SAA15699
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Hello Topbanders,
Having had a new transmit array totally ruin an excellent receive antenna,
let me share the method used to discover the interaction. The first hint
that something was wrong was getting totally killed in a contest, all the
while thinking that things were working fine.
The first step is to set up a signal source in the null of the RX antenna,
like off the back of a Beverage. It can be another local ham or a small
signal generator, whatever it takes to get a healthy s-meter reading. Then
fool with the TX antenna to detune it or otherwise reduce the current in
the TX antenna from the source. In our case this meant disconnecting an
inverted L from its feedline so that it was just a quarter wave wire
hanging in space. We saw the null on the Beverage improve drastically when
the offending inverted L was disconnected from its feedline. The permanent
solution was to install a relay at 3/4(could be 1/4) of a wavelength from
the feedpoint of the L which shorts the feedline on RX. A high impedance
is seen at the feedpoint of the L, allowing minimal(practically zero)
current in the L. The relay could also be at the feedpoint of the antenna,
opening on RX.
The better the RX antenna, the more sensitive it is to other antennas in
its aura(to borrow Yuri's analogy). It is very hard to detect effects like
this by just listening to an antenna. A Beverage may still outhear a
vertical, even if it is severely compromised by the presence of another
antenna in its neighborhood. Measurements are good things. Do them. You
will learn stuff.
Rob K2WI
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