Connect a "grain-of-wheat" light bulb across the unused coax and gradually
increase transmit power.
I know this is a rather un-scientific, crude test, but simply put, you're in
big trouble when it glows!
I tried this with a 600 foot Beverage that is, at it's closest point, 50
feet from any transmit antenna. On some bands, I could run a kW but on
others bands, the bulb was bright at 150 watts out I now have a Collins
"RF Overload Protector" (622-0908-001) permanently in the Beverage feed !
Better to use a GOOD antenna switch with published maximum isolation, or
unplug the unused antenna.
73, Charlie k3ICH
----- Original Message -----
From: "Julie Royster" <jsdroys...@nc.rr.com>
To: <elecraft@mailman.qth.net>
Sent: Wednesday, April 16, 2014 11:06 AM
Subject: [Elecraft] adjacent antennas and potential radio damage
Thanks to those who wrote me privately as well as those who replied on
this
list (KL7UW and K4NH ) regarding the possibility of damage to one radio
from another radio transmitting on a nearby antenna. For the benefit of
all
I am pasting below highlights of the useful information I received.
MANY THANKS! I learned a LOT from you kind folks.
Julie KT4JR
Dick at Elecraft said:
"At 5 watts it's usually not a problem, at 450 watts, you want to have a
bit
of room (maybe hundreds of feet) between antennas. The radios have some
protection from overdrive. Carefully planned with antennas end to end or
facing different direction with enough space it's possible to share a band
with multiple K3s. But if the antennas are coupling sufficiently, it can
be
destructive to the front ends of the radios. I can't offer any guidance
better than you've receive thus far. Every multioperator contest or Field
Day station has to deal with this problem, and there are solutions, but
they
require some effort, either bandpass filters to keep energy from nearby
transmitters on other bands, or antenna placement planning, or some
combination."
AB9CA/4 said:
"There are certain orientations which might allow simultaneous operation
of
the two stations. If one antenna is a vertical and it can be placed on a
line perpendicular to the mid-point of the Vee you should be OK. This is
when the base of the vertical lies on the vertical part of a 'T'
arrangement
with the Vee forming the horizontal top of the 'T' (looking down from the
air). Place them as far apart as possible. There is minimum interaction in
this configuration. With chokes in place and proper dress of the feedlines
you could get maybe 60 dB of isolation between them...
(On Field Day) Our club has used two stations on the same band, usually
20m,
with success. One on CW the other on Phone. We did it with two wires
arranged as I mentioned earlier, in the 'T' configuration.
Especially note that damage can occur even if one rig is turned off.
The antenna is still connected internally with power off. To be certain of
avoiding damage it is best to disconnect the antenna from any rig not in
use. And when you connect them be certain of what band the other station
is
on (and make sure you are not on it!)."
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