Land mobile radio system (LMRS), also called public land mobile radio or private land mobile radio, is a wireless communications system intended for use by terrestrial users in vehicles (mobiles) or on foot (portables). Examples are two way radios in vehicles. Such systems are used by emergency first responder organizations such as police, fire, and ambulance services, public works organizations, dispatched services such as taxis, or companies with large vehicle fleets or numerous field staff. Such a system can be independent, but often can be connected to other fixed systems such as the public switched telephone network (PSTN) or cellular networks.
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Military use
The land mobile radio system is also the United States Department of Defense's new state-of-the-art communication system. Government contractors, such as Cassidian Communications, Relm Wireless Corporation, Harris Corporation, Kenwood Communications, Motorola Solutions, Raytheon, Thales, and Tyco Electronics, provide the latest in LMRS technology to the government and military.
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Commercial use
Many businesses and industries throughout the world use LMR as their primary means of communication, especially from a fixed location to mobile users (i.e. from a base site to a fleet of mobiles). Commercial LMR Radios are typically available in two VHF and the UHF frequency bands. 30-50 MHz (sometimes called "Low VHF Band" or "Low Band"), 150-172 MHz (sometimes called "High VHF Band" or "High Band"), 450-470 MHz (called, simply, "UHF" for "Ultra High Frequencies", a term created in the 1950s). Many larger populated areas have additional UHF frequencies from 470-490 MHz, and 490-512 MHz. Low band has longer range capability, but requires mobile antennas as long as nine feet (2.7 m) tall. VHF bands works well in outdoor environments, over bodies of water, and many other applications. UHF bands typically perform better in urban environments and with penetrating obstacles such as buildings. There are also frequencies in the 800 and 900 MHz range available. Commercial, public safety and government users are required to obtain U.S. Federal Communications Commission licensing in the United States and must follow Government law.
Interference in the spectrum
In November 2005, many automatic garage doors in Ottawa, Ontario, Canada, had suddenly stopped working, due to a powerful radio signal that appeared to be interfering with the remote controls that open them.
In the summer of 2004, garage door operators noticed similar phenomena around U.S. military bases. The strong radio signals on the 390-megahertz band simply overpowered the garage door openers. One technician likened it to a whisper competing with a yell.
Repeaters
Mobile and portable stations have a fairly limited range, usually three to twenty miles (~5 to 32 km) depending on terrain. Repeaters can be utilized to increase the range of these stations. They are usually placed upon hills and buildings to increase range.
Repeaters are simply two radios tied together using either a computer based controller or a proprietary controller in commercial units. The controller takes the carrier signal from the receiver radio, converts it into a simple "On" or "Off" signal and activates the transmitter radio, all while passing the audio from the receiver to the transmitter with buffering and filters to remove unwanted noise and prevent noise from being generated within the circuit. Most controllers also decode Continuous Tone Coded Squelch System tones, which allows the repeater to activate only if the station is sending a particular pre-programmed code, preventing unauthorized stations from using the repeater. Additionally, as repeaters are placed on high locations it also prevents distant stations on the same frequency from interfering. A Morse code or a synthesized voice module may produce station ID to comply with station identification regulations; this is most common on amateur radio repeaters, some LMR stations must also identify to be legal in some areas.
Along with the repeater units, most stations utilize duplexers. These are notch filters usually in an array of six, eight, and sometimes four units. They separate the transmitter and receiver signals from each other so one antenna and coaxial line can be utilized. While this solution is very efficient and easy to install, factors such as humidity and temperature can affect the performance of duplexers, so in most configurations a dryer is installed to keep humidity out of the duplexers and coaxial cable, along with heated buildings in which they are installed in. Excellent quality coaxial cable, connectors, and antennas must also be used, as a single-antenna is not as forgiving as a dual antenna system since any RF leakage or poor connection can greatly decay the reliability and performance of the repeater. In some applications, cables going from the repeater and duplexers must be tuned to mitigate these issues.
In dual antenna systems, there are two antennas and two lengths of coaxial cable running from the transmitter and receiver. Usually, triple shield coax and or low loss Heliax are used to keep the two systems isolated. Two antenna systems are usually used if tower space is not limited, or space to build an array is available. The only issues with dual antenna systems is isolating the antennas so the receiver is not receiving what the transmitter is putting out. If this happens, it creates a loop, much like the feedback heard when a microphone is placed near a speaker. When this happens the repeater amplifies its own signal until it is either powered off or a TOT (time out timer) is expired.
To solve this antennas must be placed several wavelengths from each other in opposite vertical planes. For example, the receiver antenna is vertically polarized, while the transmitter antenna is placed one wavelength (or more) below the receiver antenna, but rotated 180° as to maintain vertical polarization. Antennas that have a null spot directly above and below them are excellent choices since another antenna can be placed in the null zone and isn't affected as much. Antennas must also be polarized the same as the stations trying to access the repeater--usually vertical polarization.
Source of the article : Wikipedia
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