On Wed, 25 Jul 2007 16:20:52 +1200, cliff wright
wrote:

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.
My approach to this situation would be to download the demo version of
EZNEC, model the vertical with the objective of de terming the needed
reactance for tuning, acquire (or have someone else run their copy) of
TLW (ARRL Antenna Handbook software) and pick the easiest matching
setup for you. If you fabricate the matching device and it is not even
close to matching then you don't have an accurate model of the antenna
in EZNEC.

Four radials will work but not real well.

John Ferrell W8CCW
"Life is easier if you learn to
plow around the stumps"