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 will hook 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. I will keep you informed!