Every radio system transmits and receives on a specific radio frequency, called the operating frequency. Allocation and regulation of radio frequencies are supervised by specific government agencies in each country with the result that allowable (legal) frequencies and frequency bands differ from country to country. In addition to frequency, these government agencies typically specify other aspects of the equipment itself, including allowable transmitter power, maximum deviation, spurious emissions, etc.
Use of the radio frequency bands in the United States is regulated by the Federal Communication Commission (FCC). Certain frequencies within each band have been designated for use by radios as well as by other services. In the United States, the frequencies used for these systems may be grouped into four general bands or ranges: low-band VHF (49-108 MHz), high-band VHF (169-216 MHz), low-band UHF (450-806 MHz), and high-band UHF (900-952 MHz). VHF represents “Very High Frequency” while UHF represents “Ultra High Frequency.”
The FCC also determines who can operate in each band and who has priority if more than one user is operating. Primary users include licensed radio/television broadcasters and commercial communications services such as 2-way radio, pagers, and cellular telephones. Radio system users are always considered to be secondary users. In general, priority is given to primary users. Secondary users may not interfere with primary users, but secondary users may be subject to interference from primary users.
Very high frequency is commonly used for FM radio broadcast, two-way land mobile radio systems, long-range data communication, and marine communications, to name a few. Except for assistive listening systems, low-band VHF is not recommended for serious applications. Due to a large number of primary and secondary users, plus high levels of general radio frequency (RF) “noise,” this band is prone to interference from many sources. Transmitter power is limited to less than 50 mW, except in the 72-76 MHz range where up to 1 watt is allowed for assistive listening systems. Finally, the minimum proper antenna size for units in this range can be over three feet long (1/4 wavelength), which can severely limit portability and/or efficiency.
In the U.S., the high-band VHF range is divided into two bands available to radio users. The first band, from 169 - 172 MHz, includes eight specific frequencies designated by the FCC for radio use by the public. These frequencies are often referred to as “traveling frequencies,” because they can theoretically be used throughout the U.S. without concern for interference from broadcasting networks.
The possibility of interference from other secondary users and general RF noise exists, but it is much less than that for low-band VHF frequencies. Additionally, although this range includes the primary users of television channels 7 - 13, there are ample frequencies available, i.e., locally unused television channels, in almost any part of the U.S.
Like the VHF region, the UHF region contains several bands that are used for radio systems. However, certain physical, regulatory, and economic differences between VHF and UHF regions should be noted here. The primary physical characteristic of UHF radio waves is their much shorter wavelength of 12 inches to 24 inches. The most apparent result of this is the much shorter length of antennas for UHF radio systems. One less obvious consequence is reduced radio wave propagation, both, through the air and other non-metallic materials, such as walls and organic matter. This results in potentially less transmitting range for comparable radiated power. Another consequence is the increased amount of radio wave reflections by smaller metal objects, resulting in comparatively more frequent and more severe interference due to multi-path dropouts.
The UHF frequencies are typically paired to provide a duplex pair for repeater operation thus giving much greater range radio to radio than possible without a repeater. There are repeaters in the VHF band as well, but the cost of duplexers and the limited availability of frequencies tilts the scale towards UHF. Since UHF tends to be more line of sight with fewer reflections of the wave from atmospheric changes, repeater systems can share frequencies at closer distances than VHF repeater systems.
The 700/800 MHz frequency band is a portion of the electromagnetic spectrum, or frequency band, that encompasses 790–862 MHz. Public safety radio systems (such as those used by police, firefighters, and emergency medical technicians) operate in several portions of the 800 MHz band, which consists of spectrum at 806-824 MHz paired with spectrum at 851-869 MHz. The 800 MHz band is also home to commercial wireless carriers and private radio systems.
To address a growing problem of harmful interference to 800 MHz public safety communication systems caused by high-density commercial wireless systems, the FCC, in July 2004, adopted a comprehensive plan to reconfigure the band. This plan was designed to protect the lives of first responders and other emergency personnel and fulfills the Commission's obligation to promote safety of life and property through the use of wire and radio communications. So, the commercial wireless carriers were segregated from Public Safety and Industrial users in a process known as rebanding. Also, in 2012, the US Congress enacted legislation to provide spectrum for public safety users in two separate 700 blocks of frequencies. One block consists of paired frequencies in the 758-763 Mhz and 788-793Mhz bands for use in a broadband nationwide network called FirstNet. This would provide data, voice, geo-location, and other enhanced services to first responders as the network is built out. The other block of paired frequencies were allocated to public safety for use in frequency efficient systems for local and statewide communications systems, I.e. P25 trunking systems.
Even though the range is lessened in the 700/800 bands, there are ways to extend the coverage with a sophisticated system equipped with multiple transmitter and receiver sites working in conjunction to provide a wide area of communications coverage. In all frequency ranges, there are new digital modulation schemes and trunking systems that allow much better coverage and much better spectrum efficiency. From Moto-turbo to NXDN, to P25 systems, there are many ways to gain spectrum efficiency and a more consistent communications path for the men and women that put their health and safety on the line to protect and serve the general population. Many of these same systems are used to provide commercial and business communications systems to provide cost advantages, efficiencies, and safety for their employees and customers. In the end, there is no one solution to fit every need. Your communications system has to be tailored to fit your needs and budget.
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