To: rsgb_lf_group@blacksheep.org
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From: N1BUG <paul@n1bug.com>
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Message-ID: <71940ee0-587b-5f59-a709-bda5ca93d45a@n1bug.com>
Date: Mon, 16 Apr 2018 09:19:02 -0400
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Subject: Re: LF: Antenna change over relay testing question
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Reply-To: rsgb_lf_group@blacksheep.org

Hello Chris,

The only "something" I can think of would be a really small fast
blow fuse. Fuses come in ratings down to 1mA at least but they tend
to be pricey and a bit hard to find. I can't be sure that would blow
fast enough to catch all potential momentary glitches which might
harm a delicate receiver. Surely someone here has a better idea!

Reversing the amp and antenna connections should not cause the
problem in and of itself. Reversing those connections combined with
failure of the relay to switch would but in that case you probably
shouldn't have been receiving anything earlier in the night since
the antenna would never have been connected to the receiver.

I don't see how delayed relay switching would have caused your
problem (unless the connections were in fact reversed) but exactly
how is this configured? Does the U3S switch on the DC-DC converter
or does the DC-DC converter run all the time with the U3S simply
energizing the relay from it? I'm wondering about start-up time for
the converter...

Good luck with it. I personally don't like using the transmit
antenna for receiving as it slams way too high a level into the
receiver, leading to possible overload issues. I suppose you could
use an attenuator.

Even with a dedicated and small receiving antenna it's wise to
provide some sort of overload protection for the receiver. I
measured 250mW coming down the line from my LNV while transmitting
on LF with 200 watts! Not good! Here the protection is extremely
crude, a pair of back to back diodes across the receiver input. That
has protected my receiver but causes its own set of issues.
Unfortunately I don't have a better solution that works at LF.

Paul


On 04/16/2018 06:24 AM, Chris Wilson wrote:
> 
> 
> Hello  LF'ers,
> 
> 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 a Russian
> http://qro-parts.com/product_info.php?products_id=102
> 
> 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.