At 08:03 AM 9/15/98 CDT, you wrote:
>Before winter sets in, I want to put up a loop RX antenna for 160M on my
>VI+. My question is a relative one, but how much abuse can the auxiliary
>antenna input take when in close proximity (25 feet) to the transmit
>antenna? At this time, I only run 100W out, so it might not be a problem,
>but I will be increasing this in the near future and don't want to fry
>anything. Has anyone burned anything out on the auxiliary antenna input?
>73/DX de Rick N9THC
Hello Rick (and all):
The front end of the Omni-VI and Omni-VI Plus receiver is protected
by two diodes at the AUX RX ANT input jack. It is possible, when
using auxiliary receiving antennas like loops or Beverages to
have transmit RF energy from your own radio come back into the
rig through your receiving antenna. It is possible to blow these
diodes doing that; they are there for protection of the receiver.
That does not lessen the aggravation of having to replace them
should you have this problem, but at least you're not going to
cook the receiver itself.
Yes, it is possible even with small amounts of transmit RF to
pop the diodes at the AUX RX ANT jack. You may want to investigate
the possibility of adding an external protection circuit between
the receive antenna and the input to the AUX RX jack. There was
an article about this in the February 1997 issue of CQ. A very
simple solution is to use a low voltage, 10 mA light bulb
in series with the receive antenna. Too much RF down the receive
antenna jack results in a burned out light bulb rather than
burnt diodes. Much cheaper and easier to replace, too!
Here's another caveat about the diodes at the AUX RX jack: It is
possible (so possible, in fact, that this happened in my own shack
last year), for receiving antennas to send a quick static discharge
down the coax. Like any other antenna, receiving antennas need
to be disconnected during lightning storms. You may encounter
a small static buildup on a receive antenna as a result of a
nearby lightning storm, that will discharge at the moment the
antenna is re-connected to the transceiver. Connecting the
receive antenna to ground immediately before re-connecting to
your transceiver can prevent this from happening.
Scott Robbins, W4PA
Amateur Radio Product Manager
Tennessee QSO Party - this Sunday, Sept. 20
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