I suggest that you consider slant wire feeding. see:
http://www.hatdaw.com/papers/Dawson_Slant_Wire_Shunt_Fed_IEEE_BTS_2010.pdf
I also suggest modeling this. Even a simple model should get you in the
ballpark for the height of the tap. You don't need to hit 50 +jX but if you're
above half of that you should be able to match with reasonable efficiently I
would think.
Wes N7WS
On 9/5/2019 1:34 PM, Roger Parsons via TowerTalk wrote:
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
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