What do you mean by Simplex, Duplex, Half Duplex, Full Duplex?

 by Mark Behrends

Two-way radios have been around a long time.  There’s a reason for that.  Radios have and continue to provide a useful function for many.  Hobbyists use them for fun.  Boaters use them for safety.  Businesses use them to streamline crew communications.  And radios are a vital piece of the public safety personnel’s communication system.

Before you can talk on a radio, you have to talk about radios to select the right radio for your application.  Yet, some terms may be unfamiliar or confusing to you.  Don’t let terminology get in the way.

There are several ways you can operate two-way radios. Here are 4 terms you may hear when ‘talking radio’ with someone.

  • - Simplex
  • - Duplex
  • - Half Duplex
  • - Full Duplex

But first, remember that two-way radios, or walkie talkies, communicate over a specific frequency, or pair of frequencies, per channel. In order for a radio to talk to another it must use the same frequency(s) in the same mode. The terms, simplex, duplex, half duplex and full duplex, address how radios use the frequency assigned.

F1100D

F1100D

Simplex – is the simplest mode of operation, hence the name.  It is also called talk-around. In the two-way radio world, simplex or talk-around is the mode of operation where the radio has only one frequency (per channel) assigned it, and the radios talk directly to one another.  No other device, such as a repeater, is between them.  A simplex radio takes turns transmitting and receiving over that one frequency.  Unfortunately, simplex shrinks the usable distance between radios.  In the radio world, the term simplex is used differently than in other communication applications*.

Duplex – in this mode, the radio has two frequencies per channel, often called a frequency pair.  One frequency transmits while the other receives.  Duplex isn’t a complete description of a mode in which radios operate.  It is a concept used to help explain Half Duplex and Full Duplex.

Half Duplex – operates as duplex with two frequencies, but in this mode the radio must switch between transmitting on one frequency and receiving on another.  It can only do one at a time.  Radios in the field, such as a radio mounted in a vehicle or a handheld radio, operate in half duplex mode when used with a radio repeater.

Full Duplex – as you might’ve guessed by now, full duplex uses two frequencies – one for transmit and one for receive – simultaneously.  Repeaters are full duplex.  A repeater receives the field radios’ transmitted frequency and transmits on the field radios’ receive frequency at the same time.  This is a bit tricky to accomplish, especially with higher powered transmitters, because the transmitter frequency must be sufficiently isolated from the low power receiver circuitry.  That is why repeaters are more expensive than field mobile or handheld radios.

FR5300

FR5300

So now, when you go to a radio person about needing radios to help your fleet logistics, you can talk to them with confidence about simplex, duplex, half duplex and full duplex modes.

*Outside the radio world, simplex is one direction only, like a one-way street.  Talk-around radios are more accurately half duplex, like a one lane road.  Traffic goes both ways, but one at a time.