All units use the SAME frequency to transmit and receive.
Because all the units in a simplex system use the same frequency or
channel, the communications are one way at a time.
All units, including the base station, transmit and receive on the same
If two units can't communicate directly, they tell the dispatcher, who then relays the message.
Hemi-duplex systems are often used by cab companies. When a request
for a taxi is phoned in, the dispatcher asks all the available taxi drivers
where they are, and gives the the cab that is closest to the phoned in
pick-up point the trip. The mobile input frequency is not repeated so that
the taxi drivers cannot hear each others location, and then lie about their
location to get the trip.
Semi-duplex repeaters are the most common form of radio repeater systems.
A repeater is a radio receiver/transmitter combination The repeater is often called "the machine" to differentiate it from the other units in the system. They are installed on a hill, a tall building, or a radio tower. The machine automatically RETRANSMITS the signal it received on one frequency (F1) on another frequency (F2).
The "Control Point" at the dispatcher's desk transmits and receives
just like a mobile radio. As far as the machine is concerned, it is just another
Police agencies often need to transmit sensitive or confidential information from the mobile units to the dispatcher without other people hearing the message. Often they reconfigure their demi duplex repeaters systems into hemi-demi duplex systems. When a mobile has confidential information to send, The dispatcher sends coded control signals over control link (F4) that turn off the machines automatic retransmission on (F1)of the mobile input channel (F2). The Control link frequencies (F3 and F4) are then used in the same way as the wire line control links of a Hemi-duplex system.
You can tell if a machine is Hemi-Demi-Duplex, if at times you hear a mobile unit saying: "turn off the repeater", and you still hear the dispatcher talking, but not the mobiles. What they are really doing is turning off the connection between the mobile (F1) receiver and (F2) while allowing the (F3)and (F4) links to continue operating.
For example a large city may have a main Control Point at police HQ, and back-up
Control Points at city hall, and the city's emergency management center.
FULL duplex repeater systems are the easiest to understand.
Cell phone systems operate in this mode.
Every message that is transmitted on (F2) is immediately retransmitted
Every message that is transmitted on (F1) is immediately retransmitted on (F4).
A Full Duplex system is a true TWO way radio systems.