Simplex, Duplex, Offset and Split

Some time ago, a ham on twitter posted about a “2m SSB simplex contact” he had made. Another ham replied with “2m SSB is just that. The word simplex is not needed.” The second ham is incorrect…not all 2m SSB contacts are simplex. Most of them are but not all. More importantly, I think this exchange highlights some common confusion about terms such as simplex, duplex and repeater operation.


Simplex – In the amateur radio context, simplex operation means the radio stations are communicating with each other directly, on the same frequency. Both stations take turns transmitting and receiving on the same frequency with no repeater or other device in between.

Duplex – Duplex operation means that a radio station transmits on one frequency and receives on a different frequency.

Full Duplex – Operating duplex with the ability to transmit and receive simultaneously.

Half Duplex – Operating duplex but having to switch between transmit and receive
[simplex, duplex]

Examples of simplex vs. duplex operations. The repeater station (lower image, on mountain) is full duplex, instantly retransmitting a received signal. Each user’s station is half duplex, unable to transmit and receive simultaneously, but shifting between the transmit and receive frequencies.

Very often simplex and duplex operation are associated with FM on the VHF and UHF bands. If you are talking to another ham directly, on one frequency, with no repeater involved, that is simplex operation.

FM repeater operation uses two frequencies: the repeater receive frequency and the repeater transmit frequency. The repeater’s job is to take the signal it hears on its receiver and retransmit it on the transmit frequency. See this article for more information on how repeaters work.  Repeaters operate in full duplex mode, because they receive and transmit at the same time. The repeater user is usually operating in half duplex, using two frequencies but switching between transmit and receive. Some FM ham radio gear can operate in full duplex mode (usually employing two different ham bands) but most equipment is half duplex only.

We refer to a repeater by its transmit frequency, which is the frequency the user listens on.  When the user transmits, the radio automatically changes frequency as required by the repeater’s offset (the difference between its transmit and receive frequencies.) The repeater offset is sometimes referred to as the repeater split.

 73, Bob K0NR

@ copyright 2018 HCARA

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The rule for offsets  for 2 meters up to147MHz: transmit 600 KHz below the receive frequency.  Above 147 MHz, the offset is 600 KHz above the receive frequency. 

On 70 cm, the offset below 445 is plus 5 MHz and  minus 5 MHz above 445 MHz. 

Questions regarding the above information are included on the Technician License exam, so new amateurs should be familiar with this info.

by Robert Burns W1LBV