Beginner's Guide to Antennas - not just for Novices
For a number of years I have been teaching a ham radio licensing class on a more or less regular basis, and as many instructors have found, the class itself is only a start.
As the students receive their licenses, they invariably return with a set of questions which point out the difficulty of translating the knowledge, newly won, into practical use. The following conversation has been repeated dozens of times and is aimed at the universal problem of selecting and erecting an antenna for a new ham station
Question: I just received my license and got a good buy on an allband rig I am out
of money, don't have much time, and want to get on the air as quickly as possible. What antenna and what bands should I start with? Answer: Put up a 40-meter dipole. This will give you a chance to make a large number of contacts, day or night, weekday or weekend. In addition, you can load it up on 15 and work DX if the sunspot cycle is in a favorable position. See Fig. 1.
Q. Can I use insulated wire? A, Sure —as long as you remove the insulation at the point where you connect the feedline. Also, make sure that the wire you select does not stretch If it does stretch, you will have to keep cutting the wire back to the proper length periodically. Your best bet is to buy copper-coated steel wire. The copper gives you good conductivity and the steel core keeps it from stretching.
Q. I have a roll of "thin" coax (RC-58 or RG-59). Can I use it instead of buying the more expensive RG-8? A. In the HF ham bands (80 through 10 meters), as long as you have a fairly short feedline run, say, 100 feet or less, you won't notice any difference.
Q. Gotcha! We learned that the feedline impedance should be 70 Ohms for a dipole and some of these coax cables have a 50-Ohm impedance. Can I still use them?
A. Again, for short runs of feedline in the HF bands, there will be no noticeable difference The swr might be a tad higher, but this won't make any difference just one caution on coax: You can buy some relatively inexpensive coax that was originally sold to undiscriminating and unsuspecting CBers. It normally costs half or 2/3 of the cost of brand-name coax, and as you can guess, there is a good reason for the lower price. Be suspicious. Cut away a short section of the outside insulation and see how much of the inside insulator is covered by braid. If there are large spaces and you can see a good part of the inside insulation, be careful- You may be inviting problems if you decide to use it
Q. But what about the swr? Can I operate with a 2:1 or 2.5:1 swr?
A. Well, for many years the most popular ham antennas used an swr of 5:1 or 10:1. These antenna/feed line systems used open-wire line, and the key here is low losses in the feed If your rig will operate with a high swr, there is no reason to fight to get the swr down to the nice round value of 1:1 as long as the loss in the feedline is low. Again, in the HF bands with short feedline runs, an swr of 3:1 won't affect your signal at all.
Q. Will my rig operate with a 3:1 swr?
A. Most rigs with tube finals will, as long as you don't keep the key down for long periods. Solid-state finals are a different problem Most have swr protection, which means they sample the swr and reduce their power if the swr goes too high. Given a choice, I would try for an antenna with a low swr. But if it means spending 5 minutes tuning up every time I QSY (change frequency), I would accept the higher swr in the interest of convenience and extend the life of the finals by not keeping the key down so long tuning
Q. What about antenna tuners? In fact, I have read about tuners (matchboxes), low-pass filters, swr bridges, and coax switches. They are all connected to the rig. In what order do you connect them and why?
A. See Fig. 2. Start at the coax jack of the rig and connect the low-pass filter with as short a coax jumper as possible. This means that harmonics will be attenuated before they have a chance to run around long pieces of coax and possibly radiate. Next in line is the
swr bridge, since you are interested in matching the rig to whatever follows The tuner is the next item, followed by the switch and the antennas themselves. Thus, you select an antenna with the switch, utilize the tuner to make the antenna and feed look like an acceptable load, and monitor the swr (and relative power out) with the bridge.
Incidentally, it might be a good idea to make up a tuning chart for each of the frequencies you use. Write down the frequency, settings of the controls on the rig, settings of antenna tuner controls, and antenna selected. When you want to QSY, simply set all controls as shown on the chart, and then tweak them to get maximum power out and minimum swr. Normally, if you operate all over a band, you don't have to log these settings any more than each 50 or 100 kHz on 80 and 40, every 100 or 200 kHz on 20 and 15, and 500 kHz on 10
Q. I live in a small valley surrounded by hills. Are there any special precautions I should take in selecting an antenna?
A. On 80 and 40, a dipole, inverted vee, or longwire will work fine. However, on 20, 15, and 10, you might not want to pick a very high gain beam or quad. A really good beam or quad radiates at a low angle, almost horizontally, and will simply pump your precious rf into the hills. Antennas such as the popular triband beams have to sacrifice some of this low-angle characteristic in order to operate on three bands. As a result, more of the rf is sent up at a slightly higher angle (up to perhaps 40 degrees, or so} and this will probably top the hills around you Alternately, seriously consider tilting the beam or quad so that it radiates up to clear the hills.
Q. I can t put up a big antenna I don't have the space or my XYL/husband or neighbors would object to a big tower. How about one of these vertical antennas? See Fig. 3.
A, Verticals, especially full-size verticals, work fine. But they do require radials. Each radial is about a quarter wavelength long, and while you would like over 100 radials, you should have as many as you can put up for each band you will operate. I suggest 2 each as a minimum on 80 and 40, and 4 each as a minimum on 20, 15r and 10.
Q. But I thought verticals were good where you don't have much space?
A. They will work with only the coax feed acting as a single radial. However, they work much better when you add radials cut to the proper length, and they work best when you have a very large number of radials Every experienced ham has a story of how he worked DX on a 10' wire hanging out the window. But for the most consistent and best results, verticals need radials, and lots of them,
Q. What about mobile whips? Can I mount one on the house and use it? Cars don't have radials.
A. On a car, the metal body is used as the ground plane in place of radials. In addition, this sort of antenna system is from 2% to 15% efficient. The physical limitations of a car make us accept this loss, but you can do much better at home
Q. Speaking of cars, I still have a standard mobile mounting ball on my car that I used to use for my CB antenna. Can I use it for a 2-meter FM rig?
A. You will have some loss and you will probably never get the swr down really low, but you have two choices First, you can buy one of the commercial 2-meter antennas which mount in the standard mobile ball thread. These are 5/8 of a wavelength long but have a loading coil which makes them look like 3/4 of a wavelength. This is an odd number of quarter wavelengths, so the input imped-
ance is about 60 or 70 Ohms and you can use the old mount and coax feeder. You also can take an aluminum or steel rod, cut to about 20 inches, and thread the lower inch to match the thread in the ball (Fig, 4). You now have a quarter-wave vertical again using the old ball mount and coax feeder,
Q, As long as we are salvaging CB antennas, surely on 10 meters I can use the 27-MHz Loudengrabber V that I have mounted on my roof.
A. You probably can use it on 10 with an antenna tuner, but it might be less efficient than simply replacing it with a 10-meter vertical. If you want to try an experiment, connect it to your 2-meter FM rig. It might make a real nice (and quick) vertical for 2m. But don't try this unless your 2-meter rig has swr protection in case the swr turns out to be very high.
Q. One final question: I have a wire which I used to listen to the ham bands before I got my license. It runs out the window, over the roof, under the apple tree, and about 10' above the ground around the garage. If I use an antenna tuner, can I make do with this wire?
A. We would all like 90' towers and large array antennas. Most hams have to make do with what they have without structural steel work. Sure, you can use the wire, as long as it is high enough so no one can touch it while you are transmitting. Generally, you want any antenna to be as high and in the clear as possible. But if you can only run a short wire, use it. It will work and you will have many hours of good contacts.
Paul Danzer - 73 Magazine 1981 -