[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
> >
> >
> >--
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