Guard Channels (was [Milsurplus] RE: Nutso (RE: WW2 B-29 VHF radio?)

[Mike]

jhaynes@alumni.uark.edu wrote:
> 
> >
> > I think that the standard "emergency" guard channel frequency of WWII
> > was 140.58 (I erroneously said "142.58" in an earlier post.)  It appears
> > Channel      Frequency
> >   A           116.10
> >   B           126.18
> >   C           140.58
> >   D           142.74
> 
> This reminds me of an old question - those peculiar numbers on the ends of
> the frequencies.  Postwar the aviation band was channelized in 0.5 MHz
> increments, then split to 0.25, currently split to 0.125.  What was the
> scheme used in WWII that results in the numbers given above?

The transmitter frequency was derived from multiplier stages from a
lower frequency crystal.  Most often the crystal frequency was
multiplied by 18.  The transmitter crystals themselves were usually
supplied in the 7 mcs range at 10 kc intervals.  Thus a transmitter
crystal frequency of 7010 kcs resulted in an operating frequency of
126.18 mcs.  The next available crystal frequency of 7020 kcs would
produce 126.36, etc.

So the odd frequencies resulted from supplying crytals in 10 kc
increments for transmitters in which the crystal was multiplied 18
times, resulting in a then military standard 180 kc channel spacing.

Mike / KK5F