A Surprise New Field Portable Antenna

My experiments to find a new antenna to use for field portable operations have been taking me in several directions. My neighbors must wonder what I am up to. Almost every day I have been out in my back yard erecting all kinds of strange wires strung up on tall poles or launched into trees carried aloft by my “sand grenades” hurled high with a dog ball launcher.

Some ideas worked, others failed but now I believe I have found the ideal candidate and it was quite a surprise to find it is a design that I had previously tried and rejected!

Just to recap; I needed a simple wire antenna that I could build myself. It had to be rapidly deployable in difficult field conditions. I prefer not to attract the attention of people who may regard my activities as suspicious so it had to be reasonably stealthy. I prefer resonant antennas but that is a lower priority since I own or have built both manual and automatic antenna tuners. Some of my operating sites are very small or in public spaces so I needed an antenna with a small footprint. The ideal candidate for my field operations should also be multiband. That is all quite a big ask so compromises had to be made.

I previously thought I had found just the right antenna. It was a good candidate. I took it out on a recent successful POTA activation. It performed well but it failed the stealth requirement. It was a 41ft random wire erected as a sloper supported by my 29ft telescoping fiberglass pole. The activation site was very quiet that day and I had erected the antenna close to some trees to keep it out of the way of hikers on the adjacent trail.

Hello, ‘ello, ‘ello; what’s going on here then?

I was well into the activation; my required minimum 10 QSOs were already in the log when I saw a motorcycle enter the parking lot. The rider made a bee line for the picnic shelter where I had set up my station. I paused to speak with him; he was particularly interested in the tall pole with wires draped from its top. Then he asked me if I had a ground. I pointed to the counterpoise. “I didn’t know people still used Morse Code” he said inquisitively. “Have you tried that ham radio mode that uses a computer?” he continued. The gentleman obviously knew a little about the hobby.

Turn On, Tune In, Drop Out

I excused myself from the conversation in order not to lose the frequency and turned my attention back to my radio. Too late, another operator was now calling CQ on my frequency so I had to QSY and hope RBN would re-spot me on the POTA system (it did).

I don’t mind being an ambassador for the hobby but clearly this antenna attracted unwelcome attention that interrupted my activation. I needed something just a little more discreet.

So Its Gotta be a Random Wire Then

Solar cycle 25 is now forecast hit a record peak in late 2023 or spring 2024. As we move toward that rapidly approaching peak the higher bands are beginning to open up. That means I need an antenna that will work on 20m, 17m, 15m, 12m and 10m. This is good because shorter wires will be needed and shorter wires are generally stealthier than long wires.

Avoid the Halfwave Trap!

Maybe that 41ft wire wasn’t needed after all. A random wire has the versatility of being tunable on multiple bands, but there is a hidden trap that random wires can easily fall into. The wire length has to be chosen carefully to avoid being a half-wavelength on any band of interest. Halfwave wires have a very high impedance when end-fed and will be difficult to tune. Fortunately, Jack VE3EED has published a list of wire lengths to avoid (and there are many). A web search will bring up that information from several sources. He also made a list of “safe” wire lengths with the shortest being 29ft.

Back in February I published a post called “What in Heaven’s Name is a Rybakov Antenna?“. I tried a version of the Rybakov (a vertical random wire) based on VE3EED’s list and chose 29ft as the length. It kinda worked but I decided although it was interesting I probably wouldn’t use it.

Then I re-read Craig WB3GCK’s post on the subject and reconsidered. Craig suggests using a 25ft or 26ft wire. A 25ft wire is 3/8 wavelength on 20m which is good – it results in a nice low angle donut-shaped radiation pattern that is good for DX. A 26ft wire is BAD – it is a halfwave on 17m – theoretically.

Out came my bruised and battered 29ft pole again. I strung up 26ft of wire, connected a 4:1 balun at the bottom and commanded my LDG Z-11 autotuner to find a match on all the bands from 40m up to 10m. It succeeded without any difficulty. I even made a few test QSOs on it. So, Surprise! A 26ft Rybakov will be joining the 80m EFHW and the VP2E in my antenna bag this year.

A Word About the Unun/Balun

In his post Craig WB3GCK links to a 4:1 unun design by IW7EHC. I have built this kind of unun but I prefer a different design that can also be used with balanced antennas. I have included the schematics for both types below. Although a coax-fed random wire antenna is unbalanced on both sides, the balun design balances the current and avoids the need for a 1:1 common mode current choke.