Max Power wrote:
AM radio multiplexing question: transmitting SSB in sync with AM (numbers
stations, clandestine broadcasting)

I understand that it is possible to multiplex SSB transmissions on top of AM
transmissions, especially on SW.
The AM transmission is decoded by the envelope or PLL coherer, the SSB
transmission is ignored.

It's not multiplexing, really, it's sort of a phantom signal. The two
sidebands both broadcast the AM signal. The difference between the
two sidebands gives you the phantom. You can probably hack up any of
the modern dual-sideband-with-reimpressed-carrier "fake AM" transmitters
to do this. The bandwidth of the phantom channel is poor, but fine enough
for CW or RTTY.

On a properly tuned SSB receiver, the AM transmission only partially
interferes with the SSB transmission.

Depends on the sort of detector the receiver has.

Some numbers stations use this transmission technique, but it just as valid
if say Radio Free Asia were to use this technique by transmitting over
Chinese domestic radio.

I am not sure why numbers stations would do this. Numbers stations _want_
folks to be listening in. One of the big things about numbers stations
is they can be used to make it look like your country has a huge espionage
network, when they actually just have a guy in a closet with a KWM-2.

1. What is the math behind this transmission working (SSB is a form of Angle
Modulation, not Amplitude Modulation)?

Check out the Kahn AM stereo system references. The system is basically
the same, with the L-R channel being carried as the phantom signal on the
Kahn system.

2. Are some receivers more affected by (interference where) SSB
transmissions superimposed over AM than others? (PLL versus SYNC vs Envelope
Detectors)

This is very well addressed in the Kahn AM stereo docs. The answer is yes.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."