Figure #1

In the simplest of radio systems, called a "simplex" system, each unit communicates directly with other units.

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.

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Figure #2

A simplex radio system works fine for large,open flat areas.
In cities,suburban areas, and mountainous regions however buildings and hills block radio signals thereby reducing the radio systems' effective range.

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Figure #3                                                 

The first solution to the problem of blocked signals, is to place an antenna on the highest point around, a hill, a tall building, or a radio tower.
The radio at the tower is called a "Base Station" and it is connected to the dispatcher's desk "Control Point" via wires called the "Control Link".

All units, including the base station, transmit and receive on the same frequency.
If two units can't communicate directly, they tell the dispatcher, who then relays the message.
Base Station

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Figure #4

Hemi-duplex systems use two different frequencies.
The Base station transmits on one frequency and the mobiles transmit back on another frequency. The mobile input frequency is NOT rebroadcast.

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.        
Hemi-Duplex Wire Line Control

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Figure #5

Every time a message is relayed, it tends to get changed or garbled. A way to reduce garbling is to allow two units to communicate directly. One method of establishing direct communications for mobile units that are out of "direct range" is the SEMI-DUPLEX REPEATER.

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 mobile unit.                     

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Figure #6

Demi-duplex systems are four frequency systems. They are regular Semi-duplex repeaters whose "control links" (F3 and F4) are two radio frequencies that are different from the mobile input and output channels (F1 and F2).
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Figure #                                                    


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.

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Figure #8

A radio system in a large city may have one or more machines that are Hemi-Demi-Semi-Duplex.
What this means is that the repeater is a semi-duplex machine that can be put into either Hemi-duplex or Demi-duplex mode usually a repeater like this has multiple control links.
In fact the machine may have several different wireline and radio control links and control points.

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.                                 

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Figure #9

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 on (F3).
Every message that is transmitted on (F1) is immediately retransmitted on (F4).

A Full Duplex system is a true TWO way radio systems.

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This web-page was constructed by Dan Yemiola-AI8O.
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