A radio repeater is a combination of a radio receiver and a radio transmitter that receives a signal and retransmits it, so that two-way radio signals can cover longer distances. A repeater sited at a high elevation can allow two mobile stations, otherwise out of line-of-sight propagation range of each other, to communicate. Repeaters are found in professional, commercial, and government mobile radio systems and also in amateur radio. Repeater systems use two different radio frequencies; the mobiles transmit on one frequency, and the repeater station receives those transmissions and transmits on a second frequency. Since the repeater must transmit at the same time as the signal is being received, and may even use the same antenna for both transmitting and receiving, 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 between input and output or as a convenience. In a communications satellite, a transponder serves a similar function, but the transponder does not necessarily demodulate the relayed signals.
Full duplex operation
A repeater is an automatic radio-relay station, usually located on a mountain top, tall building, or radio tower. It allows communication between two or more bases, mobile or portable stations that are unable to communicate directly with each other due to distance or obstructions between them. The repeater receives on one radio frequency (the "input" frequency), demodulates the signal, and simultaneously re-transmits the information on its "output" frequency. All stations using the repeater transmit on the repeater's input frequency and receive on its output frequency. Since the repeater is usually located at an elevation higher than the other radios using it, its range is greatly extended.
Because the transmitter and receiver are on at the same time, isolation must exist to keep the repeater's own transmitter from degrading the repeater receiver. If the repeater transmitter and receiver are not isolated well, the repeater's own transmitter desensitizes the repeater receiver. The problem is similar to being at a rock concert and not being able to hear the weak signal of a conversation over the much stronger signal of the band. In general, isolating the receiver from the transmitter is made easier by maximizing, as much as possible, the separation between input and output frequencies. When operating through a repeater, mobile stations must transmit on a different frequency than the repeater output. Although the repeater site must be capable of simultaneous reception and transmission (on two different frequencies), mobile stations can operate in one mode at a time, alternating between receiving and transmitting; so, mobile stations do not need the bulky and costly filters required at a repeater site. Mobile stations may have an option to select a "talk around" mode to transmit and receive on the same frequency; this is sometimes used for local communication within range of the mobile units.
Frequency separation: input to output There is no set rule about spacing of input and output frequencies for all radio repeaters. Any spacing where the designer can get sufficient isolation between receiver and transmitter will work. In some countries, under some radio services, there are agreed-on conventions or separations that are required by the system license. In the case of input and output frequencies in the United States, for example:
Amateur repeaters in the 144–148 MHz band usually use a 600 kHz (0.6 MHz) separation, in the 1.25-meter band use a 1.6 MHz separation, in the 420–450 MHz band use a 5 MHz separation, and in the 902–928 MHz band use a 25 MHz separation. Systems in the 450–470 MHz band use a 5 MHz separation with the input on the higher frequency. Example: input is 456.900 MHz; output is 451.900 MHz. Systems in the 806–869 MHz band use a 45 MHz separation with the input on the lower frequency. Example: input is 810.1875 MHz; output is 855.1875 MHz. Military systems are suggested to use no less than a 10 MHz spacing. These are just a few examples. There are many other separations or spacings between input and output frequencies in operational systems.
Same band frequencies Same band repeaters operate with input and output frequencies in the same frequency band. For example, in the US, two-way radio, 30–50 MHz is one band and 150–174 MHz is another. A repeater with an input of 33.980 MHz and an output of 46.140 MHz is a same band repeater. In same band repeaters, a central design problem is keeping the repeater's own transmitter from interfering with the receiver. Reducing the coupling between transmitter and input frequency receiver is called isolation.
Duplexer system
In same-band repeaters, isolation between transmitter and receiver can be created by using a single antenna and a device called a duplexer. The device is a tuned filter connected to the antenna. In this example, consider a type of device called a band-pass duplexer. It allows, or passes, a band, (or a narrow range,) of frequencies. There are two legs to the duplexer filter, one is tuned to pass the input frequency, the other is tuned to pass the output frequency. Both legs of the filter are coupled to the antenna. The repeater receiver is connected to the receive leg while the transmitter is connected to the transmit leg. The duplexer prevents degradation of the receiver sensitivity by the transmitter in two ways. First, the receive leg greatly attenuates the transmitter's carrier at the receiver input (typically by 90-100 dB), preventing the carrier from overloading (blocking) the receiver front end. Second, the transmit leg attenuates the transmitter broadband noise on the receiver frequency, also typically by 90-100 dB. By virtue of the transmitter and receiver being on different frequencies, they can operate at the same time on a single antenna. According to Bertrand, "Radio repeater stations nearly always use cavity filters." Their advantages include very high Q factor, and the ability to handle high power levels. These filters are cavity resonators. The cylinder provides inductance, and surrounds a capacitor with a tuning knob.
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