Topband: 1.5:1 Bandwidth of shunt fed towers.

[Tom Rauch]

>Actually, DeMaw did it first in October 1975 QST titled Another Method of
> Shunt feeding your tower He used four wires held together by 4" circuit
> board squares.
> 
> The arm is 43" long and the 47' top loaded tower is tapped 26' from the
> base. There is a 1,000 pF cap in series with the feed line. To tune the
> capacative reactance, DeMaw used a top loading wire. I can attest to you
> that a coil at the base is a much easier way to tune the antenna!

Two main factors enter the equation:

1.) How rapidly the radiator changes impedance with frequency.
2.) The Q of the matching system.

The radiator is something we are probably "stuck with" and can't 
change. 

A gamma match, with the tap point adjusted to provide 50 ohms 
with only a simple series-capacitor, will always have more 
bandwidth and higher efficiency than an omega match with two 
capacitors and a "random" tap point on the tower...all other things 
equal. An omega match always has less bandwidth and efficiency, 
all other things equal.

Look at the capacitor value used in your feed system. Lower 
capacitance "tuning-out" reactance in the shunt wire means  
the matching system (shunt feed wire and capacitor) has higher 
loaded Q. That means higher losses and less bandwidth.

A capacitance of 250 pF "resonating" a 50 ohm feed system
provides 353 ohms reactance and Q of at least 353/50 or Q more 
than 7 in the capacitor's contribution alone!

With a 250 pF capacitance on the gamma and infinite antenna 
bandwidth, 1.5:1 VSWR bandwidth would be about + - 90 kHz on 
160 meters. The actual BW will always be significantly less than 
this amount.

73, Tom W8JI
w8ji@contesting.com



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