What a radio repeater does
A repeater is a station, usually placed on a tall building or hill, that receives a signal on one frequency and retransmits it on another at the same time. Because the repeater antenna is high, handheld and mobile radios that could not reach each other directly can communicate through it.
Repeater diagram
A text version of a typical single-site repeater:
Antenna <-> Duplexer -> Receiver -> Controller -> Transmitter -> Duplexer <-> Antenna
(timer, ID, tones)
Power supply feeds all partsIn many systems a single antenna is shared by the receiver and transmitter through the duplexer; some sites use two antennas instead.
The parts, one by one
Receiver
Listens on the repeater’s input frequency. Sensitivity and selectivity are important because it must hear weak handhelds.
Controller
The controller decides when the transmitter keys up. It typically handles:
- Detecting valid signals (carrier or tone squelch)
- Timeout timers, so one user cannot hold the machine indefinitely
- Station identification, as required by the rules in many countries
- Optional features such as courtesy tones
Transmitter
Sends the audio back out on the output frequency, often at higher power than a handheld.
Duplexer
The duplexer is a set of resonant filters that allows the receiver and transmitter to share one antenna. Without it, the powerful transmit signal would overload the receiver. Quality of the duplexer is a major factor in how well a repeater performs.
Antenna and feedline
Height and gain matter most here, as coverage depends largely on how high the antenna is. Low-loss feedline is generally used; compare cables in the coax cable loss chart.
Power and backup
Many repeaters have battery or generator backup, which makes them useful for emergency communication.
Input, output and offset
The difference between transmit and receive frequencies is the offset. Common practice on VHF and UHF bands (see the 70cm band plan for an example) is a standard offset per band, but this varies by region, so check local band plans or a repeater directory. You program your radio to transmit on the repeater’s input and listen on its output.
Access tones
Many repeaters require a sub-audible tone, often called CTCSS or PL tone, so that they respond only to intended users and to reduce interference from distant signals. Digital systems may use other access codes.
Linked and digital repeaters
Some repeaters are linked over radio or the internet so users in different places can talk, as with IRLP. Others handle digital voice modes such as DMR. The core block diagram stays the same, with extra interfaces added.
Building a repeater
Building one is a project for experienced hams and usually involves coordination with a local frequency coordinator, site permission, and licensing as the control operator. If you are curious about the electronics, start with the block diagram of a radio and the diagram of a radio transmitter.
Frequently asked questions
What does a radio repeater do?
It retransmits what it hears on one frequency onto another, extending range.
Will a ham radio pick up police frequencies?
Amateur radios are designed for amateur bands, and some may receive outside them, but rules about listening and transmitting differ by location. Transmitting outside the amateur bands is not permitted with a ham license.
How do I use a repeater?
Set the output frequency to listen, the correct offset and tone to transmit, then identify yourself as required by local rules. See repeater etiquette for good habits.