Topband: High Z vs Low Z shunt feed tower
Mon, 6 Nov 2000 09:37:17 -0800

Ford, N0OQW wrote:

"Is it better to feed this type of tower at a high impedance or low
As Tom mentioned, the efficiency will increase if you simply use only one
component -- a series (gamma) capacitor.

You also mentioned that you notice two "resonant" points -- one at 1840
kHz and the other at 1935 kHz.  This is a typical observation on all
shunt-fed towers.  1840 kHz is where you have the feedpoint tuned to and
1935 kHz is the 1/4 wave resonant frequency of the structure.

Because the structure is not too far from 1/4 wave at 1840 kHz, your 2:1
SWR bandwidth should be quite good.  Best bandwidth occurs when the
structure is exactly 1/4 wavelength.  That is also the point where the
gamma capacitor value will be the greatest, by the way, and where the RF
voltage across the capacitor will be the least.  Also, it is the point at
which the 50-ohm tap point onto the tower is closest to ground level
(lower than 30 feet), which might also be considered a "plus" (less
climbing!).  If the structure is greater than or less than 1/4
wavelength, the tuned value of the gamma capacitor will be smaller, the
voltage across it will be greater, and the 50-ohm tap point will be

Although a structure which is 1/4 wave at the operating frequency is
desireable for the above reasons, it is by no means necessary.  Your
tower only needs a slight amount of additional top loading to get there,
however, and you may want to persue that.

I feel that you should find the 50 + jxxx tap point on the tower (which
will be quite a bit less than 67.5 feet in your case) and simplify your
feed system by using a single capacitor.  While you're changing things,
also replace that #14 AWG shunt wire with at least #8 AWG.

73, de Earl, K6SE

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