[ARDF] AM A2A Modulation...

[Jay Hennigan]

On Mon, 10 Oct 2005, Matthew Robbins wrote:

>     My comment about a non-PicCon Pic was basically asking the
> question, "If you are making an ARDF 2m transmitter (from whole
> cloth), first, how do you achieve A2A modulation, and second, does the
> easy availablility of a programmable audio source for a DTMF-like tone
> change any assumptions about ARDF transmitter design---Since you need
> a controller anyway, and you're likely to use a Pic, would an SSB
> radio design have advantages over existing 2m ARDF transmitters.  In
> the whole e-mail, I guess I was mixing together two thoughts---First
> about a quick way to practice using a multimode rig, and second about
> designing ARDF 2m transmitters.

An SSB transmitter and the associated audio oscillators to cause it to
produce something like a sinewave-modulated AM signal is far more complex
than building a straight AM transmitter from whole cloth.

If you already have an SSB transmitter that doesn't do AM natively,
then a dual-tone audio oscillator will give you a reasonable-sounding
signal to hunt with an AM receiver.

>     How do you look at a 2m transmitter output?  Are there
> oscilloscopes that will go that fast, or do you need something like an
> LO and a mixer?  An HT I was using right next to the antenna (with at
> least 35W out) had little squelch tails after the dits and dahs,
> although it couldn't keep up with the dits.  I figured that was
> showing very low residual signal during the audio null, but I really
> have no idea what the signal was doing in those nulls.

There are scopes that go that fast.  You can use a spectrum analyzer,
or a simple diode detector RF probe and a slow scope to look at the AM
portion of the modulation.

>     It certainly sounds like the existing ways of achieving A2A (as
> described by Dale) are simpler than an SSB design transmitter.

Yep.

>     My interest in A2A is primarily because I've hunted it, and it's a
> lot harder than FM MCW.  I want to practice before I have to hunt it
> in a big competition.

I suspect that the difference in modulation isn't the cause of the
difficulty, but the intermittent nature of the signal where there
is no carrier between the Morse elements.  I think that the PicCon
can be set to drive PTT in sync with the audio to accomplish a keyed
FM carrier which should have similar characteristics in terms of
hunting it if you're using a RSSI-type receiver designed for FM with
an audio S-meter.  If you're using a regular FM H-T with an offset
attenuator, you should still be able to hunt AM signals on an FM rig
by listening for changes in the noise much like hunting FM signals.

As you attenuate the signal into the noise and it drops out of limiting,
the AM modulation will be quite audible in an FM receiver.  Depending
on the quality of the IF, limiter, and detector, it may be audible even
at full-quieting.

In summary,

AM transmitters are easier to build than SSB transmitters and dual-tone
audio oscillators.

The difficulty in hunting is in my opinion more due to the intermittent
keyed carrier than the modulation type.

An FM receiver with an audio S-meter ("whoopie") mode or an attenuator
capable of dropping the signal down into the noise will demodulate AM
signals without a problem.  Not exactly hi-fi for speech but more than
adequate to copy the tone and determine easily which fox is transmitting.

--
Jay Hennigan - CCIE #7880 - Network Administration - jay at west.net
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