Dick Stroud, W9SR
This appeared in the May 1999 edition of "The Network News", the newsletter of the Good Sam RV Radio Network, Hal Cummings, KJ4TD Editor. Most of us don't think of TVI (or TV Interference) as a major problem today, what with the prevalence of cable TV systems. But when you're operating from a RV, where your neighbor may be parked only several feet away, and relying on an external antenna to pick up the TV signal the problem can be much more severe. This article addresses this special case.
TVI are three letters that strike fear in the hearts of hams everywhere. but if you get a complaint from a neighbor don't panic - get busy!
It is necessary that you have a good relationship with your neighbor by convincing him that you will work with him to correct the problem. Don't assume the fault is in your transmitter - usually it's not, and the interference can always be corrected!
It is important that you get your own house in order before starting to test at the neighbor's. Most amateur equipment today is well shielded and this means that interfering signals can exit your transceiver or amplifier only via the antenna or input power leads. If you have TVI on your own TV set when transmitting, remove the antenna coax from the transmitter and feed the signal into a dummy load. If the interference is still present you will need to filter the AC line (or 12 volt power input lead) to the transmitter. This can usually be done by soldering a .01 mfd., 500 volt disc ceramic capacitor from each power input lead to chassis ground. In rare cases it may be necessary to include a choke of sufficient current capacity between the transmitter circuit and the disc capacitors.
Once you are interference free into the dummy load, add a 50 ohm low pass filter between the transmitter and the antenna.
Your transmitter puts out a signal on the desired frequency and; at a much lower level, on harmonics of that frequency. It is necessary to eliminate these harmonics, some of which may fall on television channels. This is the reason for the low pass filter. If you are using an amplifier you should have a filter between the transceiver and amplifier and another between the amplifier and 50 ohm antenna as the amplifier can regenerate harmonics if it is not completely linear (which is always the case!). These filters should be of a power rating for the purpose involved and should be placed as close as possible to the transceiver or amplifier and not after a long length of cable. Filters are available from several sources, such as MFJ. (MFJ-704)
With these measures taken it is now time to turn to the television receiver. Remove the TV antenna and, while transmitting into the antenna, look again for TVI. If present you will need to bypass the AC power leads into the set, again with .01 mfd. disc capacitors direct to chassis. A choke can be formed by using a Radio Shack 273-104 snap-together core, winding as many turns of the AC line cord as possible through the core center.
When no TVI is noted without the 'TV antenna, install a high-pass filter at the 75 ohm input to the set and connect the coax from the antenna to the filter input. These filters are available from Radio Shack (15-579), MFJ (MFJ-711), or you can use an excellent unit made by Zenith (A-8477) and now marketed by AES. I carry a spare Zenith unit just for test purposes in case I get a complaint.
With these steps taken you would normally be free of TV interference. The one thing that can still show up with extremely strong signals is fundamental overload. This could happen if your transmitted signal is very strong (several volts) through the high pass filter and causes serious overloading of the receiver circuitry.
This can only happen if you are running high power and the transmit and receive antennas are only a few feet apart. The typical FET TV receiver front end will be destroyed with 6-8 volts at the input device. The signal level decreases as the square of the distance so a few additional feet of separation will make it big difference.
Even this drastic case can be corrected by installing a series trap at the transmit frequency from the center conductor of the TV input connector to chassis ground. These measures will not noticeably affect the desired TV signal level. If you are routing your television antenna input through a VCR or other device it must be handled in the same manner - bypass the AC line and filter the antenna input. Just be sure to test in steps, first with the TV receiver alone. If the television receiver uses an extension speaker these leads can function as a pick-up antenna. Try using one of the snap-together cores with the external speaker leads wrapped through the center as many times as possible. Wind the choke so as to keep the choke near the TV chassis. The leads can be bypassed to ground but with only a small value capacitor. (I would try 220pf.). Larger values may cause the audio output system to oscillate.
One problem that is unique to RV operation is that the rooftop, crank-up antenna sometimes includes a low noise wide-band amplifier as part of the antenna design. 12 volt power from the amplifier is routed through the coax cable and is switched on and off in the coach. The problem is that the wide-band characteristics of the amplifier gives gain to your transmit signal as well as the desired VHF-UHF TV signals. This means your signal will be amplified, making it harder for the high-pass filter to remove your signal and also your amplified signal can mix with other amplified signals generate frequencies, which will easily pass through the filter. An RF choke (22 uhy) can be added across the antenna elements at the amplifier input to reduce the level of the lower frequency signals. This takes some heavy topside work and should be done as a last resort.
Once you can show the neighbor that your set is interference he should be well on the way to being convinced the problem is in his TV receiver. Work tactfully through the receiver steps mentioned above. I would start by adding a high-pass filter as this is usually all that is needed. If this corrects the problem tell him where he can obtain a filter and help him install it, it is not your responsibility to furnish a filter or modify his equipment. If it is necessary to add the AC line filtering he may want to take it to a shop to have the capacitors installed. An alternative may be a plug-in AC line .filter such as Radio Shack 15-111, however these are not as effective because the exposed line cord may act as a pickup antenna. At $12.00 though it may be worth a try.
The FCC, jointly with the American Radio Relay League, publishes an 'Interference Handbook' available free from either source, that lists TV manufacturers and mailing addresses. By contacting the manufacturer you can often obtain a no-charge EMI modification kit. The Interference Handbook also discusses interference to audio equipment, telephones, etc. When this type of equipment receives your signal it is improperly functioning as a radio receiver and interference is not your responsibility; however again tact and cooperation should be the first objective. Audio amplifier problems can usually be corrected by one or more well placed by-pass capacitors and any telephone interference should be referred to the Phone Company for their thoughts. If the phone is privately owned you may have to contact the manufacturer but again a by-pass capacitor will usually do the trick.
When you know that your equipment is not generating interference to neighboring TV sets you will enjoy operating a lot more and, if you do get a complaint, you will have a solid plan of attack.
Test Equipment Stuff
Gerry Crenshaw, WD4BIS
What's all this test equipment stuff anyway! Do I need test equipment for the shack? What kind of Instruments do I need?
As we get into this hobby, sooner or later we are faced with the prospect of needing some basic test equipment. It's true that due to the complexity of our radios, the State of the Art has advanced to such a high degree that most repairs have been taken out of our hands. Do you still need any test and measuring equipment in the shack? You can always take it to the repair shop and pay $50.00 an hour to get it fixed.
More often than not, problems with a handi-talkie can be traced to a mechanical connection between the battery and a radio. Being able to prove that the battery is good with a VOM (Volt-Ohm-Meter) or DVM (Digital Volt Meter) prior to looking for a problem like this helps with the troubleshooting process. For many of us who have base radios, when a problem occurs there are usually several test points in the radio that the manufacturer may want checked before they make a suggestion as to a repair action. Some manufactures such as Ten-Tech, when calling the technical support hot-line with a problem will ask for several voltage readings. If there is one Must Have in the shack, it's a basic VOM/DVM. Many manufactures make them and the prices vary from $30.00 to several hundred dollars. Depending on your needs and skills the VOM/DVM can be of great asset in troubleshooting.
Analog Vs Digital
Of the Meters on the market what type is best for you? For those of us who have used them awhile the older Meter Movement VOM is the instrument of choice. It can be used to check AC and DC voltages, AC and DC Amperages, check continuity of conductors and connectors and Resistance's and will check most semiconductors with a front to back check. The DVM will do all of the above checks. The display reads directly and in plain numbers, but the semiconductor check is based on a front to back voltage reading rather than a resistance reading. Some of the high end meters by Fluke will even read capacitance and frequency to 10,000 hertz. Another difference with the DVM is the input impedance is very high so it won't load the circuit being tested. For most applications this is good. For others this can fool you in that it will read leakage voltages that don't show up with a VOM.
Power and SWR Meters
Another instrument in the shack that becomes a Must Have for most of us is the Power/SWR Meter. This meter is used to check the output power of a radio and also to check the Standing Wave Ratio (SWR) or Forward to Reflected power ratio of our transmitters. The name of the game here is to put Radio Frequency energy into the air. If you have a high standing wave, power is not being put into the air but the power is being reflected back toward your radio and that it has to throw off as heat. The better the SWR of your station radio and antenna system the more efficiently it is working.
Power/SWR meters come in a variety of sizes, shapes and prices from the inexpensive version sold by Radio Shack to the lab quality instruments built by Bird and Booton. Many of these instruments are designed for specific application or range of measurement such as VHF/UHF only or HF only. Depending on your use, care should be observed and follow the manufacture's recommendation. These instruments are broken up into two broad types, the Single meter and Dual Meter types. The dual meter shows forward and reflected power simultaneously and where the needles cross is the SWR. The single meter type has to a switch that changes the polarity of the sensing diode so forward and reflected power are shown as two separate measurements. Before purchasing one of these instruments consider the highest power you will be running. Many of these instruments come in Low watt versions. Test instruments for many of the basic measurements will keep the station running at peak efficiency and can warn you too of degrading or marginal performance.
Keep that station running.
Copyright 1996 Gerald Crenshaw WD4BIS. All rights are reserved.
Maidenhead Grid Squares
By Vic Black, AB6SO. This appeared in the May 1999 edition of PAARAgraphs the newsletter of the Palo Alto ARA Wally Porter, K6URO Editor.
You'll need to know your grid square in order to give the proper exchange in VHF/UHF contests. It's becoming common to provide the grid on all VHF/UHF simplex contacts, whether in a contest or not since many operators will want this information for the VUCC Award (VHF/UHF Century Club) for working I00 or more grids. Add it to QSL cards, as well.
Maidenhead designators are a combination of two letters (the field) plus two numbers (the square). The grids are named for the small village of Maidenhead, near London, where Hams invented the system in 1980. It's much easier to give a location in the Maidenhead system than in longitude and latitude, especially while operating CW. Local SF Bay area grid squares include CM87, CM97 and CM88, each covering an area of about 70 by 100 miles. The grids are slightly larger near the equator and smaller near the poles since the lines of longitude converge at the poles.
A good way to understand the Maiden head system is to visualize a world map with the equator across the middle dividing north from south and the prime meridian (zero degrees east-west) dividing the map vertically down the middle. From Northern California we could say that we're in the north west quadrant. To make the description more meaningful, the brains that invented the Maidenhead system started at the South Pole and International Date Line and further divided the world into smaller grids.
The first 20-degree by 10 degree rectangle from 180 to 160 degrees west and 90 to 80 degrees south was called 'field AA', Eighteen fields were then designated northerly in 10-degree steps through AR to reach the North Pole.
Then they moved over 20 degrees east to 90 degrees south and 60 east to 140 degrees east and named the fields BA through BR northerly. This was continued on around the earth for a total of 18 fields south to north and 18 fields west to east, ending in field RR. Starting in the south-west corner of each field, the fields were then divided into 2x1 degree 'squares' (0-9 easterly and 0-9 northerly) with longitude listed first, followed by latitude) An excellent explanation with charts is at http://www.bridge.de/~tom/maiden.htm.
There are several ways to determine your grid designator:
1. Use a GPS (Global Positioning System) personal navigation receiver to find your position. Most will indicate your location in the Maidenhead system, or you can easily convert from longitude and latitude yourself.
2. Use a grid square map. ARRL sells a grid square map of the US for $1, available at HRO. You'll also need a local map with longitude and latitude to pin-point your location so you can convert to grid squares. On the San Francisco Peninsula, CM87 includes the area from 124 to 122 W arid 37 to 38 North. Neighboring CM97 extends from 122 to 120 W and 37 to 38 N. Most road maps don't indicate longitude and latitude, but USGS topographic maps or atlases by Rand McNally, Benchmark and DeLorme show them. Many of these maps are now marked 'GPS Grids Shown', or something similar.
The 'Buckmaster World Wide Ham Call Server', searchable by call-sign, is athttp://www.buck,com/cgi-bin/do ham-call and lists longitude/latitude and grid squares. The QRZ! Web page for 'The World Radio Amateur Directory', also searchable by call-sign, lists only longitude/latitude for the center of the postal ZIP code area it's located at http://www.grz.com/wrad/directory.cgi.
3. Use a computer or Internet grid square converter. Try the interactive web-based converter 'Amsat Grid Square Converter' at http://www.amsat.org/amsat/toys/gridcon.html. Enter decimal degrees or degrees/minutes/seconds to convert to grid squares. If you enter a grid square, it will convert to longitude and latitude at the center of the grid. The ARRI Grid Locator requires only the Longitude and latitude in degrees to convert to a 4-digit grid square. If you also enter minutes and seconds, it will convert to the more accurate 6-digit grid square designator (the 'cell') which is used on the microwave bands and corresponds to a grid of approximately 3 X 4 miles in the continental US. It's located athttp://www.arri.org/locale/grid.html.
4.You can easily convert from longitude and latitude to grid squares yourself using this simple method:
Calculating Your Grid Square:
Here's a sample using the location of the PAARA Club meetings in Menlo Park. The Recreation Center is located al approximately 122 degrees, 11 minutes West longitude and 37 degrees 26 minutes north latitude.
Start with the longitude.
Now use your latitude.
2. If the latitude is north, add 90. If the latitude is south, subtract the latitude from 90. (90 + 37.38 = 127.38)
3 Divide this number by 10. (127.38/10 = 12.73). The whole number result (don't round up) determines the SECOND character of your grid as follows: O A, I=B, 2=C, 3=D. 4=E, 5=F. 6=G, 7=K 8=1, 9=J, 10=K, 11=L, 12=M, 13=N, 14=0, 15=P, 16=Q, 17=R, 18=S. Choose 12=M as the SEC-ON D character of the grid.
4 Multiply this number by 10. (12.73 X 10 = 127.3). The number immediately BEFORE the decimal point is the FOURTH character of the grid. Choose 7 as the FOURTH character of the grid.
In this example, your grid is CM87. [And now you know why most people use computer programs to determine their grid square.]
GRUMMAN AMATEUR RADIO CLUB
MINUTES OF GENERAL MEETING 10/20/99
BY Pete, N2PYV
The meeting was called to order by Pat at 5:02 PM. All present introduced themselves.
Ted read the financial statement. Finances continue to be in good shape.
No applicants showed this month. Bob handed out a document that described how to check the FCC's new database via computer.
No problems have been reported on the repeaters.
Zac reported that the Sunday 40-meter Net had 14 check-ins and good propagation. The Thursday 2-meter Net had only a few. There was only one station heard on the Wednesday 20-meter Net.
Pat reported the Ten-Tech radio had been received at the repair facility. It will be worked on soon and we expect back in about two weeks.
The January meeting will be held at the Underwriters Lab building in Melville at 6:30 PM. Maps and directions will be on the Club Website.
There was a discussion about moving the VE Exams to Melville also. Perhaps just prior to the General Meeting.
Marty, NN2C was sick and therefore could not speak about his trip to england.
It was agreed that we would provide $250 to the N/G cafeteria who would provide food for our Holiday party.
Ham Radio University 2000 was discussed and a flyer was handed out. The date will be Sunday, January 23, 2000.
There will be a discussion about a magazine article that describes how to load up the cross beam of your tower mounted antenna to us it for 40-meters.
Several members communicated that the 10-meter band was great now with very good DX.
Mike, KJ6XE announced that he would be leaving the area to go to South Carolina.
A discussion about the Grumman Memorial Park being constructed in Calverton was led by John Caruso, W2JAC The F14 has been completed and is in the hanger ready to be installed. Nine hundred memorial bricks have been sold. If you sign up now for bricks you can be assured that they will be installed prior to the dedication of the site.
In a few days I will make a set of disks available to the Club that contains a program that allows the user to read all of the articles and other information published in CQ de WA2LQO for 1998.
Included in the program are all the articles, President's pages, Editorials, Meeting Minutes and other information published in 1998. Included with the articles are any illustrations that go with the article.
The only things not included are the various cartoons we published. However, if there is a demand for these we will include them in a future version of the program.
The program will be donated to the Club and they will handle the distribution of the disks.
CQ de WA2LQO will now be available in three different versions:
The Editor, KA2FEA