[ARC5] ZB discussion!

[Michael Bittner]

Jack, I think the last sentence in your email hits the nail right on the 
head.  Different channels for different ships or channel changing for 
security puposes.  Channel switching easy at BCB freqs.   Mike, W6MAB


----- Original Message ----- 
From: "Jack Antonio" <scr287 at sbcglobal.net>
To: "Discussion of AN/ARC-5 military radio equipment." 
<arc5 at mailman.qth.net>
Sent: Sunday, May 23, 2010 9:33 PM
Subject: Re: [ARC5] ZB discussion!


>
> I have always wondered about the rationale behind the double modulation
> scheme used in the ZB.
>
> Conventional lore has it that someone tuning across the carrier would
> simply hear an unmodulated carrier, and derive no information from it.
> (Other than the fact that there was a carrier at that frequency).
>
> However, if I take a 234 Mc carrier and modulate it at 1 Mc, there will
> now be three carriers(or a carrier and 2 sidebands, if you prefer)
> 233,234 and 235.  And a receiver will be able
> to pick any of them up, without the fancy double demodulation scheme.
>
> So I offer the following as a theory, I remember a discussion
> about the ZB double modulation scheme on one of the lists a few years
> ago, but don't remember if the following was brought up.
>
> The double modulation scheme simply eliminated the need for precise
> frequency, stability and calibration of the VHF carrier, and placed
> those requirements at MF, where it was
> much more easily acheived. It made it possible to easily
> tune in with confidence signals from two different ships, or to
> pick a specific frequency of the day for security purposes.
>
> Without the double modulation scheme, it would have been much
> more difficult to provide for multiple ships or security procedures
> and have a relatively simple system for the pilot to operate.
>
> Jack Antonio WA7DIA
>
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