[CQ-Contest] What RF voltage OK at Rx in SO2R?

[k8cc]

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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