Hi Roger…
Your tower is too short for a shunt-feed. It needs to be inductive at the QRG
of interest, 90 degrees or more (1/4 WL). Your 325’ tower is only 57 degrees
long on the 630m band. (.15 WL)
To shunt feed a tower (using either a single cap Gamma or 2 cap Omega match),
we need the structure to be inductive (electrically “long" at the QRG of
interest).
The tap is located at the 50 ohm point on the tower, and the Gamma cap in
series with the coax feed tunes out the inductive reactance to leave a perfect
50 Ohm match. If the gamma wire tap is lower on the tower where the resistive
component is LESS than 50 Ohms, then the Omega match can be used to transform
the resistive part to bring it to 50 ohms. The series cap still tunes out the
inductive reactance. The Omega match will NOT work if the resistive part is
greater than 50 Ohms.
Maybe you can add some big top-loading with some wires??
I encourage you to watch my video that shows the detail of how the shunt feed
works, how to find the 50 Ohm point, the effect of wire tap height and wire
spacing from the tower and diameter.
https://youtu.be/cHlc5MTGTFM <https://youtu.be/cHlc5MTGTFM>
This video explains the Omega match and shows how the second capacitor can
transform the resistive component of the feed:
https://youtu.be/NA-Kwr09j8M <https://youtu.be/NA-Kwr09j8M>
73, de steve ve6wz
> On Sep 5, 2019, at 2:34 PM, Roger Parsons via TowerTalk
> <towertalk@contesting.com> 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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