For those that aren’t familiar with the term, a ‘primer’ was a small book in big print supplied in elementary (primary) schools, which provided a basic introduction to a subject. Initially it would be spelling or arithmetic or reading, (remember the 3R’s????) but sometimes they would be on other ‘important subjects’ such as farming or animal husbandry. 🙂
Today, I thought we could discuss another one of those subjects that don’t really get talked about in the Basic Qualification course — Repeaters. We’re certainly not going to provide the level of detail required for the Advanced Amateur Certification or make you into a Repeater Technical Guru here, but at least we can give some insight as to what they are, how they work, and how to interpret the listings you find on the MIRA website and elsewhere on the interwebs.
Repeaters are used primarily to extend the communications reach for portable radios in the VHF and UHF spectrums. Most repeaters are in place for the 2m (144Mhz) and 70cm (440Mhz) bands. While the 6m (50Mhz) band is technically part of VHF and in some places of the world there are 6m repeaters, they tend to be few and far between. The reason for this is mostly to do with demand, since there are very few 6m handhelds around any more. There have been a couple, but 6m coverage has now been relegated to the base-station transceivers for the most part. And with 100 watts, a reasonable antenna and the weird propagation available at 6m (a cross between HF and VHF), there really isn’t a need for a 6m handheld.
Repeaters are also starting to show up in the 1.25m (220Mhz) band as 2m is getting pretty crowded.
So, why do we need repeaters?? A repeater’s job is just that — to retransmit what it hears. Why would we need something to do that? Well, the primary reason is to extend the coverage/reach of our handheld radios and overcome natural barriers! Let’s take a scenario with Bob and Joe who are on their handhelds talking to each other.

In the simplest case, Bob and Joe would agree on a frequency (say 146.520 Mhz) and they would talk in what’s called simplex mode — each sending and receiving on the same frequency. Bob would say something while Joe listened, then Joe would respond and so forth. Remembering back to the Propagation section of the Basic Qualification Course, radio waves at VHF and UHF frequencies don’t ‘bounce’ off the ionosphere so Bob and Joe are limited to what is called ‘line of sight’ communications. (i.e. they can only communicate while the radio waves from their handhelds can reach the other) And also remembering that radio signals decrease in power by the square of the distance from the transmitter, then Bob and Joe also restricted on how far apart they can be to still hear each other.
What if there’s a mountain (or other tall, impenetrable object) in between Bob and Joe?

Now we have a problem. Radio signals travel in straight lines (generally) and since VHF and UHF don’t ‘bounce’ off the atmosphere, there’s no way Bob and Joe can communicate. But what if we put a receiver and transmitter pair at the top of that mountain?

Well, now we have a repeater!! The receiver section of the pair picks up Bob’s signal and re-transmits it back down the mountain to Joe. Note that, because of its high location, the land area that the transmitter can ‘see’ is large and it doesn’t need all that much power. In fact, the transmitter portion of many repeaters is usually much lower than the maximum output of a handheld! (I think that the output of the VE7RBB repeater on Mt. Cokely is less than one watt!)
Now, it is possible that the repeater could listen and retransmit on the same frequency, but to make that work we’d have to introduce some delays — wait for Bob to finish talking, store the message (somehow), then retransmit back out again to Joe. And, we’d have to be careful to shut off the receiver portion while transmitting so we don’t damage its front end. Way too complicated!!!!
Instead, what if we listened on one frequency and retransmitted on another frequency up (or down) the band a bit so that it wouldn’t impact the receiver?
This is how most modern repeaters work. In the example above, the repeater is listening on 147.780 Mhz and retransmitting what it hears back out on 147.180 Mhz. The gap is sufficient that the repeater’s receiver isn’t bothered by the transmitter. And another advantage of this arrangement is that we can simultaneously listen and transmit at the same time! No need to ‘store’ messages — we can simply feed the output of the receiver directly to the transmitter! To most of us, the very slight delays introduced would be imperceivable. Essentially, Bob would be hearing Joe in real time.
The difference in frequencies between the receiver (the repeater ‘input’) and the transmitter (the ‘output’) is called the offset and the amateur community has agreed that the standard offset for VHF repeaters is 600 Khz and for UHF repeaters is 5 Mhz.
So, once we have a repeater set up, how do we tell hams how to access it? Typically, repeater listings will look like this (example from MIRA website): VE7RBB 147.180 100/+0.6. Hmmmm… a little cryptic, right? Let’s break it down.
VE7RBB is the call sign for the repeater. Typically, the repeater will be sponsored by an individual or a club, but it still needs its own call sign.
147.180 is the frequency (in Mhz) that you should set your radio to to ‘listen’ to the repeater (i.e. the transmitter’s output frequency).
Just ignore the 100 for the moment (I’ll get back to it).
/+0.6 is the offset for this repeater. 0.6 is also in Mhz, so it’s the standard 600Khz for VHF repeaters. The + (plus or positive) means that the repeater’s receiver is listening 600Khz above the transmitter’s output frequency, so you need to set your radio’s output (transmit frequency) to 147.180 + 0.6 = 147.780 Mhz. If the listing said /-0.6, it would imply that the repeater is listening below its transmit frequency, so you would set your handheld/radio to transmit at 147.180 – 0.6 = 146.580 Mhz.
So, here’s that everything looks like diagrammatically:

OK… now back to that mysterious ‘100‘ listed above that we ignored. This is called the ‘Tone’ (short for CTCSS or Continuous Tone-Coded Squelch System) for the repeater. It’s a subaudible tone that the repeater expects to hear with your transmission in order to say it’s OK for the receiver to ‘hear’ you and forward your signal to the transmitter. It basically works the same way as the squelch control on the radio except that it’s listening for the specific tone (rather than a level of signal) before it opens up. 99.99999% of radios today have this as part of their programming capability — it’s just one of the parameters you specify when you set up the channel for the repeater. The reasons for tones are two-fold: In the early days of repeaters (about 50 years ago) the hams establishing the repeaters sometimes wanted to limit their accessibility either because of privacy or power supply issues so they set up the tone and only let their ‘friends’ know what it was. Nowadays with the bands being so crowded, the tones are used to minimize extraneous noise and help select between repeaters operating on the same input frequency.
The next repeater innovation/evolution was the development of a Trunk system where various repeaters located over a large geographical area (such as Vancouver Island) are linked together to form a single network. Here, each of the local repeaters operates as described above, but an additional communications link is provided specifically for the repeaters to talk to each other, as shown below.

This link is another receiver/transmitter pair of frequencies — usually located far away from the local repeater frequencies or in another band — which allows the repeaters to share what they are hearing with the other repeaters. In the case of the Island Trunk System here on Vancouver Island, there are 10 repeaters located throughout the island that are interconnected. The local repeater frequencies (for use by hams) are on 2m (144 MHz) and 70cm (440 MHz) while the links between the repeaters operate on 1.25m (220 MHz) — as far as I know 🙂 This gives hams from Victoria to Campbell River and out to Port McNeil access to a centralized, linked set of repeaters providing communication up and down the island. It’s great to hear hams from all over the island and the lower mainland share their morning cup of coffee together. Unfortunately, the links out to the West Coast aren’t really working right now, so things are a bit difficult for Tofino and Ucluelet, but who knows….
So, that’s basically how repeaters work. There are more ‘nuances’ when you start talking about digital communication modes (DMR, DSTAR, Fusion, etc.) and maybe we’ll tackle those in subsequent blogs. But hopefully this will get you started. Next time, we’ll talk about the ‘etiquette’ of using a repeater and of nets in general (at least voice ones for now.)
Till then, 73 and as always, please comment or send me a note at drtolson (at) outlook (dot) com if you have any questions. Have fun out there!
Don VE7ATJ