Back To Basics: Simplex, Duplex, Offset and Split by 73, Bob K0NR

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.

Terminology

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

Repeaters

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.  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.

HF Operating

The vast majority of HF operation is simplex operation. We dial up a particular frequency and chat back and forth on that same frequency. However, duplex operating is also used on HF, typically referred to as working split. A DX station may have a large number of stations calling him, creating a “pile up” on frequency. His ability to make contacts slows dramatically as this huge pile of stations calling him creates interference on his frequency. The DX station can’t hear the particular station he’s trying to work and the station he is trying to work also has trouble hearing the DX station.  A common practice is for the DX station to listen on a different frequency, typically a few kHz up from his transmit frequency. The DX operator will say something like “listening up 10” to indicate he is listening 10 kHz higher than his transmit frequency. Or he may just say his receive frequency explicitly (“listening 14.180”).  The idea is that the DX transmit frequency will always remain clear so everyone can hear him. Everyone hears the DX pull a callsign out of the pile, when the contact is complete and when he’s ready for the next call. Things get easier for the DX station as the calling stations tend to spread out and he can tune around a bit to find a particular station he wants to contact. Making this happen is a bit tricky and requires the use of two VFOs . Most modern transceivers have split mode capability with dual VFOs.  Most modern transceivers have this capability. The calling stations set one VFO to the DX station’s transmit frequency and adjust the other VFO to be on the receive frequency specified by the DX station (“up 10”). The transceiver is set to listen on the first VFO and transmit on the second VFO. This is usually called split operation in the transceiver manual. If your radio does not have split operation, it is going to be very difficult to contact a DX station running split. On the HF bands, split operation is an example of half-duplex operation. I suppose it could be full duplex under some circumstances but in most cases transmit and receive will not be simultaneous.

VHF CW/SSB Operating

Excluding FM repeater use, most VHF and UHF operating is also simplex. VHF operators can operate split just like the HF case but I can’t recall actually hearing this on the air. A strong band opening on 6m behaves a lot like HF, so a large DX pileup could benefit from going duplex. A linear translator retransmits SSB or CW signals, similar to an FM repeater but for linear modulation. There are very few linear translators being used on earth but they are a form of SSB operating that is not simplex. Linear translators actually retransmit a range of frequencies, not just one, so that multiple users can be supported by one translator. Linear translators are commonly deployed in space as satellites.

Satellite Operating

Satellites use one ham band for the uplink and another ham band for the down link. For example, the FO-29 satellite uses 145.9 – 146.0 MHz for the uplink and 435.8 – 435.9 MHz for the downlink. Similar to a repeater, the satellite operates full duplex, transmitting the signal that is heard on the receiver (uplink). Ham satellites use different modulation types, including FM, SSB, CW and digital formats. It is the most common example of “non-simplex” SSB operating on the VHF bands. Satellite duplex operations can be conducted with an HT and a dual-band antenna, usually a directional. The satellite, like a repeater, typically operates full duplex to instantly retransmit on one band signals received on the other. It is highly desirable for the satellite user to also operate full duplex (usually with headphones to avoid feedback). That way, the user can determine how well he is getting into the satellite, operate with minimum power and just do a better job of avoiding interference to other users. The FM birds can be worked with just a handheld transceiver (HT), making portable operating easy. However, only a few HTs have the ability to operate full duplex, so a lot of satellite operating is done using half duplex.

Summary

To wrap up then, simplex is a term that applies on all of the ham bands, because it is the simplest way to communicate. However, it is not the “opposite” of using a repeater. Duplex is also a term that applies in a variety of cases, including repeater operation, working split on HF or VHF and working crossband via satellite.