Topband: gamma match question

Earl W Cunningham k6se at
Sun Sep 24 16:04:40 EDT 2006

Paul, N1BUG wrote:

"I'm curious why all the shunt fed towers I've seen had the gamma section
running up only as far as the "tap" to the tower, never extending above
the tap point. Is there some reason to do it that way, as opposed to
making the gamma section longer than needed and using a movable shorting
strap to the tower somewhere along its length?
There's nothing wrong with extending a gaama rod above the tap point. 
Probably most shunt fed towers use a gamma wire (rather than a rod), so
that's why you don't see it extending above the tap point.

The "trick" is finding the tap point that is 50 ohms resistive.  Once
you're there, you make the tap permanent and all you need now is a series
capacitor to cancel the inductive reactance.

Finding that 50-ohm tap point is not too difficult if you start by
knowing that R is zero at the base of the tower and R is quite high at
the top of the tower.  Somewhere in between, R = 50.

73, de Earl, K6SE

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