[ARDF] tuning the Ron Graham RX-4 receiver

[Dale Hunt, WB6BYU]

Matthew Robbins wrote:
> 
...
>     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.


   Although the basic idea is pretty straight forward, finding a good
   implementation can be problematic.  The "coil + resistor" method
   used in the RX-4 is trickier to adjust, and requires that the
   signal from the sense antenna be stronger than that from the loop
   (since only that from the sense antenna is attenuated.)  For the
   receiver kits I used an active sense amplifier coupled to the
   main loop:  the inductive coupling provided the 90 degree phase
   shift, so only the amplitude requires adjustment.  (Obviously much
   easier, since it is simply a matter of turning one control for the
   best null.)  But the amplifier is subject to overload on strong
   signals.  (An FET probably would have been better.)  The amplifier
   also allows the use of a very short sense antenna.

   With some tinkering, you may be able to adjust the gain of the
   sense antenna as you turn down the RF gain on the set, which
   would maintain pattern as you got closer to the transmitter
   (though it would be difficult to get it to track exactly, since
   the field effects are independent of transmitter power, but
   you could make a usable improvement.)

   On the other hand, if your set had a phase detector, you could
   simply compare the phase of the loop signal with that of the
   sense antenna and tell which way to turn (a LEFT/RIGHT readout
   like a TDOA.)  That is how the old aircraft ADF units worked:
   the relative phases determined which way the motor turned the
   antenna, until it reached a null.  I have a prototype for this
   and the general principle seems to work, but it requires more
   circuitry than a simple receiver.

     There, now I've probably confused you even more!

      - Dale WB6BYU
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