While I only got space for a relative small “rotable” antenna it will always be a compromise on 20m and even on 15m. While my 80/40 QRO half wave end-fed seems to be working perfectly, I decided to make another QRO end-fed for 20m and 15m and will have a dedicated rotable antenna for 10m (maybe combined with 6m). I started off making a 2:16 (1:8) auto transformer on two stacked Fair-Rite FT240-43 toroids:
I used 1.8mm wire for the primary turns and 1.2mm wire for the secondary turns. The first two turns are bifilar turns with primary and secundary wires twisted together. Then the secundary wires makes another 6 turns on the same side. Then the wire crosses the core to the other side and there’s another 7-8 turns so there’s a 2:16, or actually a 1:8 turn ratio which will result in a 1:64 impedance transformation nicely matching the typical 3k3 – 3k5 impedance of the end of the half wave to 50 Ohms. A “Voltage fed” antenna requires very little counterpoise. On my 80/40 QRO end-fed I have no counterpoise, the coax is sufficient and just before it enters my shack I have an RF choke to avoid RFI which seems to be working fine, even with QRO power. It seems that a length of about 5% of the wave length is sufficient so I will connect 1m of wire to ground and use an RF choke (2 FT240-52 cores with 11 turns of RG58) right after the auto transformer.
For now, I have hooked up 10.2 meters of wire to test on 20m. When the transformer is OK, I will be adding a trap for 15m: a 3 uH air core inductor combined with a 20 pF high Voltage ceramic door knob capacitor. After cutting to correct length, I got a nice 1:1.2 SWR dip around 14.050 MHz and tested it out. It seems to work great and can handle the 400W PEP that I used. I worked some Canadian stations in what I believe was the ARRL contest; VY2TT, VE9HF and VO1SA. Distance is around 6000 km pretty good but…
I noticed the signals on my 80/40 end fed, that hasn’t even got a decent SWR on 20m, were 1 S-point better with less noise. I can tune the 80/40 end-fed to 20m and it seemed to work pretty well in the PACC contest and I also recentely installed a 3-band rotary dipole for 20/15/10 so the project has become kind of obsolete. Nevertheless I thought the info was interesting enough to publish on my blog.