[Lowfer] Receiver protection

[Clive Carver]

With my Merchant Navy Radio Officer hat on.  At sea we had some fairly
powerful transmitters on MF and HF and usually with the receiver aerials in
close proximity, sometimes within 20 feet. Typically 500 watts on MF and up
to 1500 watts on HF A1 and A3J. Most Marine receivers were only rated for a
max of 30 volt RF into the aerial socket. Some receivers had provision for
an antenna relay, which could work with ARQ but for Duplex SSB RT the
receiver would be on all through the transmission.  On one ship I could if
need be, run 750 watt ARQ radio-teletype whilst still be able to use the
main transmitter and receiver.

One device that worked, it must have done since I never blew up a receiver,
was quite simple.

Externally, Four or Six diodes connected in pairs back to back across the
aerial input socket and between the actual aerial and the hot end of this
back to back diode string, a lamp bulb.  One device made commercially used a
28 volt 40mA lamp, but they did use to fail. The better versions used a
100/120 volt lamp.  Some "devices" also included a fuse between the aerial
and the lamp.

The diode string will limit the voltage into the receiver at less than a
couple of volts. The lamp bulb resistance when cold, ie with just received
signals involved, is low, but when RF from the transmitter is picked up,
most of the voltage will be across the lamp bulb and less than 2 volts
across the diode string/receiver aerial socket. The bulb will light up and
the resistance of it will now increase to accommodate.

One voyage from New Zealand to Europe via Cape Horn, our Chief Officer stung
out an aerial wire  a couple of decks down from where the ships 750 watt
radio-teletype transceiver was located, his intention being to connect this
to his brand new Sony shortwave receiver so as to get the UK news. Luckily
he told me before he connected up. I wired up as described above and before
connecting to his receiver showed him how bright the lamp lit - he could not
believe it! It lit up his room when the ARQ transmitter was running. We
connected up his receiver and all was good, he could receive what he wanted
and no damage.

Obviously you need a good ground/earth connection for the receiver.  As for
wattage rating of the lamp, a bit of trial and error so as not to have too
great a cold resistance when "key up", and not too high so as to blow the
back to back diode string. Similarly, if you include the series fuse, take
into account the lamp bulb wattage, but will depend on keying cadence if
Morse or whether a continuous transmission such as FSK/MSK etc.

Also, I'm guessing that if you are taking a 40 watt lamp to full brilliance,
that must mean that those 40 watts have come from what you would otherwise
have been radiating!

Finally, the above worked for me, but no guarantees from me that it will
save the front end of your receiver.

73s

Clive 
GW4EYO



-----Original Message-----
From: lowfer-bounces at mailman.qth.net [mailto:lowfer-bounces at mailman.qth.net]
On Behalf Of N1BUG
Sent: 10 July 2018 13:04
To: rsgb_lf_group at blacksheep.org; rsgb_lf_group at yahoogroups.co.uk;
Discussion of the Lowfer (US, European, & UK) and MedFer bands;
600MRG at mailman.qth.net
Subject: [Lowfer] Receiver protection

Does anyone have ideas for receiver front end protection at LF and
MF? I am worried about my TX signal damaging receivers.

Of course the best idea is to disconnect the RX antenna from the
receiver while transmitting. Any good solution while leaving it
connected?

Now I have a pair of back to back 1N4148 diodes across the receiver
front end but I don't think this offers real protection because they
allow too high voltage before conducting. Also there is a preamp
after the didoes which can bring the RF to dangerous levels!
(Somehow nothing was damaged during last winter operating this way... :)

73,
Paul N1BUG

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