[TowerTalk] Towers and Open Wire Feeders
Dan Zimmerman N3OX
n3ox at n3ox.net
Thu Feb 1 05:40:08 EST 2007
It would (probably) add some phase shift to your tuned feeder system,
and would probably require you to trim back to length at the shack end
to get your impedance repeater back, but there's a trick that I picked
up from an old ARRL antenna book that uses small tuned loops tightly
coupled to each other.
The basic idea is that you take two copper loops of some convenient
size (maybe 0.5m diameter?) and you select capacitors to resonate
them. On the transmitter side you use a parallel resonant circuit so
that the feedline is across a high impedance except for the coupling
to the other loop. One leg of the feeder connects to each end of the
split loop and the capacitor is installed across the gap.
On the antenna side, you use two caps of twice the resonating value
(probably unnecessary, but it looks nice and balanced) and set up a
series resonant circuit. Each leg of the feeder is connected to an
end of the loop by a capacitor.
I've not used it for antennas myself, but I did use it to couple the
signal from an ultrasound velocity measuring instrument onto a
rotating experiment. The best signal transfer from one side to the
other was seen with the parallel resonant circuit on the TRX side and
the series resonant circuit on the transducer side (I tried
parallel-parallel as well, more phase shift and loss)
In our case we needed continuous rotation at a good clip and didn't
have any RF capable slip rings. The frequency was 4MHz and I used 8"
copper loops and ceramic caps. You'd want to go bigger and higher Q
on everything for high power transmitting use.
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