Radio repeater
From Wikipedia, the free encyclopedia
A radio repeater is a combination of a radio receiver and a radio transmitter that receives a weak or low-level signal and retransmits it at a higher level or higher power, so that the signal can cover longer distances without degradation. This article refers to professional, commercial, and government radio systems. A separate article exists for Amateur radio repeaters.
In dispatching, amateur radio, and emergency services communications, repeaters are used extensively to relay radio signals across a wider area. With most emergency (and some other) dispatching systems, the repeater is synonymous with the base station, which performs both functions. This includes police, fire brigade, ambulance, taxicab, tow truck, and other services. The civilian GMRS service in the United States and UHF CB service in Australia also use repeaters in much the same fashion as amateur radio operators do; however, the Canadian GMRS service is an expansion of the Family Radio Service and is designed to operate simplex-only.
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[edit] Full duplex operation
A feature that defines the repeater is the fact that it transmits and receives at the same time, (simultaneously). A duplex repeater uses two radio frequencies; an "Input" frequency, which it monitors for signals, and an "Output" frequency, on which it retransmits the received signals at a higher power or higher altitude.
The input is the frequency on which mobile radios, or hand-held radios, transmit to reach the repeater. The output is the transmitter frequency for the repeater which repeats the audio from the received signal on the input.
The fact that the transmitter and receiver are on at the same time means some electrical 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 blinds the repeater receiver. Example: 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.
Part of isolating the transmitter and receiver is keeping the input and output frequencies distant. The further apart these two frequencies, the easier it is to keep the transmitter from interfering with the receiver. This is an engineering problem with many possible solutions.
[edit] 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.
The narrowest documented spacing found in preparing this article is a system dismantled in the 1980s. One channel used on the Riverside County, California Sheriff's Department system, which was replaced by an 800 MHz trunked system, used 158.760 MHz as input and 159.090 MHz as output, a spacing of 330 kHz. The station callsign was KMH971. It is unusual to see systems with input and output spaced this closely. It is believed working systems have been spaced as close as 175 kHz.
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 US 144-148 MHz band usually use a 600 kHz (0.6 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.
- Amateur repeaters in the US 420-450 MHz band use a 5 MHz separation with the input being either on the higher or lower portion of the 440-450 segment of the band (the standard changes regionally). Example: output is 444.400 MHz; input is 449.400 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.
These are just a few examples. There are many other separations or spacings between input and output frequencies in operational systems.
[edit] Same band frequencies
Same band repeaters operate with input and output frequencies in the same frequency band. For example, in 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.
[edit] 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 input leg while the transmitter is connected to the output leg. If the right specifications are chosen, the duplexer has a narrow-enough filter to prevent the repeater's receiver from being overloaded by its own transmitter. By virtue of the transmitter and receiver being on different frequencies, they can operate at the same time on a single antenna.
Any anomaly or fault with the antenna or antenna feed cable will reflect transmitter power back into the receiver, possibly causing the receiver to be overloaded. The reflected power will quickly exceed the duplexer's filtering ability.
There is a relationship between the frequencies on which a duplexer filter is utilized and the physical size of the filter. A band-pass duplexer for 45 MHz will be much larger than the same design built to work at 860 MHz.
[edit] Combining system
There is often not enough tower space to accommodate a separate antenna for each repeater at crowded equipment sites. In same-band repeaters at engineered, shared equipment sites, repeaters can be connected to shared antenna systems. These are common in trunked systems, where up to 29 repeaters for a single trunked system may be located at the same site. (Some architectures such as iDEN sites may have more than 29 repeaters.)
In a shared system, a receive antenna is usually located at the top of the antenna tower. Putting the receive antenna at the top helps to capture weaker received signals than if the receive antenna were lowest of the two. By splitting the received signal from the antenna, many receivers can work satisfactorily from a single antenna. Devices called receiver multicouplers split the signal from the antenna into many receiver connections. The multicoupler amplifies the signals reaching the antenna, then feeds them to several receivers, attempting to make up for losses in the power dividers (or splitters). These operate similarly to a cable TV splitter but must be built to higher quality standards so they work in environments where strong interfering signals are present.
On the transmitter side, a second transmit antenna is installed somewhere below the receive antenna. There is an electrical relationship defined by the distance between transmit and receive antennas. An desirable null exists if the transmit antenna is located exactly below the receive antenna beyond a minimum distance. Almost the same isolation as a low-grade duplexer (about -60 decibels) can be accomplished by installing the transmit antenna below, and along the centerline of, the receive antenna. Several transmitters can be connected to the same antenna using filters called combiners. Transmitters usually have directional devices installed along with the filters that block any reflected power in the event the antenna malfunctions. The antenna must have a power rating that will handle the sum of energy of all connected transmitters at the same time.
Transmitter combining systems are lossy. As a rule of thumb, each leg of the combiner has a 50% (3 decibel) power loss. If two transmitters are connected to a single antenna through a combiner, half of their power will reach the combiner output. (This assumes everything is working properly.) If four transmitters are coupled to one antenna, a quarter of each transmitter's power will reach the output of the combining circuit. Part of this loss can be made up with increased antenna gain. It is important to note that 50 watts of transmitter power to the antenna will make a received signal strength at a distant mobile radio that is almost identical to 100 watts.
In trunked systems with many channels, a site design may include several transmit antennas to reduce combining network losses. For example, a six-channel trunked system may have two transmit antennas with three transmitters connected to each of the two transmit antennas. Because small variations affect every antenna, each antenna will have a slightly different directional pattern. Each antenna will interact with the tower and other nearby antennas differently. If one were to measure received signal levels, this would cause a variation among channels on a single trunked system. Variations in signal strength among channels on one trunked system can also be caused by:
- failed parts in the combiner,
- characteristics of the design,
- loose connectors,
- bad cables,
- mistuned filters, or;
- incorrectly installed components.
[edit] Cross-band frequencies
Consider the possibility that Amateurs are likely to have been the first to experiment with, and to use, cross band repeaters.
[edit] Modern
Cross-band repeaters are sometimes a part of government trunked radio systems. If one community is on a trunked system and the neighboring community is on a conventional system, a talk group or agency-fleet-subfleet may be designated to communicate with the other community. In an example where the community is on 153.755 MHz, transmitting on the trunked system talk group would repeat on 153.755 MHz. Signals received by a base station on 153.755 MHz would go over the trunked system on an assigned talk group.
In conventional government systems, cross band repeaters are sometimes used to connect two agencies who use radio systems on different bands. For example, a fire department in Colorado was on a 46 MHz channel while a police department was on a 154 MHz channel, they built a cross-band repeater to allow communication between the two agencies.
If one of the systems is simplex, the repeater must have logic preventing transmitter keying in both directions at the same time. Voting comparators with a transmitter keying matrix are sometimes used to connect incompatible base stations.
[edit] Historic
In looking at records of old systems, examples of cross-band commercial systems were found in every US radio service where regulations allowed them. In California, specific systems using cross-band repeaters have existed at least since the 1960s. Historic examples of cross-band systems include:[1]
- 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.
[edit] 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.
[edit] 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.
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.
[edit] Siting as part of system design
Radio repeaters are typically placed in locations which maximize their effectiveness for their intended purpose:
- Low-level" repeaters are used for local communications, and are placed at low altitude to reduce interference with other users of the same radio frequencies. Low-level systems are used for areas as large as an entire city, or as small as a single building.
- High-level" repeaters are placed on tall towers or mountaintops to maximize their area of coverage. With these systems, users with low-powered radios (such as hand-held "walkie-talkies") can communicate with each other over many miles.
[edit] See also
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[edit] References
- ^ From historic records of Federal Communications Commission licenses.
