Author Topic: Testing an antenna change over relay question  (Read 398 times)

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Offline Chris Wilson

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Testing an antenna change over relay question
« on: April 16, 2018, 11:19:57 pm »
After  an  expensive  accident whereby my home brew antenna change over
relay  put  nearly  a  kW  of  RF up the RX antenna jack of my Kenwood
TS-590  i want to safely long term test the relay. It may have been my
lack  of  foresight  in using two identical N type sockets for the amp
input  and  the  antenna  on the relay box and they got reversed...The
relay is Russian:

It  needs 24V to reliably change, that is sourced from 12V via a DC to
DC  converter  that  is  energised via a safely long delay from my U3S
exciter so TX is delayed 100mS   It's connected like this:

 The output of my 1 kW amplifier to NO connection and receiver to NC
 connection of the same relay, with COM port to antenna), then
 de-energized I'd always be in RX mode and energized I'd be in TX mode
 and there's no way the TX should ever be connected to the RX no
 matter what failed. (Theoretically! I am not sure what reversing the
 amp and antenna connections may have caused, but I am concerned some
 other issue caused the RF to appear on the RX port....) I don't think
 the  pre  LPF voltages from the amp would be too high for this relay,
 but  can't  be  1005  sure  of  this given what occurred. In any case
 there's  also a 40 Amp 24V automotive "safety relay" that grounds the RX output jack
 when  in  TX  mode. The failure of this to protect the TS-590 further
 suggests  I  might  have  had  the  antenna  and amplifier connectors
 reversed,  although it all worked fine until the early hours when the
 WSPR spots ceased and I found the bad news.

Anyway, I need a "something" to put across the RX antenna output BNC
socket that wil blow or somehow tell tale if RF is injected there. I
don't have a high power enough dummy load other than a huge water
cooled 10kW Bird one which means flowing mains water through it to an
outside gutter overnight or longer. My none water cooled one won't
take high power for long at all. If a FET blows in the amp I can
swallow that, rather than my current Red Pitaya receiver getting RF up
it :) So what could I put across the jack instead of a receiver that
will tell tale anything nasty happening during a prolonged test
please? Thanks.
Best regards,

                 Chris Wilson.

Offline dmills

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Re: Testing an antenna change over relay question
« Reply #1 on: April 17, 2018, 01:13:13 am »
50 ohm low power resistor wrapped in a bit of fireproof material, you will be able to tell if it gets a kW up it, and if the amp has decent protection it should survive the reflected power when the resistor explodes.

Sticky relays are a thing, and there is something to be said for running some DC thru the contacts (choke on each side) and using that to gate the amplifier keying (So the amplifier cannot be keyed unless the relay contacts have really changed over).

On the RX side, you are usually going to be sky noise limited anyway (on HF) so a modest pad (3dB or so) helps robustness quite nicely (But will not make the RX survive a kW), sensitivity is seldom a limiting factor on a modern HF set!

Regards, Dan.

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