Radio repeater
A radio relay station that extends communication range over obstacles.
A radio repeater is a device that pairs a receiver with a transmitter to pick up a signal and send it out again, extending the range of two-way radio communications. When placed at a high point, like a mountain or tower, it lets two mobile radios that cannot see each other due to distance or obstacles still connect. These systems are used in professional, commercial, and government mobile networks, as well as by amateur radio operators.
Repeater systems rely on two separate radio frequencies: mobile units transmit on one frequency, and the repeater receives that signal and rebroadcasts it on a second frequency. Because the repeater must transmit while receiving, and often uses the same antenna for both tasks, frequency-selective filters are needed to stop the transmitted signal from overwhelming the receiver. Some repeaters use two different frequency bands to improve isolation or for convenience. In a communications satellite, a transponder does a similar job, though it may not demodulate the signals it relays.
A repeater operates automatically as a radio relay station, typically located on a hilltop, tall building, or tower. It allows communication between base, mobile, or portable stations that cannot talk directly due to distance or obstructions. The repeater listens on one frequency (the input), demodulates the signal, and instantly retransmits the information on another frequency (the output). All stations using the repeater transmit on the input frequency and receive on the output frequency. Because the repeater sits higher than the other radios, its coverage is much larger.
Since the transmitter and receiver run at the same time, isolation is critical to prevent the transmitter from degrading the receiver. Without good isolation, the repeater’s own transmitter desensitizes its receiver—similar to trying to hear a quiet conversation at a loud rock concert. Separating the input and output frequencies as much as possible makes isolation easier.
Mobile stations using a repeater must transmit on a different frequency than the repeater’s output. While the repeater site handles simultaneous reception and transmission on two frequencies, mobile radios switch between receiving and transmitting one at a time, so they do not need the bulky, costly filters used at the repeater site.
- Frequency separation examples
- Amateur repeaters in the 144–148 MHz band use 600 kHz separation; in the 1.25-meter band, 1.6 MHz; in the 420–450 MHz band, 5 MHz; in the 902–928 MHz band, 25 MHz.
- Example input output 450 470 mhz
- Input 456.900 MHz, output 451.900 MHz (5 MHz separation, input on higher frequency).
- Example input output 806 869 mhz
- Input 810.1875 MHz, output 855.1875 MHz (45 MHz separation, input on lower frequency).
- Military spacing suggestion
- No less than 10 MHz spacing.
Lore & Background
Repeater systems use two different radio frequencies: mobiles transmit on one frequency, and the repeater receives those transmissions and retransmits on a second frequency. Because the repeater must transmit simultaneously with reception, and may use the same antenna for both, frequency-selective filters are required to prevent the receiver from being overloaded by the transmitted signal. Some repeaters use two different frequency bands to provide isolation or as a convenience.
In same-band repeaters, isolation between transmitter and receiver is created using a device called a duplexer, a tuned filter connected to the antenna. The duplexer has two legs: one tuned to pass the input frequency, the other to pass the output frequency. The receive leg attenuates the transmitter's carrier at the receiver input by typically 90–100 dB, and the transmit leg attenuates transmitter broadband noise on the receiver frequency by a similar amount. Cavity filters are nearly always used in repeater stations due to their high Q factor and ability to handle high power levels.
At crowded equipment sites, repeaters can be connected to shared antenna systems. A receive antenna is usually placed at the top of the tower to capture weaker signals, and a transmit antenna is installed below it. The distance between them can provide isolation similar to a low-grade duplexer (about −60 dB). Transmitters are connected to a single antenna using combiners, which are lossy—each leg typically has a 50% (3 dB) power loss.
Reader's Guide
Radio repeaters are fundamental to extending the range of two-way radio communications, particularly in environments where direct line-of-sight is blocked by terrain or distance. By placing a repeater at a high elevation, mobile and portable stations that cannot communicate directly can do so through the repeater. The requirement for simultaneous transmission and reception on different frequencies drives the need for careful isolation design, typically achieved through duplexers or cavity filters. In shared sites, combining systems allow multiple repeaters to use a single antenna, though with inherent power losses. The frequency separation between input and output varies by band and regulatory convention, with examples ranging from 600 kHz in the 2-meter amateur band to 45 MHz in the 800–900 MHz band. Repeaters operate automatically and are a staple of professional, commercial, government, and amateur radio systems.
Did You Know?
- A repeater receives on one frequency and simultaneously retransmits on another frequency.
- Duplexers in same-band repeaters can provide 90–100 dB of isolation between transmitter and receiver.
- In the US, amateur repeaters in the 420–450 MHz band use a 5 MHz separation between input and output.
- Transmitter combining systems typically lose 50% (3 dB) of power per leg.
More in Radio Electronics, Part 2 1-24
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