RANDOM WIRES REVISITED
As a young novice operator back in 1977, I tried a variety of antennas–verticals, slopers, loops, dipoles, inverted vees, and random wires. All of these antennas worked well, except for the random wire which exhibited a wide range of SWR readings and an uncertain radiation pattern. After careful trimming and the use of a decent earth ground (with a real ground rod driven into the earth) and counterpoise, most of the in-shack rf and erratic SWR readings were eliminated. After a few days of getting “rf bite”, I realized that so-called “random wires” really aren’t so random–they required careful attention to length and grounding.
On the plus side, random wire antennas can be quickly built and erected. With a good variable L-network tuner, counterpoise, and earth ground, a low impedance can be presented to the transmitter.
With this in mind, I recently made an emergency “long wire” antenna which will serve as a backup should my inverted vee or sloper fall victim to the elements.
MATERIALS AND PRECAUTIONS:
One of the first things I did was to determine the quarter wavelength of the lowest band I wanted to use. Since my lot is rather small, I opted for the 40-meter band with a chosen frequency of 7.088 MHz. That is the frequency of the daily Hawaii Net which meets at 0200 UTC. Using the general formula for a 1/4 wavelength (234/f (Mhz)), I came up with a length of 33.01 feet (10.06 meters). I rounded off the length ato 33-feet (10.06 meters). I cut another wire for the same length. That wire would serve me as a counterpoise and would be attached to the ground connection on an old MFJ-941E tuner. Your could also use a commercial L-network tuner such as the MFJ-6010.
Based on further study, I later changed the antenna and counterpoise length to 36-feet (10.97 meters) to avoid the “half wavelength” problems described in the next few paragraphs by N4UJW, VE3EED, and AB3AP.
I used #14 AWG housewire for the antenna and counterpoise. If you want this antenna to stay up for awhile, I recommend the use of copperweld wire (copper coated steel wire). This wire seems to resist environmental degradation fairly well.
According to articles written by N4UJW, VE3EED, and AB3AP, one should avoid a half wavelength of wire, ” because a halfwave will give us a very high impedance and resultant SWR into a 50 ohm transmitter.” These amateurs recommended the following wire lengths to avoid the high impedance feed problem:
29 feet (8.84 meters), 35.5 feet (10.82 meters), 41 feet (12.5 meters), 58 feet (17.68 meters), 71 feet (21.64 meters), 84 feet (25.60 meters), 107 feet (32.62 meters), 119 feet (36.28 meters), and 148 feet (45.12 meters).
I chose a length of 36 feet (10.97 meters) for my “random wire.” A counterpoise of the same length was attached to the ground terminal of the old MFJ-941E tuner.
Next to the ground stake beneath the shack window, I installed a 5-foot ( 1.52 meters) wood stake. This stake would support the wire to the shack window. A clip lead would attach the random wire to a short piece of #14 AWG housewire comming from the random wire connector on the antenna tuner. The clip leads would allow me to disconnect and ground the antenna when operations were done for the day.
Two ceramic insulators were used to tie off each end of the random wire.
I used a slingshot to launch the antenna wire to a nearby tree. A 30-foot (9.16 meter) piece of monofilament fishing line and sinker were attached to the antenna wire and served to guide the antenna to a notch in the tree approximately 35-feet (10.67 meters) above ground. The fishing line and sinker were tied off to a nearby fence post, making the line fairly straight but not overly tight. A little slack was built in to account for the bending of the tree limb by wind.
All connections, save the clip-to-clip attachment near the shack window, were soldered and protected from the weather by clear nail polish and several layers of vinyl electrical tape.
Thanks to the installed ground system, the attached counterpoise, and the old MFJ-941E antenna tuner, the SWR was kept below 1:5 to 1 using 20 watts of power on SSB and 10 watts CW. Daytime reports around Hawaii were acceptable–56 to 57 on SSB and 579 to 599 on CW. At night, SSB reports varied between 53 and 56, with CW reports holding between 559 and 579. The antenna works from 40 to 10-meters. The old tuner gets quite a workout as I change bands, but there is no stray RF in the shack and all equipment seems happy with the arrangement. This antenna will not bust a pileup or dominate a band during a contest. But it will get you on the air with minimum cost. Most of the materials can be found at a hardware store or home improvement outlet.
Once the school week is over, I plan to assemble an identical antenna for my emergency “go kit” in the van.
I had fun building this simple, inexpensive antenna. You may find this “skyhook” just right for your situation.
Random Wire Antennas–Best Lengths to use for Random Wires. http://www.hamuniverse.com/randomwireantennalengths.html.
How Do I make a HF longwire antenna? http://forums.radioreference.com/hf-mw-lw-genral-discussion/20270.
The End-Fed half-wave antenna (Yates, Steve, AA5TB). http://www.aa5tb.com/efha.html.
Silver, Ward (2007). The ARRL General Class License Manual, 6th Ed., ARRL, pp. 6.6.
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Aloha es 73 de Russ, KH6JRM–BK29jx15–along the beautiful Hamakua Coast of Hawaii Island.
Filed under: Amateur Radio