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[aprssig] How About 40M APRS

Dave Baxter Dave at emv.co.uk
Tue Jun 3 08:11:08 UTC 2008

> The indicated "dial frequency" on SSB is the suppressed carrier
> The suppressed carrier frequency is NOT transmitted. 

Point of technicality....   Your "Suppressed" carrier IS still
transmitted!   Maybe (hopefully!!) many dB down on the wanted sideband
signals, but it is still there.  If you have some 40dB suppression, &
many "Amateur" rigs when more than a few years old are not even that
good!  Just tune around 40m or 80m on a Sunday and listen..

If you have 100W PEP (+50dBm) sideband tones (one tone at a time for
AFSK of course) then your so called "Suppressed" carrier is still 10mW
(+10dBm).  And as any QRP operator will tell you, that is often enough
to get you into trouble, if it's in the wrong place.   OK, not exactly
good operating practices, but you get the idea.....

Hopefully most modern radios are much better than that, but I have to
say I've not made that measurement on my radios of late, and yes I have
access to the kit to do that.

Then there is also the unwanted "Other" sideband, usually well
suppressed too, but.....   Plus any in (audio) band IP3 (in the case of
multiple simultaneous tones) plus other modulation artefacts and
unwanted products etc....   With some multiple conversion rigs, the
situation can be quite interesting at times.

Please don't fall into the oft quoted trap, that all you get out of a
SSB TX, if you feed it a pair of tones, is "only" the (suppressed)
carrier + or - those tones. 

Take care...   Especially with the choice of carrier frequency, as
indeed you do need knowledge of the actual audio tones fed to the radio,
as per WA8LMF's mail, to determine the actual RF signal frequencies.

As for 30m APRS being in wide use in the EU/UK...  Yes, I know of a
French 30m I-gate that can easily be hit from here, but I wouldn't say
activity is widespread from the UK on 30m.


Dave G0WBX.
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