The G4GKH Doublet
Antenna has been used in various guises for several years -
although since we moved to Stone in November 1999 I am making do
with a half-size G5RV. I rescued the traps from the previously
installed antenna and they seem to have survived the elements,
surprising since they had been on the roof for nearly ten years.
The MS word 6.0 file provides you with the maths so you can see that this is not based on alchemy. This really is an example of where the theory goes right into practice without tweaking. In other words it worked first time with good SWR.
The method of implementation can be as simple or as complex as you like. The use of suitably protected/coated cut lengths of broomhandle is the cheapest and all I have ever used.
35mm Film cases filled with epoxy give good protection for the 'traps' but I have only ever used very thick coats of varnish. It lasted for ten years so it must have worked! The graphics below speak for themselves. You can dispense with the ATU if you have a very good 6:1 balun. I have not found one yet that does the job and would be very grateful for anyone who can find a source. Most 6:1 balun designs give extremely poor performance.
The ATU design gives a reasonable 'Q' factor so you will have to tune it regularly. Eurotherm PID controllers and a stepper-motor can make a good auto-ATU. Decouple everything to earth on high power once you have a match though as you will zap the PID controller. They really do not like RF all that much! Note that dimensions are in feet.