Topband: High Z vs Low Z shunt feed tower

[i4jmy@iol.it]

If the tower were self resonant, the current distribution would be 
linear and a voltage shunt feed will be just a folded unipole.
When the radiator is not resonant, but loaded, the current distribution 
is not linear and it is a good advice to keep the shunt tap as much as 
it is possible in the antenna portion where the current distribution 
approaches the linear one. (With very short radiators it's not 
raccomandable to shunt feed, but rather to series feed only)
To cancel reactance and match 50 Ohm we have to consider the tap 
position, the ratio between tower and gamma rod and the gamma capacitor 
capacitance.
The pratical solution for any tap position is single but useful tap 
positions are many.
Last but not least, althought true that an omega match is theorically 
lossyer than a gamma, the wrong capacitor type in a low Z shunt feed 
gamma can be much lossyer than an omega with proper capacitors (i.e 
vaacum types) at higher Z.

73,
Mauri I4JMY








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