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 Previous: Sputnik 41/RS-18 Now QRT |  Next: Hams and their Antique Automobiles |
By Richard H. Arland, K7SZ
December 15, 1998
Making the leap to HF can be a challenge if your hamming experience has been on VHF or UHF FM. In this second of a series of installments, an experienced HF ham radio author and QRP (low-power) aficionado offers the benefit of his advice and expertise in hopping over to HF without busting the budget. This installment takes a broad look at what can be the most important aspect of a station installation: the antenna.
Procuring an HF transceiver is only the beginning. To make some contacts, you'll have to invest in an antenna. Simple HF antennas are the topic of this installment.
While pictures of tall towers and big beams abound in various ham radio magazines, most folks operating HF use simple antennas, usually made of wire. Erecting an HF beam and tower can be a very expensive, labor-intensive project. Wire antennas on the other hand are inexpensive to make and can yield outstanding results.
Chapter 20 of The ARRL Handbook for Radio Amateurs offers a good insight into antennas and has several projects designed to get you on HF with minimum hassle. If you want a more in-depth look at antennas, I highly recommend the 18th edition of The ARRL Antenna Book. For those interested in a practical, no-nonsense approach to HF antennas, try Your Ham Antenna Companion from the ARRL bookshelf.
Over my 30-plus years as a radio amateur, I've used a multitude of wire antennas. Some performed very well, while others. . . well, let's just say I would have been better off using my dummy load! My best performing wire antenna all have been of the dipole variety. Without a doubt, the venerable dipole or one of its variants will yield very good performance and provide thousands of contacts at a fraction of the cost of a tower and beam antenna installation.
Figure 1--The physical construction details for a typical, horizontally mounted coaxial-cable fed dipole antenna. (A) shows the end insulator connection, while (B) shows the complete antenna. This is a balanced antenna system. [from The ARRL Handbook for Radio Amateurs] |
Single-band dipole antennas are very easy to build and erect, and they are resonant antenna systems, requiring no additional "RF ground." Figure 1 shows the physical construction details for a typical, horizontally mounted coaxial-cable fed dipole antenna.
You can use the tried-and-true halfwave dipole formula to determine the approximate length of the antenna (both halves). The formula is: Length = 468/FMHz.
When you're cutting your antenna to length prior to actually putting it up, always cut the wire a few inches longer than the formula calls for, then trim it to the correct point for resonance after you've gotten the antenna up in the air and have an idea where it resonates (here's where the antenna analyzer comes in handy; see below).
If you recall your basic theory, you'll remember that an antenna's resonant frequency--usually indicated by minimum SWR--depends on its length. If you find your antenna resonates at a higher-than-desired frequency, you'll need to add wire. If it resonates at a lower-than-desired frequency, you'll need to cut wire. This "cut-and-try" method can be tedious and time-consuming but it yields results. (Besides, it builds character!--Ed)
Once you figure out what length works for your particular installation environment, you can "reverse engineer" the formula to come up with a number that yields results that are closer to reality for your set of circumstances. By the way, many "single-band" dipoles will work on other bands. For example, a typical 40-meter dipole often will resonate (or come pretty close) on 15 meters as well.
Figure 2--An inverted-V dipole antenna. In a practical installation, it's a good idea to keep the ends at least 10 feet off the ground, for safety's sake. The end of a quarter-wave element can carry substantial RF voltage. [From The ARRL Antenna Book] |
I prefer installing the dipole in an inverted-V configuration (see Figure 2), where the feedpoint is elevated above the ends of the antenna and the legs come away toward the ground in an upside-down V shape. This can offer an omnidirectional radiation pattern and reduce the overall end-to-end distance needed to erect the antenna. It also has the advantage of needing just one high point of attachment. You'll likely discover that the actual length of your inverted-V will be somewhat shorter than the "formula" length for a horizontal dipole because the ends of the inverted-V often are closer to the ground and the dipole legs closer together.
Figure 3--An off-center-fed dipole antenna. Using the measurements shown here, this antenna system will work on the 80, 40, and 20 meter bands. The balun transformer at the feedpoint is a 1:4 or 1:6 step-up current balun. [From The ARRL Antenna Book] |
Off-center-fed (OCF) antennas (see Figure 3)--sometimes called "Windom" antennas after the fellow who came up with the original, single-wire feeder design--have become quite popular in the last few years. Antenna manufacturers produce several varieties of skywires based on the OCF principle. Since the feedpoint of this kind of dipole is not in the center of the antenna, it can make it an ideal choice for certain installations. Also, some OCF designs will work on more than one band and, because RF currents can be present on the feedline, the system can produce both horizontal and vertical components to the radiation pattern.
Figure 4--An "invisible" end-fed wire antenna using lightweight, enameled "magnet" wire. You will need an antenna tuner of some type (depending on the overall length of the antenna plus feedline), and you will need to provide a good RF ground when using a single-wire antenna like this. For a relatively low feedpoint impedance, try to make the overall length of the antenna and feedline an odd multiple of a quarter wave at the design frequency. [From The ARRL Antenna Book] |
Wire antennas are easy to disguise. This offers an additional advantage to the "antenna challenged" who are prohibited from erecting outside antennas, perhaps because of deed restrictions or other restrictive covenants. An end-fed wire antenna, made with small diameter wire (#24 AWG or smaller) and tuned with a simple antenna tuner can produce lots of contacts (see Figure 4).
The #24 AWG or smaller magnet wire often is available at hamfest flea markets or salvaged from old transformer windings or TV picture-tube yokes. The main thing to remember is that this sort of very small diameter enamel-coated copper wire is not robust and can come down in even a moderate wind. If you go this route, plan and execute your antenna installation carefully.
Keep in mind that a single-wire antenna requires some kind of RF ground to work properly. The typical cold water pipe "safety" ground usually is not sufficient. You'll need to back this up with 1/4-wave radials or counterpoise wires at the ground point. Sometimes, even a single 1/4-wave wire, cut for the band(s) you operate and laid out on the ground or around the baseboard of a room, will do the trick. But be careful! The ends of 1/4-wave elements get very "hot" with RF, so insulate or isolate them carefully.
Also, to obtain a relatively low feedpoint impedance, make the overall length of and end-fed wire an odd multiple of a quarter wave at the design frequency. A quarter wave is one-half the length of a dipole according to the "classic" formula for determining the approximate length of a halfwave dipole (see "The Simple Dipole," above, for formula).
Here's an idea for high-rise apartment or condo dwellers: Mount a flagpole holder on your balcony railing. During the day, hang out Old Glory. At night, slip a broomstick or other nonconductive pole into the holder (or just use the flagpole if it's nonconductive) and use it to secure an end-fed wire away from the building. A small (3/4 to 1-ounce) fishing sinker will add the weight necessary to keep the antenna wire taut (but keep it away from windows and people below--Ed).
Trail the wire downwards, parallel to the outside of the building, connect a feedline to the end from your transmitter, make sure you've got a suitable RF ground (see above), and load it up using an antenna tuner. What you have is a top-loaded vertical wire. Don't laugh. It works! Some apartment dwellers have managed to make do with end-fed metallic gutters and downspouts, too.
Trees are your friends. The nice thing about most wire antennas is that they were made to be supported by trees. Any nearby tree will suffice to anchor the legs of an antenna or to hoist the center of an inverted-V into the air. In short, trees are ready-made antenna supports. Some ops use a bow-and-arrow or a slingshot to shoot some lightweight fishing line over available tree limbs, then use it to haul up the actual support rope.
Wire for antennas comes in many forms. Classic dipoles and OCFs ("Windoms") that are intended to be left up for extended periods call for #14 AWG stranded wire or heavier, or, for severe-weather installations, copper-clad steel wire (commercially known as Copperweld). Unfortunately, copper-clad steel wire can be extremely difficult to work with (because of the "spring" factor), but it is extraordinarily strong.
For low-profile installations or for portable, temporary antennas used for backpacking and camping, the wire need not be that robust. For example, #18 or #22 zip cord speaker wire sold at Radio Shack and elsewhere works well for these applications, and a 100-foot roll is pretty inexpensive. Determine and measure the "per-leg" length you need (using the formula for the approximate length of the whole dipole and dividing that number by 2), cut the wire and split it apart and you have both halves of a dipole!
Remember: Your antenna is the single most important part of your HF station. The time you spend in planning and installing a good antenna system will result in better signal reports and more contacts--especially on the low bands where rotary beams are less common to start with. Again, for more information on practical antennas, antenna tuners, and antenna theory and design, see The ARRL Antenna Book (18th edition) and The ARRL Handbook for Radio Amateurs.
See you on the air!
Editor's Note: Rich Arland, K7SZ (ex-K7YHA), of Wilkes-Barre, Pennsylvania, has been a ham since 1963 and holds an Amateur Extra ticket. He's a retired US Air Force Master Sergeant with 20 years in military communications, including long haul and tactical units. While in the service, he got to be DX during his three years in the Azores (CT2BH) assigned to the 1936 Communications Squadron. Rich also has worked as a broadcasting engineer He currently teaches basic and advanced vocational electronics at the State Correctional Institution--Dallas, Dallas, Pennsylvania, and he serves on the state board that oversees video surveillance upgrades for all Pennsylvania prisons. He is the author of Low-Power Communications, Vols 1-3.
"Beginners Only: HF on a Budget--Part 1" can be found in the November 16, 1998 issue of the ARRLWeb Extra.
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