[TowerTalk] Topband: Receiver protectors.

[Rob Atkinson]

Last year I purchased an Airspy HF+ Discovery SD receiver.  Eventually
I got around to trying to use it with a second hand Windows laptop.  I
was concerned about blowing out the front end on transmit because I
had heard of that happening with these receivers that are not designed
exclusively for ham radio.  A lot of the SD receivers are designed and
manufactured by non-hams for lots of uses other than ham radio.  They
are not made to be  used in an environment where they are in the near
field of a transmitting antenna excited by a few hundred watts or
more.

I looked at the DX Engineering product and saw this specification:
Max Output Level: RG-5000HD:+14 dBm at 10 W input. RG-5000: +10 dBm at
10 W input
The corresponding specification for the Airspy rx is:
+10 dBm Maximum RF input

These are sensitive receivers.   The DX engineering products were
marginal so I did my own thing.

I put the rx in an aluminum box with a UHF jack mounted on it an a
grommetted hole for the USB cable's exit.  I mounted a pair of relays
in the feedline to the receiver inside the box and put RF chokes in
series with the DC line to the relay coils (24 v. DC) at the entrance.
I figured two relays in series in the line would add some protection.
Amazingly none of this  helped much as a 20 w. carrier on 160 m.
produced a disturbingly strong signal trace on the receivers
panadaptor.  I wanted to see little or no signal at a few hundred
watts to feel comfortable about operating at higher power.  I was
using typical ice cube style relays.  The contact spacing isn't much
so I think the relays were just acting like low value air dielectric
capacitors in the line.  RF went right through them.

What finally did the trick were a pair of small DowKey style coaxial
feedline relays with SMA jacks on them.  Something kind of like these:
https://www.ebay.com/b/SMA-General-Purpose-Relays/36328/bn_7684024  I
found them at a hamfest and bought two because they had 12 v. coils so
I figured I could put two in series with their coils in series for my
24 v. keying system.  The same hamfest also had a guy selling small
coax jumpers with SMA males on them.  Perfect.  I wired them up  and
put them in the aluminum box with the rx and all the other stuff.
That did the trick.  I see little or no signal on transmit and the SDR
is safe at high power.  I really didn't want to zorch a $170 throwaway
item by making an avoidable mistake.  It took a long time of fiddling
and experimenting to get it right so I'm posting these notes to save
others some time.

73
Rob
K5UJ