What's in a Repeater?

Foundations of Amateur Radio

Today I'm going to talk about repeaters. These invisible services that sit on a particular frequency and do magic things to your signal.

First of all, the best way to think of a repeater is to think of it as two radios. One is the receiver, the other the transmitter. The way it works is that the receiver hears your signal and sends that audio to the transmitter which sends it out over the air.

For this to work, there need to be two frequencies in use, the one that you're transmitting on and the one that the repeater is transmitting on.

From this simple idea, many different things flow. There is no rule that states that the receiver and the transmitter need to be in the same place, let alone on the same band; if they're on different bands, it's called a cross-band repeater.

If the receiver and the transmitter are on the same band, the system needs to deal with the fact that a strong signal is being transmitted by the repeater right next to where the receiver is. If you're not careful, the transmitter will overwhelm or de-sense the receiver, making it harder to get your signal into the repeater.

Several techniques are used, a contraption called a cavity filter is set-up to specifically let either the receive frequency through, or to block all frequencies except the transmit frequency. Some combine both of these techniques to make the repeater hear weak stations better.

If the receiver and transmitter are on the same band, the difference between the two frequencies in use is called the offset. It varies per band. On 2 meters, the offset is normally 600 kHz, but it varies, on 70cm the offset is 5 MHz, but on 10m, the offset is 100 kHz. So different bands use different configurations and of course each of these is subject to local variation. There may be local interference on the standard offset, so it may be varied.

There are some other things going on with repeaters. You can have a repeater that receives and transmits on the same frequency, it's called a parrot repeater and it sits there waiting for you to transmit, stores the incoming audio for a set period and then when you stop transmitting, it sends out the audio on the same frequency. This is useful to see how you sound on-air.

Other techniques include adding computers to create IRLP, Echolink and AllStar Link. Essentially the receiver is connected to a computer which sends the audio across the Internet to another computer which in turn sends out the audio to another transmitter. After you stop transmitting, the chain is reversed and the other station can talk to you via a reverse path.

There are also specialised repeaters that can listen in one mode, like FM and transmit in another, like AM, or SSB. This allows a 2m user to use HF from their FM hand-held radio.

If all that's not enough, there are other things possible with repeaters. You can use a special tone to identify to the receiver that your signal is a valid audio signal. This is used in environments where noisy local signals often trigger the repeater, resulting in ongoing kerplunking of the transmitter.

Next time you key up your local repeater, have a think about what's happening when you key-up your radio and say thanks to the owner of the repeater who spent time and effort, not to mention money, to make this invisible friend on the air work for you.

I'm Onno VK6FLAB