On Sun, 3 Apr 2005 13:24:27 -0700, "John Smith"
wrote:

[snip]

The "Monopole" I had pictured in my mind was a 1/2 wave end-fed, but, I was
not sure if the gamma would even be suitable for this use--and I attempted
to leave my original question open-ended to catch this, if that was the
case.

I am glad you pointed out the center of a driven element, such as in a yagi,
is "dead" and can be attached directly to ground. If a 1/2 end-fed monopole
can be matched with gamma--would that still be the same case, only here the
"end" could be attached to a grounded mast?

The gamma is normally used to feed the center of the driven element of
a Yagi where the feedpoint impedance is lower than that of the
transmission line and it is desired to have an unbroken element.

Another application is to shunt feed a grounded monopole, such as when
a grounded tower is used as a radiator. In this case too, the base
impedance is lower than the transmission line Z.

Both cases use the gamma as an impedance "step-up" transformer.

Because you suggest an end-fed half-wave, where the impedance is much
greater than 50-70 Ohm, I see no compelling reason to opt for a gamma
feed, other than the case when you want to ground the end of the
radiator (which may be what you have in mind).

While it is convenient to think of the gamma as a system where a "50
Ohm point" is located on the radiator and a tap made to it, the
reality is that the system is *much* more complicated and involves
mutual coupling, transmission line effects, etc, between the gamma rod
and the radiator. A moments reflection (no pun intended) will make
this obvious. If a "50 Ohm point" is the only goal then the gamma rod
diameter and spacing would not be factors.

Additionally, there is a multiplicity of combinations that will
present a match, at one frequency. Each will have a different
effective bandwidth, however.

Suggested reading:

"A New Look at the Gamma Match", QEX, May/June 1999, pp 23-31