In literal definitions, Simplex means one way communication, Half Duplex
means two way communication but not simultaneously, and Full Duplex means
two way communication simultaneously. However, in radio, these terms are
typically used as follows:

Simplex: Transmit and Receive on the same frequency, not simultaneously.
Duplex: Transmit and Receive on split frequency pair simultaneously.
Half Duplex: Transmit and Receive on split frequency pair, not
simultaneously.
VOX: Voice Operated Transmit, has nothing to do with the mode of operation.

Most marine VHF channels are simplex (transmit and receive on the same
frequency). The Public Correspondence (marine operator) channels are duplex
(transmit and receive on different frequencies).

All FRS channels are simplex (transmit and receive on the same frequency).

For hands free operation, you want a radio with VOX. A decent radio with VOX
will have adjustments for VOX sensitivity and "hold" time.

Eric N3EF

"Peter" wrote in message
...
I thought there was either Simplex or Duplex.
At least that's how they described the hands free car kits a few years
back.