What Is Simplex Operation?

Duplex and Simplex Operation Explained

Bill VY2WM

A newer ham asked me if I could explain what was meant when operators spoke about ‘working simplex’. It’s sometimes easy for those of us who have been in the hobby for a number of years to forget that some of the terminology we use can be confusing to newer hams. So I’ll explain what is meant by ‘going simplex’.

I think the best way to start is to talk about what simplex is not. Let’s talk about duplex operation first. Most of us these days talk to other hams on 2 meter FM through one of several local repeaters. Most of us do this without really giving a thought to how the repeater works. We just know that we can talk to other hams over fairly long distances, even from our vehicles, often with moderate to low power output.

The reason we can do this is that the repeater is normally located at some high vantage point, geographically, with an antenna system many meters above ground. Remember that for the most part 2 meter FM communications are limited to ‘line of sight’. The higher the antenna the longer the range. As the name implies, a repeater listens for incoming signals, and retransmits (or repeats) those signals out again on a different frequency in the same band, usually on VHF (2 meters) or UHF (70 centimeters). The repeater takes advantage of its high antenna and higher power output to extend the range of the stations using it. This mode of operation is known as duplex. The repeater listens on one frequency and transmits on another at the same time.

Your 2 meter radio transmits on the listening (input) frequency of the repeater, and receives on the transmitting (output) frequency of the repeater. The difference in frequencies between the input and output frequencies for the repeater is known as the ‘offset’. On 2 meters the standard is 600 Khz. The input of the repeater can either be higher or lower than the output, so the offset can be said to be either plus or minus. If the repeater offset is minus, this means the repeater is listening 600 Khz down from the frequency the repeater transmits on, ie. the output.

With most modern 2 meter radios, we don’t even have to think about programming the two different frequencies into the radio. We just enter the receive frequency (the repeater output) and the radio automatically knows what frequency it needs to transmit to be heard by the repeater input. It can do this because there is an accepted set of standards around repeater frequencies and their offsets.

So now that we know what duplex operation is, and we understand this is really the mode of operation most of us use when working other stations on 2 meters, let’s talk about simplex.

Simplex just means that we transmit, and receive on the same frequency. The station we are working must also be ‘tuned’ to the same frequency as our station. Often we say that we are ‘working direct’ when we are in simplex mode, because we are not using the repeater to be the middleman for us. Obviously in most cases the range in which we are able to work stations in simplex mode is going to be reduced substantially. We must be able to send and receive our signals with the other station without the help of the lofty antenna and extra power that the repeater can provide.

The range of our simplex communications can vary quite a bit, since it depends on the type and height of the antenna we are using, our geographic location and the power output of our radio. It also depends on all of these variables for the station we are trying to work. One way to determine if you and another ham will be able to successfully talk to each other on simplex is to listen to their signal on the ‘input’ frequency of a repeater when they are using that repeater. Most radios allow you to do this quickly and easily by using a button they provide, often called ‘Rev’ for reverse. Pressing the Rev (or your radio’s equivalent) switches from receiving on the output frequency of the repeater, to receiving on the input frequency of the repeater. If you can hear your fellow ham when you press the Rev button, then you should be able to work his station simplex, or direct.

There are a number of designated 2 meter simplex frequencies. The accepted standard ‘calling’ frequency is 146.520 Mhz. Longer QSO’s on simplex should be done on some of the other ‘channels’ such as 146.550 Mhz or 146.580 Mhz. The complete 2 meter band plan with repeater and simplex frequencies can be found on the RAC website here.

There are a number of advantages to working another station in simplex mode.

  • You are not tying up the repeater, which frees it up for others to use
  • Your QSO is a bit more private, since it’s not being repeated over the wider range of the repeater
  • Since you are not using the repeater, there is not an automatic time out feature to worry about
  • You are able to appreciate more the capabilities and range of your station

In fact it is highly recommended that you use simplex whenever possible. Too often we hear local hams within the city limits, talking on the repeater for 30+ minutes, tying up the repeater, when they could just as easily have been working each other directly on simplex.

Two more things to add. Everything I’ve explained here in reference to 2 meters, repeaters and simplex, goes for the other VHF and UHF bands as well. For example the 70 cm FM calling frequency is 446.000 MHz. Secondly it can be said that normal operation procedures on the HF frequencies are always ‘simplex’ QSOs, since both stations are transmitting and receiving on the same frequency.