[ARDF] AM A2A Modulation...

[Dale Hunt, WB6BYU]

   Matthew -

      Applying a single tone (assuming it is a clean sine wave) to
   the mic of an SSB transmitter actually produces CW (A1) modulation,
   not A2.  But if the PicCon puts out a square wave (as most
   microprocessors do) then the fundamental tone probably acted as
   a carrier modulated by the harmonics, so it probabl would be
   audible on an AM receiver.

     The most common method of creating A2A is to amplitude modulate
   the power lead to the final amplifier of a CW transmitter.
   If you put a switching transistor in this lead and set it for
   half the normal collector voltage, then apply audio to the base
   of the switching transistor (in addition to the normal bias)
   it will do the job.  You can also then key the DC bias to the
   same transistor to shut off the power between characters.
   (Although you may have to control two stages to reduce the 
   backwave during off periods.)

       One problem with this approach is that the modulation percentage
   will change with the load on the amplifier stage.  Several
   Europeans I have talked to said that they simply keyed such a
   stage with a square audio wave: the switching transistor is off
   except during the peak of the audio waveform, when it is switched
   full on.  So it really is a high-speed CW transmitter being keyed
   at an audio rate, but I don't expect this mode to be practical
   on an unmodified commercial rig.

       But if your multi-mode 2m rig has an AM mode, then feeding
   an audio tone into the mic and keying the PTT in AM mode would
   be the right approach - presuming the PTT circuit responds fast
   enough, which you will have to find my experiment.  You may be
   able to change the weighting of the CW (the dot/dash ratio)
   to correct for the slow PTT of the rig.


      The clarification that keyed or continuous carrier A2 is
   permitted was added with the recent rewrite.  Keyed carrier
   has been tradition because it saves on battery power.

     - Dale WB6BYU



Matthew Robbins wrote:
> 
> The ARDF Region 1 Rules for 2m require:  Mode A2A (keyed carrier
> modulated by AF tone *or* continuous carrier modulated by keyed AF
> tone). {emphasis added}
> 
> The second way seems to be similar to an AM radio station (RF out all
> the time?).  It should be similar to the FM MCW signal we Americans
> tend to use for ARDF, at least in the sense of it being on the whole
> minute.
> 
> Saturday (October 8th), I was the fox for a local mobile hunt.
> Because 3 of the hunters are planning to go to Raleigh next April, I
> proposed attempting to use A2A modulation (the first type listed in
> the rules), even though I had no idea how to do it with my radio.
> 
> My first thought was to apply an audio tone to the microphone input
> and key the PTT.  My first thought is often wrong, so I ran it by some
> serious radio guys I know, and they recommended using SSB.  That way,
> I could key the PTT, and key an audio tone into the mic input.
> Between the keyed bits of audio, the RF out drops to zero
> (supposedly).  Fortunately, a PicCon can do what is needed.
> 
> Long story short is---The hunt went well.  Everyone agreed it was much
> more difficult than FM.  The ARDFers who have hunted A2A thought it
> was representative for ARDF, at least in the difficulty of taking
> bearings..
> 
> I don't know how bespoke ARDF 2m transmitters achieve A2A, and I don't
> know if an SSB circuit makes any sense for ARDF, but I do know it
> works with a SSB 2m rig, and it's pretty simple if you can figure out
> how to attach a PicCon.
> 
> (Another thing---I'm pretty sure no one used an FM radio for the hunt,
> so I/we have no idea how difficult it is to hunt using one.)
> 
> Matthew
> AA9YH
> Cincinnati, Ohio
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