Before my QRP- pedition I have searched for
an effective field antenna for 40 meters. I need antenna which
can radiate both as vertical as horizon wave, which does not need
antenna tuner and the same time which could be a resonant antenna.
I have found such antenna. This one is designed by G3XAP. The
antenna works well at my mountain QRP- pedition.
Figure 1 shows a design of the antenna. Table 1
gives data for ranges 20, 40 and 80 meters.
The maximum gain of the antenna is directed to side opposite L2. The
difference in gain forward/back is small it is only near 3-dB.
Two counterpoises have length 0.25 lambda each. For improving
antenna efficiency use as more counterpoises as you can. The maximum
of radiation lays in sector of 30 - 70 degrees. I get SWR 1:1
(at SWR meter of my FT- 817).
Tuning of the antenna is very simple. Reach minimum SWR with help of
base capacitor C. I used a variable capacitor while tuning. Then
change the variable to fixed one. The capacitor should be protected
against atmospheric forcing.