[ARDF] tuning the Ron Graham RX-4 receiver

[Matthew Robbins]

Dale,

    Thanks for the very detailed response.  I've only read it quickly,
and I will definitely spend a lot more time on it, but what struck me
immediately was that the instructions in the manual are completely
different from what you recommend.  Yours makes a lot more sense. 
(Oh!)  I'll check out the RG website as well.  Again, thanks.  I knew
adjusting sense antennas was a trial-and-error proposition, but it
really didn't register until I got out there trying to do it.

    One thing I thought of was to set up several transmitters at
specific distances from my adjustment point.  I think the long walk is
still necessary to do a full check, but having several distances
without moving might help.  Next time out will still be 1 T, though. 
I'll definitely try your 1 wavelength (80m?) recommendation.  From
what you've written, it sounds like my approach of starting a long way
away was the wrong way to do it.  Starting closer should save a lot of
walking.

    Gyuri once attempted to explain to me (he explained, I attempted
to understand) why some sense designs are problematic.  If I
understood correctly, there are sense circuit designs which take care
of this.

Don't ask me to explain anything---
It went "fooosh" over my head.

(Which is a pretty cool 2-line haiku.)

Matthew




On Wed, 16 Feb 2005 19:38:45 -0800, Dale Hunt, WB6BYU <wb6byu at arrl.net> wrote:
> 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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