In the past I’ve made some attempts to homebrew a simple L-tuner circuit that I wanted to use to tune random (actually “convenient”) lengths of wire to a certain frequency. The idea of course was to be able to find the ideal L/C ratio to tune a holiday vertical (wire in a fishing pole) to my favorite 40m band.
On a ham fest, I bought a small (10 uH?) roller inductor and I had a 250 pF variable capacitor with sufficient plate distance lying around the shack. I soldered ‘m in series and did some tests with random lengths of wire but I never actually took this “solution” into productie. So for my holidays, I used my double coil holiday end-fed; about 8m of wire with two coils; a 15 uH coil acting as 10m trap and loading coil for 20m + 40m and a 30 uH coil acting as a 20m trap and loading coil for 40m. This antenne worked fine on 20m and probably great on 10m, but the efficiency on 40m was very limited.
This autumn, we will have a short holiday with our caravan and I want to test a “random” length of wire inside my 10m fiber glass mast, tuned by a decent LC circuit. To my surprise, I saw a UPC unique wire tuner (USA 70’s) for sale. Price was fair so I contacted the seller and bought it. I hoping it to arrive somewhere this week. Basically, this is an L/C. circuit with inductor first, than vcapacitor to ground. It’s from the 70’s, made from good quality parts and built like a tank. It’s not possible to switch the vcapacitor before or after the inductor so it will only tune high impedances. While 10m of wire will provide low impedance to the tuner, I’m planning on having a 10 – 20 uH loading coil in the center (like mobile whips do) so that the physical length will be around 14m. This will provide a relatively high impedance on 20m, 40m and 80m so the L/C circuit should be able to tune it down to 50 Ohms. I’m really curious what the results will be, I will keep you informed. Some pictures from the internet: