Topband: Folded dipole vs gamma match

[Tom W8JI]

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