The step up ratio of a folded dipole occurs because the fed conductor
extends parallel to the un-fed conductor for the entire length of the
antenna. The element or element halves form 1/4 wave shorted stubs in
differential excitation mode, but the current divides by ratios of effective
diameters. The feedline, in effect, just samples a portion of the total
current causing radiation.
With a shunt feed system, the mechanism is different.
The shorted stub formed by the gamma section is not 1/4 wave long, and
parallels the feedpoint. Also, the gamma does not parallel the antenna
length. There is actually not much change in the real part of impedance as
the gamma rod changes ratio compared to element size. The slightly larger
change is in reactance.
For example, a 3" diameter gamma rod on a 1 inch diameter resonant 160 meter
element at 40 feet produces an impedance of 289.6 + J 57.92 ohms
Changing it to 0.1 inches results in 454.7 + J 130.7 ohms
130.7/57.92 = 2.26 ratio in reactance for a diameter ratio change of 30:1.
454.7/289.6 = 1.57 resistance ratio for the 30:1 change
If I adjust the tap point to a good match (at 14 feet above ground) I have:
3 inch diameter gamma conductor 32.68 + J 65.45 ohms
0.1 inch gamma diameter 52.04 + J 111 ohms
1.6 ratio in resistance and 1.7 in reactance for a 30:1 change in rod
diameter.
The primary benefit in a larger diameter gamma rod is lower Q and lower
voltage across the tuning capacitor.
If I shorten the element, I can gamma match without a capacitor.
73 Tom
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