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Blog - Simplex, Duplex and Repeater Types

Last week I got a FB question that prompted me to question the answers I thought I already knew. And the further I got into it… the more confused became.

It was a very simple question, really. The question was “Is the YL repeater a simplex repeater?”

“Well... uuuuuuuuuh…” Simplex? That’s not at all what I thought simplex was…

Luckily, I have a very gracious and patient Elmer who answers all my questions. Unfortunately, the more answers he supplied, the more questions I came up with. In the interest of preserving our friendship, I bailed for the Googlator. I had to figure it out myself.

This cleared up one misconception right from the get-go.

Google is *NOT* faster than Elmer.

Highlighters and printouts in hand, I sat down with my downloaded articles and went in search of the truth. A girl's gotta do, what a girl's gotta do. After it was all said and done, I had my answers and enough research to do a topic. After all, why waste good research?

What started as a quest for simplex information turned into a realization that there are all types of repeaters.

Digitpeaters, as we talked about several weeks ago, are a type of repeater used in packet radio. This can be a stand-alone sender-to-receiver direct connection packet transmission, or in the case of APRS, digipeaters may operate on a one-to-many broadcast. But the key piece of information is: they receive and transmit packet data on the same frequency. Receive, store, forward.

Slow Scan Television Repeaters relay SSTV signals. A SSTV repeater is equipped with an HF or VHF transceiver and a computer with a sound card to serve as a demodulator/modulator. Contrary to what the name might imply, this is not video. Hams has SSTV to send pictures to other hams. The key word here is… slow.

Duplex repeaters are the most common repeater type and the type with which you are most familiar. These repeaters receive a signal on one frequency and re-transmit on an offset frequency. They are called duplex repeaters because they use a device called a duplexer to keep the input receiver from hearing the output transmitter. A pretty tough job when you consider the receiver and the transmitter sit side by side within the repeater box. This is like not hearing your noisy apartment complex neighbors on Saturday "morning" after bar night.

“Duplex” also means two-way communication -- the ability to listen and hear at the same time. A skill I definitely do not have. Duplex repeaters, however, do. Your radio does not. How do I know this? If you have the PTT engaged on your radio, you cannot hear. In order for a conversation to actually be considered two-way, the repeater must be duplex and your radio must be duplex.

And finally, full circle to answer to the original question.

A simplex repeater uses a transceiver and a voice recorder to record short-duration recordings (usually less than 30 seconds) and repeats on the same frequency. You can see why from this description simplex repeaters are sometimes called parrot repeaters. The YL NTX repeater is a duplex repeater. If there is an offset on a repeater—it’s a duplex repeater.

In the strictest sense of the word, simplex means one-way communication. This is where I got confused. If you say simplex to me, I think “close-distance FM simplex communications”.

In the case of simplex repeaters, I talk, the repeater records, the repeater repeats on the same frequency, you listen. In the case of close distance simplex communications, there’s no repeater in the middle, but the process is the same -- I talk, you listen, you talk, I listen -- on the same frequency.

If you’ve never tried simplex, let me enlighten you. It’s fun! I highly suggest you give it a try just for experience. It’s fun discovering who’s near you. The other thing is, if you have a good case of mic fright, try throwing your callsign out on simplex frequencies just to get over it. The likelihood of anyone hearing you without making a “date for simplex” is… pretty much nil. But throwing your callsign out there will get you past the pushing-the-key down phase of mic fright.

The key to close distance simplex communications is (duh!) distance. If you would like to know if you are within simplex distance of a particular station, try listening to them on your local duplex repeater on the output frequency. That sounds very complicated, but really, there’s a cheat. Leave your radio programmed exactly as it would be for talking, but hit the reverse key on your radio. If you still hear the station talking, you are close enough to try a simplex QSO.

When you do try simplex, turn off any squelch tones you have set, turn off the offset and turn the squelch knob up so that you hear that static. (If not, you may squelch out weak signal stations.) You will notice two things very quickly. The sound is incredibly clean. No white noise, pops, static, or intermod to interfere with that beautiful sound. The other thing you will notice is... no squelch tail.

I cannot let this opportunity go by without giving a plug for the JC Simplex Net on Sunday nights. This net kicks off at the end of the JC Training Net, which is held on 145.49 on Sunday nights at 7:30. After that net ends, change to the simplex frequency of 146.48 and the simplex net begins. It’s fascinating stuff, really. From my Burleson location I can pick up stations on simplex as far as the Cleburne State Park and Mansfield. I’ve also heard other stations as far as Weatherford and Crawford who had very tall towers or good elevation.

There you have it. Everything you need to know about repeaters, duplex and simplex but were afraid to ask. To recap: Simplex is one-direction communication on the same frequency. Repeaters can be duplex or simplex. Simplex, like tone, is one of those concepts that took awhile to wrap my head around. And even after it sunk in, I still questioned myself. Turns out, I just had the definition of simplex wrong.