cliff wright wrote in
:

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.

Cliff,

I have written an article on unloaded verticals as a multiband antenna,
it is at http://www.vk1od.net/multibandunload...ical/index.htm .

If I scale the model results for a 13m vertical to 7.5m, it suggests that
you would have a base feed point impedance around 150+j300. You could
start by designing a matching network for that sort of impedance and fine
tune the network for low VSWR.

I am not clear on whether you are using this vertical as a mast for
something else. If so, all bets are off.

Owen