[ARC5] ZB discussion!

[Jack Antonio]

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