 12), a second receiver or transmitter, and a synthesized transmitter/receiver option, or combinations of the above. The E-line radios were about 5 inches thick and very heavy. The first digit of the combo number would be an E on those radios. They were painted the same medium olive gray. Some that were in UHF IMTS mobile phone service have four-can UHF mobile duplexers in them. Depending on which model you find, there may be other options that can be determined from the combo number. A GE Master II manual has most of them. Moving on to the control head: there were three basic versions, with many options and variations. The most common control head was the C-500 or clamshell-style head. It can handle 12 channels and has an option of a one-channel priority scan. The C-800 was the most common scan head made for the Master II. It was big, at 9x5x7 inches, and it would scan up to eight channels, with several options available. Last is the C-900. You think the C-800 is big until you see one of these! it was half again larger than the C-800. It has buttons and options such as 12-channel scan, multi-Channel Guard (CTCSS), siren, gun release, lights and more. C-900 heads aren't often found in good condition. Manuals for the Master II often show up at hamfests, or can be begged or borrowed from your local two-way shop or friends, and people who sell them often advertise in ham magazines. As a 1970s-vintage radio, the Master II uses a transmit and a receive crystal for each channel you want to work. When you order the crystals, buy good ones from a company that will warranty them for life. The prices range from $9 to $35, depending on the dealer, the speed of the service and the tolerance of crystal. Because the crystals take time to make, the best price is usually on four-week delivery. This will get you better stability because the crystals are burned in and tested better than ones ordered by requesting 48-hour service. Master IIs use crystal holders called ICOMs, which come in several models. For mobiles, use ones marked EC on the transmit side and EC with one 5C on the receive side. The 5C is a frequency-compensation ICOM and helps adjust the frequency with changes in temperature. Don't use more than one 5C. It's best to put the 5C on the receive side. The ICOM's are marked for transmit and receive, so try to keep that sense correct. By buying good crystals and setting up the compensation correctly, you shouldn't have trouble with frequency changes. Other trivia needed for trolling for Master II's are knowledge of the vintage of Channel Guard Boards, SAS boards, Exciter boards and the types of cables used. This information applies mainly to the Master II radio; the MVP and Exec II radios are similar in design, but packaged differently. I focused on the Master II because I know it best, but much of the information on combo numbers and options apply to the MVP and Exec II. The MVP was an under-dash-mount radio and the Exec II was a trunk-mount version. Some of these radios have been put through the mill and are long overdue for retirement. Buy the radio from someone who will back it up if it doesn't work. If it looks beat up, has corrosion on it or is kludged up from several different radios, don't buy it. I hope these tips on GE Master II hunting help you troll the hamfests for the bargain of the show. These radios can serve you for years and escape the fate of the scrap pile. Sources for crystals include International Crystal Manufacturing Co Inc, 701 W Sheridan, PO Box 26330, Oklahoma City, OK 73126; 800-725-1426, and JAN Crystals, 2341 Crystal Dr, PO Box 06017, Ft Myers, FL 33906-6017; 800-526-9825, 813-936-2397, fax 813-936-3750.