[CQ-Contest] What RF voltage OK at Rx in SO2R?
Mike Gilmer - N2MG
n2mg at contesting.com
Wed Sep 26 06:45:22 EDT 2001
Even if you don't care about the "re-radiating" of crud, if you are trying to actually listen with the radio that has been protected this way, AND the diodes are conducting - from the TX sigs from your other radio(s) - you might be a tad disappointed.
The circuit is a failsafe, but not an SO2R-enabler.
Mike N2MG
On Tue, 25 September 2001, k8cc wrote:
>
>
> While the circuit G4BUO described will most certainly protect the receiver
> front end, it will also likely have the nasty side effect of generating
> spurious signals all over the spectrum. When the diodes conduct from the
> RF coming in the antenna, the spurious signals are re-radiated out the
> antenna. If you live out in the country (as I recall Dave does from my
> visit to his QTH in 1990) perhaps this is no big deal. In a suburban area,
> I would think that every time you transmit you'd wreck every television, or
> amateur, over a wide area.
>
> Back when I first got going at this QTH, I had a 2L 40M quad switchable
> NE/SW using a relay on the tower between the loops. When the quad was in
> the default (relay relaxed) position, there was a S9+20dB harmonic on
> 14.200 (not even the band of the quad!) which absolutely disappeared when
> the quad was switched SW (relay actuated). The problem turned out to be a
> diode across the relay coil to protect against "flyback" transients when
> the relay was de-energized. The 300' of twisted pair control wire from the
> shack to the relay picked up enough RF from a 10 KW BC station *three miles
> away* on 1420 KHz to generate a *tenth* harmonic on 20 SSB. It also caused
> the only two TVI complaints I've ever had from this location in fifteen years.
>
> Don't use diodes to protect front ends unless you're also going to have
> filtering to stop the spurious signals generated.
>
> Dave/K8CC
>
> At 03:57 AM 9/25/01 -0400, Dave Lawley wrote:
>
> >Here's an RX protector circuit I made up a couple of years
> >ago, following a posting here on the reflector by Tom N4KG.
> >Brought it with me to WRTC2000 and we used it on the second
> >rig, though the voltage levels didn't really warrant it.
> >
> >Here at home I managed to squeeze a 260ft beverage
> >diagonally across my garden, but it was only about six feet from
> >my 40m 4-square at the nearest point. The bulb lights brightly
> >when I'm transmitting on 40m but the front-end of the FT1000MP
> >has held out fine.
> >
> >Here's the circuit. The bulb is a 60mA grain of wheat type and
> >the whole thing is mounted in a metal box with a small hole so I
> >can see the bulb when it lights.
> >
> >Dave G4BUO
> >
> >
> > antenna --- bulb --- 0.1uF --- 33ohm 2W ------------- rx
> > ! !
> > ! !
> > ! ! back-to-back
> > ! ! 1N914 diodes,
> > ! ! 4 in each string
> > ! !
> > ground --------------------------------------------- gnd
> >
> >
> >--
> >CQ-Contest on WWW: http://lists.contesting.com/_cq-contest/
> >Administrative requests: cq-contest-REQUEST at contesting.com
>
>
> --
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