Topband: High Z vs Low Z shunt feed tower

[Tom Rauch]

Hi Ford,

> Question:
> Is it better to feed this type of tower at a high impedance or low
> impedance?

In most cases ground losses will dominate system losses, and so 
feed system errors won't make a noticeable difference. But if you 
are splitting hairs the following is true.

The system will have less loss and more bandwidth when a simple 
series C gamma with very high capacitance is used. This is 
accomplished by placing the tap on the correct point of the tower, 
using the right spacing, and using a fat conductor or cage of 
conductors for the feedwire. This makes the Q of the feed system 
the lowest possible value, and reduces reactive power in the 
system. Reactive power causes loss, does not contribute to 
radiation, and reduces bandwidth.

An omega match has more loss and less bandwidth, all other 
things equal, than a simple gamma match with only a series C. A 
gamma with a high value of series C has more bandwidth and 
higher efficiency that one with a lower value of C, all other things 
equal.

73, Tom W8JI
w8ji@contesting.com


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