[Milsurplus] Aircraft comm/nav specs

[gl4d21a at juno.com]

Jim:

I think you missed an order of magnitude there.

Ken:

When, in the late 1930s, VHF aircraft communications were originally fielded, it was not practical to insure that transmitters were closer to nominal channel frequency than 35 kHz (~.025%) or so.  Crystal tolerances, circuit tolerances, vibration, etc. were all limitations. The technology was just not there to build stable frequency control within the size, weight, ruggedness and cost constraints which existed.  So, aircraft channelization was set at 0.1 MHz (100 kcps in the lexicon of the day).  That was reduced to 0.05 MHz (50 kHz) sometime in the late 50s or early '60s IIRC, where it stood for a while, due to private aircraft owner reluctance to spend big money for better radios.  With present technology (read synthesizers) it is possible to build transmitters which will hold SSB frequency accuracy (think hams on two meter SSB or 432 or 1296 SSB for that matter) at VHF, and aircraft standards are slowly moving out of the dark ages.  Commercial aircraft have had radios which could channelize at 7.5 kHz for years, which is about the width of an AM signal, but AOPA, well, they resist and they represent a lot of private pilots. 

HTH & 73
George
W5VPQ

---------- Original Message ----------
From: Jim Haynes <jhhaynes at earthlink.net>
To: Ken Kinderman <scr274 at gmail.com>, milsurplus at mailman.qth.net
Subject: Re: [Milsurplus] Aircraft comm/nav specs
Date: Tue, 24 Aug 2010 14:52:30 -0500 (GMT-05:00)

-----Original Message-----
>From: Ken Kinderman <scr274 at gmail.com>
>Sent: Aug 24, 2010 2:14 PM
>To: milsurplus at mailman.qth.net
>Subject: [Milsurplus] Aircraft comm/nav specs
>
>What happened on Jan 1, 1997 to affect aircraft comm specs? Anything to do
>with acceptability of plain old AM as in ARC-3, ARC-1 operation? Something
>tells me it was related to minimum channel capacity?
>
Probably that's when the channel spacing was shifted to 0.25 KHz from 0.50 KHz


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