2007-06-02
End-Fed Half-Wave for 2 Metres
I threw this antenna together while playing with the Fredbox transmitter. I wanted a reasonable radiator for stuffing around, but without tying up the main half-wave on the balcony.
It is simply a half-wave radiator fed at the end with a matching network comprising a tuned tank. A balun is provided to decouple the feed-line, and the match to coax is affected by tapping into the tank coil near its cold end. It is hung by a string when used.
The tuning is pretty sharp, but once tuned has reasonable bandwidth and offers an excellent match to 50 Ohms. The radiator is a piece of mains cable earth (multi-strand), cut for the harmonic centre of 2 metres. The coil is a bare piece of the active conductor from the same cable, 5 turns well spaced on a AA battery. The trimmer is a 2-9 pF air-space unit from Rockby.
Power handling is likely limited by the trimmer plate spacing, and the balun core saturation. It works fine at QRP levels.
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