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From: haynes@ucscc.UCSC.EDU (99700000)
Newsgroups: sci.electronics
Subject: Re: Re: economical to modify a scanner radio ?
Message-ID: <5431@saturn.ucsc.edu>
Date: 10 Nov 88 16:48:58 GMT
References: <4156@tekgvs.GVS.TEK.COM> <182@sunset.MATH.UCLA.EDU> <3014@vice.ICO.TEK.COM>
Sender: usenet@saturn.ucsc.edu
Reply-To: haynes@ucscc.UCSC.EDU (Jim Haynes)
Organization: California State Home for the Weird
Lines: 36

In article <3014@vice.ICO.TEK.COM> georgep@vice.ICO.TEK.COM (George Pell) writes:
>
>Actually, the reason aircraft radios are AM is that it is an international
>standard. All (commercial) aircraft (with radios) worldwide use the 118 to
>136 Mhz AM band as their primary communication channel (and by the way, all

And the reason for this is history.  At the beginning of World War II FM
was in its infancy.  England had developed AM VHF (108-152 MHz) for aviation
use.  The U.S. Army Air Corps needed to be able to operate out of England.
As there was little or no U.S. equipment for the purpose the British
air-to-ground radios were copied and built in this country.  After the
war I presume there was so much surplus radio equipment available cheap,
and the AM VHF technology had achieved so much momentum, that it wasn't
practical to change.  Later the military moved out of 108-152 to the
225-400 MHz band and for reasons unknown to me stayed with AM.  This
required that the big civilian airports and air traffic control facilities
also be equipped for "UHF" (225-400) since military planes use them.
The Air Force initially had a policy of shunning VHF (no new equipment
designed or purchased).  This was not totally wise, since occasionally
civilian planes make emergency landings at military bases and have to
talk to the tower, which is using 1937-model VHF radio equipment.

The ground forces, in contrast, had the luck to be introduced to a line
of FM equipment that had been designed for police cars.  They decided
this was a lot better than the AM HF gear they had planned to use,
and bought into the concept.

Hence the forces on the ground can't talk to airplanes.  Helicopters
have to carry FM radios to talk to the ground forces and AM radios
to talk to airports and air bases.
haynes@ucscc.ucsc.edu
haynes@ucscc.bitnet
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"Any clod can have the facts, but having opinions is an Art."
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