Simplex Made Simple

What is SIMPLEX and how is it different from using a REPEATER


SIMPLEX is when you transmit and receive on the same frequency.

Using simplex is really simple and it is just two or more ham stations using mobile or hand held radio transceivers to communicate on the same frequency without a repeater re-transmitting your signals.

So SIMPLEX means Same Frequency

Every Band in the VHF and UHF and the Microwave bands has several SIMPLEX channels / frequencies that are specifically designed to relieve repeater congestion.

The opposite of Simplex is DUPLEX (a type of repeater operation)

REPEATERS do NOT transmit AND receive on the same frequency. With a REPEATER you transmit on one frequency and you receive (Listen) on another frequency.

That is why Repeaters are known as DUPLEX because they do NOT transmit and receive on one frequency but on TWO …. hence the name DUPLEX (DU as in Dual)

Repeaters use one frequency to transmit and another frequency to receive. In contrast to SIMPLEX (as in simple — same — one frequency)

We can talk about repeaters some other time if you are interested…. but for now you only needed to know the difference between

Simplex (use one frequency)


Repeaters (use 2 frequencies)

So back to SIMPLEX

SIMPLEX OPERATION is when you transmit and receive on the SAME Frequency.

When you use SIMPLEX you don’t have to worry about + or – offsets, or tones, or shifts, or other frequencies — and you don’t have to worry about timing out a repeater because you are not using a repeater–

so with SIMPLEX you just enter the one frequency and you are “good to go.”

BUT remember — when using SIMPLEX

1. Even with simplex you have to give your call sign every 10 minutes or less

2. You can talk as long as you want without having to worry about timing out anything

3. BUT If your radio starts getting warm in your hand, you are talking too much, so give it a rest and listen to your friend(s) instead of talking all the time. : )

You can use SIMPLEX when both (or more) stations (radios such as handi talkies) are within a few miles of each other (i.e. line of sight with no obstructions blocking the path).

And, yes, it can be more than two stations — any station that is within the line of sight can participate in simplex conversations. All that is required is that they be within line of sight. So you can have group conversations just like on repeaters IF you are all within line of sight of each other.

As you recall — line of sight is usually about 5 to 7 to 10 miles and then the curvature of the earth begins to limit your ability to transmit to another person by simplex.

Line of sight simply means that there is a “radio horizon” from your radio — stretching out to the earth’s horizon (if you have a clear path) — and after your signal passes the “radio horizon” (curvature of the earth) it shoots on out into space.


if your signal is still within the curvature of the earth, your line of sight can be blocked by things or structures such as buildings, mountains, etc. Such things can block or attenuate the radio signals before they ever got to the curvature of the earth situation.

Imagine your radio signal as a light beam (as if from a flashlight) reaching out to the other station’s radio and antenna, —

anything in the way of your “light beam” will tend to block or shield it from the other station’s antenna and vice versa.

As just mentioned, some things involved in blocking or attenuating the signals are tall buildings with lots of metal in them, high hills, mountains, local “ground clutter” near you or a combination of all those things.

If you are in a tall building or on a mountain — your line of sight will be greater and you might be surprised to see how far simplex will go.

Also the higher and bigger your antenna (even though rubber ducks will work) the farther and better your signal will be.

Anyway — if you want to make contact with a person or persons and you are within 10 miles or less, check and see if you can use Simplex instead of a Repeater.

You might, for example, be neighbors and just want to chat — use Simplex.

You might be traveling in 2 or more cars or boats or motorcycles or bicycles and have hams in all the vehicles and want to chat or discuss where you want to stop and get gas or to eat, etc. Use Simplex

You might be in a shopping mall or at a sporting event or camping or at any number of places where you want to be in contact with family or friends who are nearby — use simplex.

You might be participating in an event such as the Special Olympics where there is no need to tie up a repeater because you are in line of sight of each other– use Simplex.

Anytime you can use simplex you can localize your transmissions and free up repeaters that are needed for more distant contacts.

But if repeaters are better for transmissions over distances large and small, why would you want to use Simplex?you ask.

Well, for several reasons, as we mentioned, but the bottom line main reason is

If you use Simplex when direct contact is possible, you avoid tying up repeaters.

Ok Great. you say…. I understand the reason to use simplex — but if I am talking on a repeater and wonder if I could use simplex instead….. how do I find out if I can use Simplex instead of the repeater?

Well, it is really simple — you check the repeater INPUT ( the transmit / talk ) frequency of the repeater to see if you can hear the other station.

How do you do that?

Well, good news — it is easy.

Almost all handi talkies (HTs) as well as mobile and base station UHF VHF FM radios have a small button usually labelled REV or MON or MONI or some such name — that you can press to shift (change) to the input (talk/transmit) frequency of the repeater.

I am assuming that REV means reverse — i.e. shift (change) to the other frequency — and

MON or MONI means ‘Monitor” — and who knows what your radio button says but the meaning will be generally the same — i.e. you are checking /shifting/ monitoring to see if you are close enough to hear without having to go through the repeater.

So be sure to check the manual for your radio to find out the Name of the button and where it is located. Usually it is very close to the PTT (Press to Talk Button) but one never knows for sure until you check your manual.

Once you have found the correct button and its location — then while listening to someone talking on the repeater — momentarily push that button and it will cause your radio to quickly shift (change) to the input (talk) frequency of the repeater.

This lets you determine whether or not you can pick up the other station directly.

If you can hear them when you push that REV or MON or MONI or ‘whatever’ button, it means they are near enough to you that you could communicate by simplex.

And if you can, and if you just want to talk to them without tying up the repeater, you can switch over to a SIMPLEX frequency and talk with them on the Simplex Frequency.

BUT An important word of WARNING —

when you switch from using a repeater to going to a Simplex frequency, be sure that you go to one of the authorized / recognized simplex frequencies.

If you just pick any frequency, you could land right on a repeater input frequency — which would really interfere with the Repeater use and will result in many unhappy repeater users.

So How do you know what the authorized SIMPLEX frequencies are?

Well, there is a list that you can find in various books, study guides, internet sites, etc. but to save you the trouble of hunting down one of those websites here is a link to one of the many internet sites that lists them.

I have taken a picture of the list and put in in the picture folder on my cell phone.

I also have a printout of it in the “go box” where I keep my radio ‘how to’ information.

You might want to do the same.

And one more interesting thing to know about SIMPLEX frequencies

Each of the VHF and UHF FM Bands (i.e. 2 Meter, 1.25 Meter, and 70 cm, etc etc ) has a National Simplex Calling Frequency.

So What is the National Simplex Calling Frequency? you ask

Well that is the designated frequency that you can listen to and call on — that folks use to see if anyone is out there on simplex and wanting to chat or make contact with other hams on simplex–

so you would use that frequency if you were trying to do a “radio check” or to call to see if anyone was listening. If so…. they will answer and then you can go to another simplex frequency to chat after you make contact.

The National Simplex Calling Frequency for the

  • 2 meter band 146.52
  • 1.25 meter band 223.50
  • 70 cm band 448.0
  • 33 cm band 906.5
  • 23 cm band 1294.5