Topband: RX loop antenna RX protector

[Rob Atkinson]

You probably have a preamp on your rx loop.  The first thing to do is
kill the dc to it on transmit to shut it down.   I have an Airspy
HF+Discovery SDR which has a fragile front end.  To keep tx RF out of
it, in addition to shutting down the preamps, I put a pair of coax
relays in series, the little ones with SMA jacks on them, between the
antenna feed and the SDR.  I used a pair because each had 12 v. coils,
and my sequencing system runs on 24 v.  I also put the SDR in a metal
box and put pie wound chokes in series with the DC control lines into
the box.   As I recall, that was enough to protect the SDR.  A lot
depends on prox of tx antenna to rx antenna and also tx antenna
relative to shack.  If all are spread out, job is much easier.  I also
used an ice cube relay in the feedline to break open both the center
conductor and shield to interrupt RF on the outside of the shield.   I
looked at the commercial rx protectors (not the one posted here by a
fellow in the U.K.) and the specifications were not sufficient for the
Airspy.

Rob
K5UJ