Owen Duffy wrote:
"Wayne" wrote in
news:srSpi.4065$9A6.827@trnddc01:


"Owen Duffy" wrote in message
...

"Wayne" wrote in
news:cbKpi.3766$7w.1151@trnddc05:

...

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.

Whilst this system might work for an antenna with a capacitive
feedpoint, will it work for an inductive feedpoint such as this ~3/8
lambda attena?

Owen

Just add enough coil to bring it to 3/4 lambda resonance. Wouldn't
that work?


I don't really understand exactly what you mean, though I know it is a
popular proposition.

If the antenna feedpoint is around 100+j300 (see my previous posting),
the simplest matching network is an L network, and there are two
solutions to it, but both L networks in this specific case require an L
and a C, and both use practical values of L and C. For example, a 84pf
capacitor in shunt with the feedpoint, then a 2.4uH inductor in series
from the feedline to the feedpoint will deliver that transformation with
reasonable efficiency and with one side of the capacitor grounded.

Can you propose the values of a two inductor L network, or a tapped
inductor that will transform 100+j300 to 50+j0.

Owen
Thanks Guys!
That has already given me a few ideas.
The G5RV is supported by the mast but it is electrically isolated and
about 300 mm away from the mast itself.
Of course there will be coupling effects but the idea is to tune them
out as much as possible for single band use. Operation is mainly on CW
at the low end of the band.
This then might give me 2 antennas on a rather restricted section, and
an alternative especially for receiving.
Many years ago I got very good results with a pair of 1/4 waves
verticals on a bonded iron roof fed with tapped coax to give a cardioid
pattern. It was a superb 20M rx antenna and I worked a lot of
"antipodean" stations in the western Med area even when the band was
apparently dead back in the 1970's.
Can anyone suggest a good text on matching network design?
Now I'm retired I no longer have easy access to Auckland University's
library for reference and will need to make special arrangements.

73's
Cliff Wright ZL1BDA ex G3NIA