Path: utzoo!attcan!uunet!lll-winken!lll-tis!helios.ee.lbl.gov!pasteur!ucbvax!bloom-beacon!gatech!mcnc!unccvax!dya
From: dya@unccvax.UUCP (York David Anthony @ WKTD, Wilmington, NC)
Newsgroups: sci.electronics
Subject: Re: economical to modify a scanner radio ?
Message-ID: <1200@unccvax.UUCP>
Date: 8 Nov 88 13:52:38 GMT
References: <4156@tekgvs.GVS.TEK.COM> <182@sunset.MATH.UCLA.EDU>
Organization: Univ. of NC at Charlotte, Charlotte, NC
Lines: 39

In article <182@sunset.MATH.UCLA.EDU>, hgw@julia.math.ucla.edu (Harold Wong) writes:
> In article <4156@tekgvs.GVS.TEK.COM> jans@tekgvs.GVS.TEK.COM (Jan Steinman) writes:
> ><...the public safety services all seem to use FM (even Highway Patrol at 42 
> >MHz) but aircraft tend to use AM. (would anyone care to explain why?)...>
> >
> >Capture effect.  You don't want to lose your contact with a 747 on final 
> >because someone nearby turned on an FM broadcast radio that happend to be tuned 
> >10.7 MHz higher!
> 
> Wouldn't noise and losing contact be more of a problem?  Just think of your
> AM radio in the car.  

	Naaahhh. The AM radio in your car suffers from a variety of man-made,
(both desired and undesired) and natural interference.  AM radios aboard aircraft
are fairly immune from interference; all but the strongest fields seem to give
it grief.

	Back in the darker ages, AM was a piffle to implement, while direct-FM
and indirect-FM exciters were a pain in the derriere to build.  Basically, to
do an aircraft radio, you desire a very stable carrier generator (so that 
channelized switching will work instead of having a radio operator aboard).
Every car radio has a suitable synthesizer on a chip now; but the FM transmitters
from even the 1950's used direct crystal excitation and multiplication up to
the 108-136 mc band.  6-10 vacuum tubes and their attendant tuned circuits are
just too much of a drag. (Note that old aircraft radios use multiplication as
well, but had overtone crystal oscillators...Generally, the higher the deviation,
the more multipliers required. Broadcast transmitters generally start in the 115 kc
range...oh well, another story)

	Plate modulated AM transmitters are trusty, then as now.  With just a few
watts at 108-136 mc, multipath which would not affect AM all that much (changing
the character of the speech being transmitted) can obliterate an FM transmission.
FM that is worth a damn occupies a wider channel bandwidth than AM; for that matter,
the C/N's required for adequate narrowband FM and AM service are about the same.

	Besides, AM in and of itself isn't so bad.  The current US implementation
here on the East Coast of AM broadcast is horrbile, though...

York David Anthony
WKTD Wilmington, NC (uh, 1120 kc on your AM dial)