[TowerTalk] Switching ladder line

[Bill Putney]

Jan,

Don't do it! Ladder line is a balanced transmission line. If you common all of
one side and only switch the other legs you'll unbalance all the lines to say
nothing of coupling in one leg of every antenna you have connected.

The coolest balanced line switch I ever saw was at the Delano Voice of America
site. They had a fenced off area just outside the transmitter building. There
were a bunch of balanced rotary switches inside this yard that were driven by
motors (selsyns I suppose). The antenna ladder lines came in to these switches
and terminated on brass balls on one hemisphere and a straight rod with a wiper
on it went through an insulated shaft attached to the motor. The transmitter
ladder line attached to brass balls on the opposite side of the switch. Two of
these switchs were ganged on the same motor shaft so that both sides of the
ladder line were switched at the same time. They had arranged these switches in
trees so they could switch any antenna to any group of transmitters.

Of course the VOA was using 10KW - 500KW transmitters at the time. Our
requirements might be a little more modest. I wonder if a person could build a
small version of this arrangement inside a piece of PVC sewer pipe. Then the
whole thing could be weatherproofed by putting caps on the ends. The ladder
would just attach to screws on the circumference of the tube on the outside and
the screw heads or a small ball (like table lamp shade nut) on the inside. The
wiper on the inside wouldn't be too hard to fashion. A rod with piece of a
switch leaf stuck in both ends. Something springy and durable. You'd have to
scale the whole thing to try to keep the ladder spacing all the way through the
switch but it sounds like it might work. You could use a stepper motor and have
an index contact on the shaft to find the first antenna contact then just count
the steps from there. You get two complete contact sweeps for every revolution
of the shaft.

Makes you just want to go put up half a dozen rhombics pointed in your favorite
directions and work the world QRP. :) Of course my lot is only 1/4 acre... But
I'm sure the neighbors would cooperate.

- Bill

   On May 18,  8:10am, Jan Ditzian wrote:
> Subject: [TowerTalk] Switching ladder line
>
> I am interested in remotely switching among multiple balanced lines (450
> ohm).  There are latching vacuum relays that I can use, but they cost
> from $9 to $15 each and are mostly spst.  I originally thought to use
> two
> relays per antenna, one spst on each side of the line.
>
> I would like to know whether one can switch balanced lines the way one
> might switch coaxial lines, by leaving one side common and switching
> only the other side.  This would, of course, mean that I would need only
> half
> the number of relays that I would need for switching both sides.
>
> Thank you for any information.
>
> 73,
> Jan, KX2A
>
>
>
>
>
>
>
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>-- End of excerpt from Jan Ditzian



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