Hi Adrian,

The feed point will depend on the geometry of the gamma match wire.
A large cage requires a shorter gamma wire than a thin gamma cage.

For my tower, a 6" diameter 3-wire cage spaced 2-1/2 feet from the tower
was about 34 feet high at the 12.5 ohms point. It was replaced with a 1"
diameter
aluminum tube and the feedpoint moved up to 42 feet high at the 12.5 ohm
point.
This is for topband. The wire cage kept breaking due to wire fatigue (we
have a lot
of wind here), the aluminum tube is much more reliable.

The 12.5 ohm feedpoint at 1850 KHz turns out to be a 50 ohms feedpont
at about 3750 KHz. Using a 4:1 transformer, one can thus select this gamma
feed for either 160 or 80 meters.

A 50 ohm feedpoint on topband would be *rougly* twice the lengths quoted
above.

The gamma capacitor is a 15 kv vacuum variable with a homebrew stepper motor
to turn the capacitor. Tuning the capacitor allows full coverage 1.8-2 MHz.
and 3.5-4 MHz.
An assortment of wide-spaced air variables all arced badly at 1.5 KW power
level.


-- Tom










"Adrian Rees (M1LCR)" wrote in message
...
Hi
I have just got planning permission for a 75 Foot tower. I plan on putting
a
Force 12 C31 XR (10-15-20) as well as a lightweight 50MHz and 144MHz
yagis,
and a rotable 7MHz Dipole up.

I plan on running top band by using a Gamma match to feed the tower
against
a good carpet of buried radials and a chicken wire mat.

My question is, does anyone here have experience of shunt feeding a tower
for top band, and, does anyone have a formula for calculating the match ?
What are you experiences, how good is the solution ?

(The formula is important as it will determine the type of tower I order,
in
terms of seftion length as I don't want to order a tower that although
telescopic, has the gamma match feed pint half way up a telescoping
section).

Thanks

Adrian Rees
M1LCR