On 9/5/19 1:57 PM, David Gilbert wrote:
Shunt feed essentially involves building a transmission line comprised
of the tower and the shunt wire. Your 50 feet equates to not much more
than 2% of a wavelength at 475 KHz, so yeah ... it sounds to me like
you're pretty much just trying to feed a shorted transmission line.
exactly.. wavelength at 475k is 631 meters (about 2100 ft), and your
tower is 325 ft, about 1/8th wavelength.
If it's an AM BC tower, with an isolated base, maybe you can just feed
it at the base as a "short monopole" with an appropriate LC matching
network.
the Z will be about 11 -157j ohms, so that's not too bizarre to match.
73,
Dave AB7E
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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