|[AMRadio] RE: AMRadio Digest, Vol 49, Issue 35|
Clarke, Tom AIR4.0P NATOPS
frederic.clarke at navy.mil
Mon Feb 11 15:56:38 EST 2008
> I seem to remember that the aircraft radios we used during the 60-80s
> in P-3s/C-130s etc (ARC-94/102 aka 618T) had a selection called AME
> (AM equivalent). It was explained to us (pilots) by the techs that it
> wasn't a high level AM, but USB with carrier. It sounded OK, but then
> again, it was designed for communications and not "easy listening"!
> All the airways communication now days is SSB, so the AME position
> rarely gets selected.
> Perhaps someone is familiar with that mode and can expand on it.
> 73 Tom/W4OKW
>>Aircraft VHF radios still use full carrier AM. I have a small Radio
Shack portable with the aircraft band, and there is >>a lot of AM
traffic on it. I purchased it mainly to use as a tool for sniffing out
power line noise. The reason they use >>AM instead of FM is the capture
effect of FM. With AM, when a strong signal is on frequency, a weaker
one can still be
>>heard under the stronger one. It would be a catastrophe waiting to
happen if aircraft used FM radios and a stronger
>>signal completely overrode, particularly near a busy airport. Also, I
seem to recall something about SSB being less
>>than satisfactory due to the shift in frequency due to the Doppler
effect, with high speed jet aircraft.
It probably would have been helpful if I had prefaced my comments with
"HF Airways (long range over water) comms"!!
I suspect that the Doppler effect on HF is minimal or SSB wouldn't be
universal for HF air to ground today. Back in the day, 6567 KHz was a
nearly universal AM frequency shared by Miami, New York and San Juan
radio. I don't see any AM frequencies listed in the DOD In-flight
Handbook or the ARINC listings. The weather broadcasts like 10051.0
(New York, Gander, Shanwick) are sideband too.
I have looked inside several Collins VHF xcvrs (although not the newer
Pro-Line stuff) and sure enough they used high level Plate (collector!)
modulated AM. I imagine the newer ones use "computer generated" AM or
some such magic!.
We (pilots, military and commercial) were taught that FM was not used,
as you say, due to the capture effect. With AM when someone "doubles",
it is pretty obvious. Safety of flight issue.
73 de Tom/W4OKW
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