"cliff wright" wrote in message
...
Good day all.
Have run into a bit of a problem matching my G5RV's support mast as a
vertical on 20 metres.
The mast is 7.5 metres high and insulated from ground with 4 radials at
ground level but insulated from ground by installation along fence lines.
Obviously a typical 1/4 wave vertical would be ~5.1 metres high, but I had
hopes of base loading the 7.5 metre mast to act as a sort of "short 5/8
wave" vertical on 20. Feed is 50 Ohm coax buried up to the matching box.
After a lot of research both in books and on the web I have found very
little information on the design of an antenna like this.
Anyone had any experience of such an idea?
I have plenty of airspaced inductor stock and TX type capacitors for the
network, and as I usually only run about 150 watts here that shouldn't be
too critical.
Any ideas gratefully received!

73's Cliff Wright ZL1BDA ex G3NIA.

You might try the matching system used on the old "Gotham Vertical". This
uses a tapped coil from the base of the mast to ground. The top of the coil
is tapped down until resonance is found. Coax braid is to ground, and the
coax center conductor is tapped up from the bottom of the coil for best
match.

Repeat the tapping, and measuring for best settings, as there is
interaction. Then you can consider removing unused turns off the coil. Or
you can find the taps that match other bands. This same matching method is
often used on mobile antennas, and may still be in the ARRL handbook.