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. •
Solano County Fire, (former Fire Radio Service): 46.240 input; 154.340 output. This system was dismantled in the 1980s and is now a same-band repeater. • Mid-Valley Fire District,
Fresno, (former Fire Radio Service): 46.140 input; 154.445 output. This system was dismantled in the 1980s and is now a same-band repeater. •
Santa Clara County Department of Parks and Recreation, (former Forestry Conservation Radio Service): 44.840 MHz input; 151.445 MHz output. This system was dismantled in the 1980s and is now a same-band repeater. •
State of California, Governor's Office of Emergency Services, Fire, (former Fire Radio Service): 33.980 MHz input; 154.160 MHz output. In commercial systems, manufacturers stopped making cross band mobile radio equipment with acceptable specifications for public safety systems in the early 1980s. At the time, some systems were dismantled because new radio equipment was not available. Sporadic E ionospheric ducting can make the 46 MHz and below frequencies unworkable in summer.
As links For decades, cross-band repeaters have been used as fixed links. The links can be used for remote control of base stations at distant sites or to send audio from a diversity (voting) receiver site back to the diversity combining system (voting comparator). Some legacy links occur in the US 150–170 MHz band. US Federal Communications Commission rule changes did not allow 150 MHz links after the 1970s. Newer links are more often seen on 72–76 MHz (Mid-band), 450–470 MHz interstitial channels, or 900 MHz links. These links, known as
fixed stations in US licensing, typically connect an equipment site with a dispatching office.
Vehicular repeaters Modern amateur radios sometimes include cross-band repeat capability native to the radio transceiver. In commercial systems, cross-band repeaters are sometimes used in vehicular repeaters. For example, a 150 MHz hand held may communicate to a vehicle-mounted low-power transceiver. The low-power radio repeats transmissions from the portable over the vehicle's high power mobile radio, which has a much longer range. In these systems, the hand-held works so long as it is within range of the low power mobile repeater. The mobile radio is usually on a different band than the hand-held to reduce the chances of the mobile radio transmitter interfering with the transmission from the hand-held to the vehicle. • Motorola, for example, marketed a vehicular repeater system called PAC*RT. It was available for use with 150 MHz or 450 MHz hand-helds and interfaced with some Motorola mobile radios. • In the 1980s, General Electric Mobile Radio had a 463 MHz emergency medical services radio that featured a 453 MHz vehicular repeater link to a hand-held. There is a difficult engineering problem with these systems. If you get two vehicle radios at the same location, some protocol has to be established so that one portable transmitting doesn't activate two or more mobile radio transmitters. Motorola uses a hierarchy system with PAC*RT, each repeater transmits a tone when it is turned on, so the last one on site that turns on is the one that gets used. This is so several of them are not on at once. Vehicular repeaters are complex but can be less expensive than designing a system that covers a large area and works with the weak signal levels of hand-held radios. Some models of radio signals suggest that the transmitters of hand-held radios create received signals at the base station one to two orders of magnitude (10 to 20 decibels or 10 to 100 times) weaker than a mobile radio with a similar transmitter output power. ==Siting as part of system design==