RadioBanter - AM Or FM For These Freq's ?

On Thu, 19 Aug 2004 07:47:44 -0400, "Robert11"
wrote:

Can anyone perhaps verify that for me, and irrespective of whether they
are, or not, please tell me whether they are AM or FM:

123.100 - U.S. Coast Guard - VHF S.A.R. Primary
122.900 - U.S. Coast Guard - VHF S.A.R. Secondary
381.800- U.S. Coast Guard - UHF S.A.R.
282.800- U.S. Coast Guard - UHF S.A.R.
126.200- U.S. Coast Guard - Ground-to-Air Operations

All of the above would be AM.

Are "all" aero related freq's over 30 MHz always AM ?

Aero related, not necessarily. Where aircraft are actually
communicating by radio, though, this is usually the case.

i say this because there are freqs above 30 MHz where the comms are
aero related (ground support functions at airports for example) but
are not AM, they're usually NFM. The FAA's freqs in the government
bands are another example - certainly aero related, but not AM.

The reason aircraft radios are in AM is that back in the early days AM
was the standard, and by the time FM became a standard for two-way
comms, there were just so many aircraft out there with AM radios that
replacing them all would have been a major nightmare, for financial
and other reasons...especially for commercial airline companies that
would have hundreds of radios to replace.

One benefit of using AM is that most AM receivers are susceptible to
receiving harmonics of the frequency they are tuned to. Thus, at an
air traffic control tower, a receiver tuned to the civilian aircraft
emergency frequency of 121.5 will pick up transmissions from nearby
military aircraft transmitting on the military air band emergency
frequency of 243.0 (often referred to as "guard") as well.

I've noticed a few posts from you along similar lines, so as a general
guideline:

Aircraft transmissions in the civilian air band (108-136 MHz) and the
military air band (226-400 MHz) are generally going to be AM. You will
hear other signals in these ranges...there are navigation systems
operating in these bands, and in the milair band there are military
satellites that use wide band FM and digital modes and/or are
encrypted. Air Force One often uses wide band FM here as well, but
when talking to civilian towers on VHF they will be in AM, which is
the standard for that.

You'll also find AM outside those ranges, in use by military aircraft.
If you're scanning in the 148-151 MHz range, for example, you may well
hear military comms in AM mode there...assuming you have a receiver
that allows the user to select the AM mode here (many commercial
scanners default to FM here and if the user cannot override the
default choice of FM, he/she experiences buyer remorse if he/she is a
milair buff).

FWIW, below 30 MHz most aero comms are on upper sideband (USB).
However, that's a discussion for rec.radio.shortwave. :-)

Hope this helps.

John Kasupskim Tonawanda, New York
Amateur Radio (KC2HMZ), SWL/Scanner Monitoring (KNY2VS)
Member of ARES/RACES, ARATS, WUN, ARRL
http://www.qsl.net/kc2fng
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