Is there an advantage of a UHF wireless system over a VHF system other than frequency availability?
Frequency Bands for Wireless Microphone Systems
Every wireless microphone system transmits and receives on a specific radio frequency, called the operating frequency. Allocation and regulation of radio frequencies is 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. These specifications differ from one band to another and from one user to another within a given band. For these reasons, it is not possible to select a specific frequency or frequency band that is legally usable in all parts of the world. Furthermore, it is not possible to design a single type of wireless equipment that will satisfy the specifications of all or even most of these agencies around the world.
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 wireless microphones 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 = "Very High Frequency". UHF = "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. Wireless microphone 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.
Please note that while manufacturers must be licensed by the FCC to sell wireless equipment, it is the responsibility of the wireless microphone system operator to observe FCC regulations regarding its actual use.
Described below are the VHF and UHF radio frequency bands. Each has its own advantages and disadvantages for wireless microphone operation, based on the designated users of the band, the physical characteristics of the band, and the regulatory limitations of the band.
At the beginning of the low-band VHF range is the 49 MHz region. This region is used not only by inexpensive wireless microphones, but also by cordless telephones, walkie-talkies, and radio controlled toys. The 54-72 MHz region is occupied by VHF television channels 2 - 4. The 72 MHz area is used by "assistive listening" type wireless systems. 76 - 88 MHz are assigned to VHF television channels 5 and 6. At the top of the low band VHF range, 88 - 108 MHz is the commercial FM radio broadcast band. All of these regions have been used at one time or another for wireless microphone systems. Allowable deviation limits (typically up to 15kHz) can accommodate high-fidelity audio. This is the same deviation as for FM radio broadcast. The propagation of these RF waves through the air is very good, as is their ability to pass through many non-metallic substances. This is a result of the relatively long wavelength which ranges from 9 feet to 20 feet. The most attractive feature of operation in the low-band VHF range is low equipment cost.
Except for assistive listening systems, low-band VHF is not recommended for serious applications. Due to the 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.
The high-band VHF range is the most widely used for professional applications, and in which quality wireless microphone systems are available at a variety of prices. In the U.S., the high-band VHF range is divided into two bands available to wireless microphone users. The first band, from 169 - 172 MHz, includes eight specific frequencies designated by the FCC for wireless microphone use by the general 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 broadcast television. Legal limits of deviation (up to 12 kHz) allow high quality audio transmission. Power is again limited to 50 mW. Propagation characteristics are good and antenna length is more manageable at about 20 inches for a 1/4 wavelength type.
Unfortunately, the primary users in this "traveling" band include many business band and government operations such as forestry, hydro-electric power stations, and the Coast Guard. Since the secondary user category is not restricted, the potential for interference from both primary and other secondary users is always present. Also, general RF noise is still fairly strong in this band. Due to the limitation of available frequency bandwidth, and the spacing of the prescribed eight legal frequencies, it is only feasible to operate two or three units simultaneously on traveling frequencies. These frequencies are not generally legal outside of the U.S. or Canada.
The larger part of the high-band VHF region is 174-216 MHz. The primary users of this band are VHF television channels 7 - 13. Each occupies 6 MHz of bandwidth: channel 7 (174 - 180 MHz); channel 8 (180 - 186 MHz), etc. Once again, high quality audio is possible within legal deviation limits (up to 15 kHz) The 50 mW power restriction is the same; propagation losses are still minimal; and 1/4 wavelength antenna sizes range down to less than 14 inches.
The possibility of interference from other secondary users and general RF noise exists, but it is much less likely than for low-band VHF frequencies. In addition, 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. In some other countries, the high-band VHF range extends to 230 MHz.
Like the VHF region, the UHF region contains several bands that are used for wireless microphone 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 consequence of this is the much shorter length of antennas for UHF wireless microphone systems. One less obvious consequence is reduced radio wave propagation both through the air and through other non-metallic materials, such as walls and human bodies. 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). However, diversity receivers are very effective in the UHF band and the required antenna spacing is minimal.
While the regulations for users and for licensing are essentially the same in the VHF and UHF bands, regulations for the equipment allow two potential differences. For FM signals in the UHF band, greater occupied bandwidth is allowed. This effectively permits greater deviation for potentially wider frequency range and wider dynamic range of the audio signal. In addition, transmitter power up to 250 mW is allowed. Finally, the available radio spectrum for UHF wireless microphone system use is almost eight times greater than for high-band VHF. This allows for a much larger number of wireless microphone systems to be operated simultaneously.
In practice, the effectively greater deviation limits of UHF are not generally used because of the resulting reduction in the number of simultaneous systems that may operated: the corresponding increased occupied bandwidth of each system uses up more of the available frequency range. Also, use of increased transmitter power is rare due to the resulting severely decreased battery life and to the increased potential of mutual system interference. However, even for UHF systems with less than the maximum limits of deviation and power, the capability for an increased number of simultaneous systems is a significant benefit in certain applications. This is especially true since UHF systems can generally be used in conjunction with VHF systems at the same location without interference.
The primary economic difference between VHF and UHF operation is the higher cost of UHF equipment. Typically, it is more difficult and more expensive to design and manufacture UHF devices. This is a consequence of the behavior of high frequency/short wavelength radio signals. Current UHF systems cost substantially more than comparable VHF systems. This cost differential applies to antennas, cables, and other accessories as well as to the basic transmitter and receiver. Until the economies of scale extend to the UHF band, this premium is likely to remain. However, the trend is toward a lower range of UHF equipment prices.
The low-band UHF range of frequencies may be considered as two overlapping bands: low (450-536 MHz) and high (470-806 MHz). The primary users of these bands are business services such as land mobile radio and pagers (450-536 MHz), and UHF television channels 14 - 69 (470-806 MHz). As in the high-band VHF region, unused television channels are allotted for wireless microphone system use by broadcasters and video/film producers. Business and television are the primary users in the low part of this band, but interference from primary users is rare in the high (non-business) part of the band, due to the relatively small number and shorter range of UHF television stations. Other secondary users and RF noise are also less likely at these frequencies.
Like high-band VHF, licensing is required in this UHF band. The required minimum 1/4 wavelength antenna size for UHF radio waves is 4 - 7 inches. Equipment is moderately expensive and diversity systems are strongly recommended, but high quality audio can be achieved along with a large number of simultaneous systems.
The high-band UHF range, above 900 MHz, includes studio-to-transmitter links (STL) and other primary users. This band offers additional channels and potentially less interference from RF noise, as well as antenna lengths of only 3 or 4 inches. Other operating characteristics are similar to low-band UHF. It should be noted that the allocation of these bands is always subject to governmental change as demand for spectrum increases. In the U.S., proposals to designate unused VHF TV channels in major urban areas for use by land-mobile services are being considered, as are proposals to designate additional bandwidth to existing stations for High Definition Television. Technologies such as spread spectrum transmission may also further limit the availability of certain bands for wireless microphone use. These allocations vary from country to country in an even less predictable pattern than in the U.S.
This, along with other information about how wireless microphones work, can be found in our online publication Selection and Operation of Wireless Microphone Systems.