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

Marijan Miletic, S56A artinian at siol.net
Wed Sep 26 10:42:48 EDT 2001

G4BUO circuit with 4 diodes would limit at 3 V RF peaks.  This equals about
50 mW of power distributed over the wide frequency range.

In order to reach back to the antenna, RF should go thru bulb which has
certain inductance and makes a nice lowpass filter.
However, additional bandpass filters are strongly recommended.

Even the best medicine has some unwanted side effects!

73 de Mario, S56A, N1YU

P.S.  Relays can be dumped with a large capacitor which also kills RF!
  ----- Original Message -----
  From: k8cc
  To: Dave Lawley ; Reflector
  Sent: Wednesday, September 26, 2001 3:15 AM
  Subject: Re: [CQ-Contest] What RF voltage OK at Rx in SO2R?

  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
  I would think that every time you transmit you'd wreck every television,
  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
  shack to the relay picked up enough RF from a 10 KW BC station *three
  away* on 1420 KHz to generate a *tenth* harmonic on 20 SSB.  It also
  the only two TVI complaints I've ever had from this location in fifteen

  Don't use diodes to protect front ends unless you're also going to have
  filtering to stop the spurious signals generated.


  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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