One of my favorite single band portable antennas is the end fed halfwave antenna. Such an antenna requires a special coupling network to properly couple my 50 ohm coax to the antenna's high impedance (~2500 ohms). For a number of years I just used a circuit that I threw together with large components that I had in my junk box. For portable use however I was in need of something much smaller. In my junk box I also had several Mitsumi Polyvaricons. This are the small encapsulated variable capacitors that helped revolutionize the pocket radio industry and can be found in almost all analog transistor portable radios. They are not intended for transmitting applications and I wasn't sure if one would hold up to the high RF voltages that would be across it when used in the parallel tuned circuit that is used to couple energy into the end fed halfwave antenna. Since all of my operation would be at QRP power levels I thought I would give one a try. The circuit works great at the 5W level and I've successfully used it at 20W with no problems. In fact, during a test transmission at 20W for several minutes nothing failed and there was no detectable heating of any of the components. This indicates high efficiency of the circuit. Of course, the LED SWR detector circuit described below will require larger resistors (R1-R3) to handle 20W.

http://www.aa5tb.com

Schematic: