[TowerTalk] Shunt Feeding a Vertical

[Roger Parsons]

I thought I understood shunt feeding, but I clearly do not. I am pretty au fait with determining gamma matching parameters, but for this application I want generic shunt feed.

I have use of a 325' grounded former AM broadcast tower (on which I have various 160m antennas). I now want to feed it on 475kHz as well. I had been of the opinion that one could tap at any point on the tower, bring a wire down from that point, and end up with some impedance which could then be matched. Whilst that may be generally true, there is clearly more to it. My first attempt was a tap at 50', spaced about 18' from the tower. The impedance measured at the bottom of the wire was almost zero - that is it appeared as if it was the tower itself, and that was not useful.

As gamma matching clearly works, and as other people have successfully used shunt feeding, it must be possible. It has been suggested that I should go higher up the tower (say 100'), and space the wire further out. I could do this, but would like some confirmation that it is likely to work first.

73 Roger
VE3ZI