Playing cuckoo with redundant antennas!
Tue, 2 Feb 1999 21:23:59 +1300
Hi there Andre'
I have played 'cuckoo' at a couple of old BC tx sites now, Gebbies Pass bing
the most recent. Hot feed points are quite dangerous when open but fairly
safe once grounded(obvious!!) so a ground stick is a must! I would
recommend using an adjustable series inductor to ground off the 1200 footer
as it should take care of most of the safety problems. I have used a large
tapped inductor and a smaller variable one in series, at the ground end, the
latter only temporarily to get the thing on tune. You can remove the smaller
one and measure it to see where to go with adding or changing the inductance
in the main inductor. Getting the overall inductance close to the right
value can be done by using a receiver with a 1 or 2 turn link at the ground
end and tuning in a test signal from a site not too close in. Setting the
tapping point can then be done with a return loss bridge, provided there is
not too much rf coming back at you from off the tap! A good high voltage
rf voltmeter (eg HP 410C and probe) is useful to see what you are getting.
A nominal 50 ohm termination low pass or even band pass filter can be setup
before hand and can be used to keep the krap out of the measuring setup when
tuning, as well as when eventually operating. It does disguise the load Z
when tuning though, as if it don't look near to 50 ohms the other end of the
filter can go almost anywhere, Z wise! So getting the inductance close to
right with a rx is a good way to start. Then you can fiddle with the tapping
point or use an L match on a fixed tap or whatever, but the grounding effect
of the main tuning inductor is quite comforting!!