[ARDF] tuning the Ron Graham RX-4 receiver

[Dale Hunt, WB6BYU]

Matthew Robbins wrote:
> 
>...
> First, I was adjusting the sense antenna on a Ron Graham receiver that
> had absolutely no fore-aft directivity...



  Matthew -

     I've managed to do a fairly good job of getting my Ron Graham
  80m sniffer sense antenna properly adjusted.  It isn't easy, and
  I was the one who suggested that Ron drill the holes in the top
  of the case.  If you take the cover off and pull out the controls
  and connectors, the plastic should pivot on the battery cable
  gromit until it is out of the way enough.  Then put the connectors
  and controls back in and reassemble the radio.  Looks funny, but
  the plastic cover does help to keep rain and debris out of the
  receiver.

     There are two adjustments for the sense antenna: a pot that
  sets the level, and a coil that sets the phase shift.  You are
  trying to make the output from the sense antenna be equal to, and
  90 degrees out of phase from, the output from the loop antenna.
  Unfortunatly, you can't use any external instrumentation for this,
  since anything near the case or the whip will affect the level
  of the signals picked up.  So the only way I have found to do it
  is with a transmitted signal.  (But a continuous carrier is easier
  than CW.)  This is why he recommends a metal back on the case:
  since the whip extends through the interior, both the signal pickup
  and the capacitance to ground of the whip part in the case will be
  different if there is not a cover on the box.  And these affect
  the settings.

     Now, there are some quirks to watch for.  First, the signal level
  picked up on the whip is a function of the whip length and the
  height above ground at which the receiver is held.  With a
  telescoping whip it has to remain the same length at all times.
  (I use a flexible wire hanging down from the set instead, since
  I usually break telescoping whip antennas rather quickly.)  And
  you have to tune it up while holding it at the height that you
  will normally use it for competition.  So sitting at a picnic table,
  as inviting as it may sound, probably is not the right approach.

     The other thing that you seem to have encountered is that, while
  the ratio between the electric field (sense antenna) and the magnetic
  field (loop) is constant in the far field of the antenna, they are
  NOT when you get in the near field, closer to the antenna.  The whip
  will pick up more signal relative to the loop, so you no longer have
  a null in sense mode where they exactly cancel each other.  If the
  signal from the loop is 5mV and the sense is 15mV, then there will
  only be a 6dB difference in strength between the "null" and "peak"
  when the sense antenna is enabled.  Any closer and the difference
  is smaller, to the point where it is impossible in practice to hear
  any difference.

      The advice I was given is to tune up the sense antenna about
  1 wavelength from the transmitter, which seems to give the best
  balance between near and far field settings.  Then you might consider
  replacing the sense switch with an ON-OFF-ON toggle and use the
  extra position to add some further resistance in series with the
  sense antenna.  With some experimentation you probably could
  adjust this for a good null at, say, 30 to 50m from the antenna.
  Otherwise all you can do is to use the loop without the sense and
  keep track of signal strength to make sure you aren't running the
  wrong direction.  (At least if you are 20m away, you should be
  able to tell this before you run too far.)


     Now, for the adjustment procedure (which I think Ron has on his
  web site under "user feedback".)  First, make sure the loop is
  properly resonated: you will have to readjust the sense any time
  you retune the loop capacitor.  (Yes, I discovered that the hard way
  in Hungary.)  Then, holding the receiver at about the height you
  would in normal operation, turn the null of the loop to the transmitter,
  turn on the sense, and adjust both the sense coil and resitor for
  maximum signal strength.  (The resistor will be at one end of travel.)
  This tunes the sense antenna to resonance using the coil, and gets you
  close to the proper setting.

     Turn off the sense antenna and turn the receiver for maximum signal.
  Make a mental note of how loud this signal is.  Try to do this without
  turning down the RF gain if possible, as that effects the settings
  somewhat.  But it won't be a major problem.  Now, remembering how loud
  this signal sounded, turn the end (null) of the loop to the transmitter,
  turn ON the sense, and adjust the resistor to get the same signal
  strength.  (I think I've tried to do this setting using a voltmeter
  on the audio output, but for some reason I don't think it worked very
  well.)

     Now the sense should be close to resonance, and about the same
  strength as the signal from the loop.  Now all you need to do is to
  tune the coil slightly off of resonance to get the desired phase
  shift, and fine-tune the signal levels.  Aim the cardiod null side
  of the receiver at the transmitter and gently tweak the coil to one
  direction or the other, looking for a drop in signal strength.  If
  you go to far the signal will drop because you are no longer resonant,
  so don't move it too much.  Alternately adjust the resistor for 
  minimum and go back and forth until there is no more improvement.

    Now you can go back to the 700m mark and walk towards the transmitter
  checking the null.  The receiver should have a distinct F/B ratio up to
  pretty close to the transmitter, but you will hear as the sense signal
  pickup changes and the null disappears.  This doesn't mean that the
  pattern has a perfect null, but the difference between the two directions
  should be distinct.


     Granted, the Ron Graham RX-4 does have a few issues.  Mine is
  rather deaf.  I haven't decided if this is due to the noise level
  of the MC3340, or the close shielding or choice of ferrite mix on
  the loop antenna.  However, given how much the signal from a 6" sense
  antenna needs to be attenuated to match the signal out of the loop,
  and the fact that my first attempt at building an air core loop
  required a sense antenna 4 FEET long to equal the loop output,
  I suspect that the loop could use some improvement.  (That was
  why I used an amplified sense antenna in my receiver kits, as
  otherwise it required an inconveniently long whip to pick up
  enough signal.)  Perhaps one of these days I'll acquire the
  proverbial "Round Tuit" and try out some improvements...
  However, it does work, and I could hear all the transmitters
  on the course in Hungary with it, and I didn't finish in last
  place.

     If anyone wants to try some improvements, let me know and
  I'll give you some ideas.

    Good luck

         - Dale WB6BYU



Matthew Robbins wrote:
> 
> I spent a couple of hours in Mt. Airy Park today.  I put up a single
> 80m transmitter with a 4 minute on --- 1 minute off cycle.  (I choose
> the 4-1 for no good reason.)  I put it up about 700m more-or-less down
> a road, since the ground was a little mushy.
> 
> I was testing two things.
> 
> First, I was adjusting the sense antenna on a Ron Graham receiver that
> had absolutely no fore-aft directivity.  I sat down at a picnic table
> in the sun, since it was just a beautiful day out, and followed the
> directions.  I was able to get a really nice sense adjustment made at
> least for 700m.  Then the acid test:  I walked 700m to the transmitter
> checking the receiver every couple steps.  Around 300m from the T, I
> couldn't detect a difference between front and back.  It went on for
> about 100m, so I decided to stop and adjust it there, and see what
> happened.  So what happened?  Mostly disappointment.
> 
> I didn't follow the directions exactly.  They say to place a metal
> shield over the part of the sense antenna inside the case when the
> metal case cover is off.  I didn't take anything with me, so I just
> did without.  I noticed holes that are drilled in the cover for this
> adjustment, but they're covered by a label and a piece of clear
> plastic.  I've decided to open up the holes because the adjustment
> (which I plan to continue experimenting with) seems like it's going to
> take a while, and require lots of trial and error.
> 
> I was hoping someone knows or could explain some way to do this
> adjustment with measuring equipment.  Are there measureable quantities
> here?  I don't need "the answer" as far as what number to set the
> quantities to, because I don't mind making up a matrix of values and
> testing them---Although I'll have to think about ways to mitigate the
> 700m walk to check it at different distances.
>
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