Amateur Radio Repeater Communications - SARCNET

This activity introduces the concept of repeater communications and provides practice in its use.

Amateur Radio repeaters provide the infrastructure required, for Line-Of-Sight (LOS) radios, to communicate around the city, the state, the country and even overseas. They operate on permanent, fixed channels within the VHF/UHF Amateur Radio bands; are located on high-spots, like city skyscrapers and mountain tops; are fully automatic, unattended stations; catering for either analog or digital signals, and sometimes both, and they are issued with their own callsigns.

Repeaters work using split frequency, or duplex, operation. They simultaneously receive a signal on one frequency and re-transmit the same signal on another frequency. Careful RF design ensures that their powerful transmitter does not interfere with their sensitive receiver, even if the frequency split is only 600kHz. Some repeaters cleverly use diplexers so that they need only one antenna.

Repeaters only transmit when they receive a signal on their input frequency. However, because analog repeaters usually have a very wide receiver capture area and no Digital Carrier Detect (DCD) mechanism, they are more susceptible to receiving Radio Frequency Interference (RFI). It would be very distracting to anyone monitoring a repeater to hear it transmitting every time it received some RFI. So, to differentiate legitimate signals from RFI, analog repeaters often use a simple security access mechanism called Continuous Tone Controlled Squelch System (CTCSS). Effectively, all this means is that your radio must transmit a continuous, sub-audible, access tone to identify you as a legitimate repeater user.

Repeaters keep transmitting for a short time, even after the input signal has disappeared. The time period is set by a Tail-End Timer. The reason for this repeater function is that sometimes mobile signals are affected by mobile flutter, caused by multi-path fading, or rapid fluctuations in path loss, caused by intermittent shielding. If the repeater was to stop transmitting instantly when this occurred: All receiving stations would not only hear the flutter on the input signal, but also experience the even more disruptive sound of their squelch circuit activating. Instead, the repeater keeps transmitting for a while to give the input signal time to recover. A side effect of this function is that if you briefly press the PTT button on your radio: When you release it, you will hear the repeater transmitting for a second or so longer. This gives you an indication that you are in range of the repeater: It can hear you and your can hear it. Unfortunately, many operators use this test far too frequently, which is why you often hear many, short, unmodulated transmissions on a repeater channel.

Repeaters have another mechanism for averting being locked out by continuous RFI or, just as annoying, an operator sitting on the microphone PTT button while driving along, oblivious to the fact the radio is transmitting. Repeaters will not transmit continuously. They have a timeout, maybe several minutes or more, and will then transmit a warning tone and shutdown. They will start up again when the input signal disappears. So, to avoid getting timed out, you need to let the repeater "drop out" before starting your own transmission, or periodically if you intend to have a long over.  

Some analog repeaters, in fact all around the world, are linked by the Internet Radio Linking Project (IRLP). This means that you can connect to your local IRLP repeater and have a QSO with a station connected to their local IRLP repeater, somewhere else. According to the IRLP website: "IRLP operates a worldwide network of dedicated servers and nodes offering very stable worldwide voice communications between hundreds of towns and cities." To use IRLP you have to identify your intention to do so on your local IRLP repeater - when it is not it use by others - then enter the code of a remote IRLP repeater on your radio's Dual Tone Multi Frequency (DTMF) keyboard. Wait while your connection is announced at both ends and listen to see if the remote repeater is in use. While using IRLP you need to press the PTT button then wait a few seconds before speaking. When finished, simply hang up the connection by pressing the PTT button and sending code 73. Note: We are not covering digital P25 and DMR repeaters and talk groups here.

To program a modern VHF/UHF radio to use a local repeater, first read the manual: Look for information about repeaters, duplex operation, frequency splits and tone coding. Every radio is different.