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http://www.qsl.net/kb2seo/antenna |
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ANTENNAS– Simple Ideas that WORK. |
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The antenna should be sought like the Holy Grail for your station. Hotly contested theories over the years have made it’s share of fables, foibles and just plain goofy stuff. “Another Service” which will remain nameless here, has adopted a mind set that more power equals more signal. This is not the whole story. A antennas, it’s transmission lines and the way it is put up in the air is all part of the SYSTEM. You can buy 15 pound books that explain every nuance of antennas, This page is just for a few common sense approaches for the new comer or Old timer to think about for their station. The name of the game is having a MATCHED antenna system and an antenna system that can offer GAIN. You can use a simple formula, or some variation of the original to build many Wire or tubular types of antennas. I group the antennas into two groups” Vertical and Horizontal. Which is better? The short answer is “It DEPENDS”. Verticals are great antennas. For Local use, especially if you’ll be talking to Local Mobile stations, A vertical would be the best choice. It offers a Low angle of radiation– Very important if you need to be heard at a distance, And since 98 percent of mobile antennas are vertical, you would be best heard using the same . They radiate the same way and at local areas, will hear that way better. Before I go much further Let’s put a long standing argument to bed– ONCE AND FOR ALL. Your Antenna is not shot if your SWR is above a 1.2 SWR!!!. Read that again and again. With the Tuner (transmatch) and with most rigs at least at home, and cut to true size, (Not loaded) a SWR as high as 2:1 is still an effective and good working antenna! Next -This is especially for the New Comers from “that other service”: the big coils, cans, and the antenna with Wire wrapped all the way down and only 5 feet long IS NOT a great antenna. In fact you may be dissipating more than half your power into that goofy huge loading coil in the middle of the antenna. Sure– you might be able to transmit the full 5 KW into it– But you STILL are only getting less than half that out as real RF . The rest is just heating up the antenna( and Coax and the car and your brains). I will challenge any “truck stop tech” to a battle of the Calculators I’ll even use one from the dollar store– EVEN with him using the misleading Claims of DBi– you can’t change the math. The Shorter the antenna is made compared to a true size quarter wave, the less effective it is. Some of these little stubby antennas people stick on the car or RV or whatever may only be effective to a third of the RF they could be using. It’s Physics, Get over it or get a 8 foot whip. Now Then. Common sense antennas anyone can use and have a good sounding station… Verticals all have similar components, a vertical radiator and some sort of COUNTERPOISE component. The counterpoise is to allow the RF signal to radiate both parts of the sine wave. Remember we are not dealing with D.C. energy we are working with a Sine wave All Sine waves have a rise above 0 and a “valley” Below 0. BOTH of which needs something to radiate from. Antenna jargon you’ll hear it said the antenna needs to “work against ground” In the case of the Vertical. The 2 or more elements that are connected to the outer part of the Plug, or shield of the coax or simply the second wire in a balanced feed needs to be there. To simple stick a long pole in the air and drive a ground stake next to it and attach the shield, IS NOT GOING TO WORK WELL. Some sort of counterpoise is needed. Commercial Verticals used matching components to “fool” the signal into radiating as it should. Sometimes, the Shield itself becomes part of the counterpoise, and depending on the power, Frequency, Where the next door neighbors phone line is to this coax can spell trouble if not handled correctly. The simple Quarter-wave ground place can be built for any frequency: that is– as long as you are able to put it up in the air– The Formula is simple grade school math that can be figured out on a calculator . I always try to go with a frequency that is Centered on the range of Frequencies I plan on using: We’ll look at 10 meters, Just above the C. B. Band (Hint: CBer’s can use these antennas too) The First Formula is simple 4th grade Math: the length of each part of the antenna is figured out by Dividing the CONSTANT (Same number all the time) 234 by the Center Frequency (In this case we’ll make it so we can use the CW/Digital areas of 10 up to the top of the Novice/ technician portion of 10 meters: 28.500– that puts us around 28.300 SO: 234 divided by 28.300 equals a element length of 8 feet 3 3/8 inches. I always add at least 2 inches to each element for “Pruning”- You can always trim a element, It’s a lot tougher to put it on. The antenna would be constructed of heavy stiff copper or very stiff aluminum wire on a small metal plate. The center would be either a antenna mount like the SO-234 to 3/8x24 threaded stud used for mounting through a roof or found on mirror mounts and such. You could set the element in a 3/8x24 threaded stud, or simply put the element in and fill with solder. An easier was would be a SO-239 through the Plate with the wire soldered directly into the “backside”. You could use there means, but the once part of this that cannot be omitted: the center Vertical radiator MUST be insulated from the mounting plate. The Center of the coax attaches to this element. Although this is a A/C type of signal, think opposite poles. If the elements were all attached with out this, You would simply short out your coax. You then need to attach 3 “or more” wires on a downward 45 degree angle off the metal plate. 3 will give a good pattern. While 4 or more would make the pattern smoother– Not something too many should worry about– This antenna is great for Local repeaters, Local rag chewing with friends or as a Rapid response or replacement after a disaster. It’s cheap, It offers Unity gain, that is to say any power going in will result in a signal of that same power going out. The trick to these antennas is to get it as High in the air as possible. Scanner Buffs could use 3 or more different sized vertical radiators in the Same Center attachment point. Each Radiator cut for the Band they wish to monitor : So for Low VHF(46-54 MHz), it would be 4.6 feet, Mid VHF would be around 1 1/2 feet and UHF would be 6 inches. The Radiator that is most closely matching that Frequency the scanner is trying to pick up will be the one that is most active. It is possible That the antenna would use more than one radiator. Just be sure to use some sort of non-metallic light material to hold the 3 elements apart. |
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THE HORIZONTALS: The next Antenna that the New Ham, SWL or even CB’er would want to Consider are Dipole wire antennas. A dipole is a 1/2 wavelength long at the Frequency you wish to speak (or listen) on. The Formula is again, VERY simple: 468/Frequency= Length in feet If you have not immediately picked up on that number: It’s constant is twice the constant of the Ground plane. 234 x 2 = 468. The Dipole and the Ground plane share a few common traits: on ODD multiples of the wavelength, The antenna will work and Show Some GAIN over the Unity gain of the Dipole. The most famous example is on 40 meters. It is also a Great antenna on 15 meters! On 80, adding a small Figure 8 loop of wire about 81/4 feet out from the center allows the 10 meter band and the 80 meter band to work on the same wire. Also because it’s Horizontally polarized, Some noise from powerlines and other sources can be knocked down considerably. This is a great antenna for the CB’er to use on Single sideband. If you and the friends you are talking with want to have great signals between yourselves, and a antenna that will allow you to work farther distances, The Dipole is a great antenna for this. Make sure each antenna is pointed broad side to the area they wish to speak with, and try to get the dipole as high as possible! Back to Basics though, the Formula is calculated, Lets take 40 meters. Length in feet =468/f SO: 468/7.150 (center of the band) = 65’ 5 1/2” And again, add 4 inches to the antenna for attachments and “Pruning”. |
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Some Variations of the Dipole are Double Zepps “DEZ” or Double extended Zepps (A dipole cut to multiple wavelengths of a specific band) and Long Wire Dipoles. Some Hams refer to a Long end fed random wire antenna as a Long wire– this is not a true Long wire antenna. A true Long wire is a Dipole where the “driven” side, or where the center part of the Coax would go, Is 1 wavelength long Or Odd multiple wavelengths of the Lowest frequency used) while the “short side” where the braid would attach remains at 1/4 wave on the band you wish to use. It is an EXCELLENT antenna, and offers Gain, at longer than 1 wavelength, You can figure out the patterns so you can orientate the antenna so the Strongest part of the signal favors the area you want to get to. For 10 meters, I used 8 feet 6 inches for the Short side, And the Long End was 99 feet 6 inches , attached with RG 59 CATV coax. The Signal comes off the sides of the Wire like a rail road crossing sign. About 30 degrees or so. Pointing the Wire Due North and South allows me to put a “lobe towards the North end of the UK, Norway and parts of Russia, While the other one pointed toward Spain and the Indian Ocean areas, The west side put a lobe towards Montana, Western Canada and Alaska and the South west was towards Nevada, then So. California and Oceania If you used the Same antenna with Ladder Line and a tuner, You would have an interesting Antenna that would provide options all the way to the 80 meter band! Another Version I would recommend you try especially for Dx ing on 20 or Lower would be the same antenna fed with 300, 450 or Home brew ladder line and a wide range tuner. This antenna is a great choice if there is a support issue or running the Center Fed dipole would be impractical . It is also a good way to add some stealth if you are in a situation where a Big wire antenna will draw the ire of neighbors, antenna covenants, or working from a Apartment building and can at least feed an antenna from the end nearest you. |
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A word on Feed lines: Coax is a good feed line if you are going to use a single dipole ONLY on the band intended for (except for cases like 40 and 15m together or a parallel dipole ) If you want a “Multiband” antenna from a single wire, Your best bet then is to build the dipole as long as you can(135 feet cut in half for 80 meters as an example) Try using a Manual or Automatic tuner and feed the antenna with twin lead. On the Lowest frequency, the pattern is basically a Omni/dipole pattern. On 20 we show that the pattern has lobes that exhibit gain in certain directions. Now some folks like to think of the old twin lead that was used on TV antennas, and yes, it works and WELL! Be careful of how much is going in as far as power with TV twin lead. The stuff you find at the hardware store may not handle much above 200 watts, When you calculate the Voltage potential on some of the Frequencies, Watch out! Other types of Feed line is 450 Ohm Window line and 600-900 Ohm “ladder line” which is basically thin wires spaced a certain distance apart and kept that way by some sort of spacers. I have seen plastic sticks, fiberglass rods, one Ham collects the adjustment rods off of window blinds. As long as It is an insulator and won’t change if wet (Ice pop sticks will work, You need to seal them in Paraffin (dangerous stuff to do indoors) or well sealed in Varnish, Spar or Polyurethane. Some hams even use heavy duty lamp or Speaker cord it’s around a nominal 72 ohms. Although there are serious mismatches with this line and antenna, the twin-lead family is so efficient, the loss is nothing in the Feed line. Where SWR is a significant factor on COAX loss, There are frequencies you can load through the tuner that may exceed 200 to one SWR and STILL be radiating most of the RF out of the Antenna. For 450 Ohm line– the loss per 100 feet is less than half a db. It is the big reason many Hams are re-discovering the beauty, low cost and versatility of twin lead. Nearly all of the MFJ and Vectronics tuners can handle it. MFJ is now making a tuner dedicated to the twin lead feed. I have been using twin lead now for over 5 years. It can be an excellent solution for getting more bang for the buck. Nearly any band above the lowest band cut for will allow tuning and use. Check the ARRL website or many antenna books to get an idea of SWR vs. Band. One important thing about all twin lead– it cannot be buried, It will not work well if drooped over your aluminum gutters or taped to the Galvanized Pipe either. You can get “Stand offs” to keep the wire at least 4 or more inches away from any nearby metallic objects. A great way to avoid some of the hassles of trying to get twin lead into the shack– which could introduce RF radiation and can detune the wire if you don’t handle it properly is to run the twin lead to the house entrance– as close to the operating position as possible, then add a 4:1 Balun (Balanced to unbalanced transformer) or better yet a higher ratio (like 6:1 or even 9:1) then run as short a run of Low loss coax– RG-213 would be good choice– or even LMR series to the shack– tryu to keep the length under 30 feet if possible. Another feed line many Hams and CBer’s shy away from with out good reason is the MILES of CATV out there. If you use the more modern type RG6 or RG11, You have a GREAT low loss feed line! Station KB2SEO uses CATV on the 2 Meter Beam (it’s only 30 feet or so ) And the Long wire Dipole. The RG 11 on the 10 meter beacon has performed excellent now for 4 years straight. If I have to replace it, the local CATV guy will strip me off a wad and I am underway again! Most Dipoles exhibit more along the Lines of 70 Ohms Load at practical heights of trees and 50 foot towers– The CATV will handle high power as long as there is minimum mismatch involved. You can even use “f” connectors and F to PL 259 adapters to the rig and the Antenna if suck a connection is needed (Center insulators with coax connectors or Baluns/Unun needs. Many of these Antennas are found on the ARRL website and other Ham radio sites. These Antennas are Staples of the Hobby in various forms. They are inexpensive to build and are great ways to get your feet wet on HF. |
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Think it through –Before you Do! |
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As you can see, Construction can not be more simple. A few noteworthy things: First be SURE to use insulators on the ends if you plan of transmitting more that 20 watts. Something to keep the center wires apart is a MUST. Also, if you need an antenna for Multiple bands, But can only run one feed lines, simply add another dipole to the center. Once all the dipoles are set on the insulator, use this pipe or stiff plastic rods to makes spreaders. Starting with the Longest Dipole at the top, work your way down to the smallest set of wires. Space each antenna about 4 inches from the other one. Use some stiff heavy fishing line like 80 Lb test to connect the bottom of the spreaders to the end insulator, this will act as a back stay to keep the wires spread and open. (see Photograph) again, Coax is the better choice for feed line here. |