Frequency Spectrum for Wireless Systems

There seems to be such a mystery about what makes a wireless system work, but sometimes it's easy to think of a wireless system as a small radio station. Like a radio station, a wireless microphone operates on a specific frequency or carrier, and this carrier can be identified by its cycles per second (Hz). Remember that Hz stands for Hertz or vibrations per second, and kHz stands for kilohertz or thousands of cycles per second, and MHz stands for megahertz or millions of cycles per second. The commercial FM radio band operates between 88 MHz and 108 MHz or 88 million cycles per second to 108 million cycles per second.

The frequency that a wireless system operates on is rather unimportant provided the operating frequency is above atmospheric problems. Generally as long as the operating frequency is above 50 MHz, atmospheric problems from weather and sunspots will not affect performance. Over the years, some frequencies have developed reputations for problems, but this is due to the allocation of that frequency by governmental agencies rather than the physics of the frequency. There really are no "bad" frequencies in the sense that a quality wireless system can utilize almost any frequency range.


Low Band (49 MHz)

This very common band has been used for years for such things as toys (walkie talkies), cordless telephones, remote control radio equipment and others. This range of frequencies tends to be "polluted" and unusable due to the number of devices that operate in the band. In addition, the lower frequencies experience difficulty attributed to radio frequency interference (RFI) produced naturally by the sun during the eleven year sun spot cycle.

Mid Band (72-76 MHz)

With the advent of the Americans with Disabilities Act (ADA) this range of frequencies has come into widespread use for hearing assistance systems. This range is sandwiched between TV channels 4 and 5 (channel (4) 66-72 MHz, and (5) 76-82 MHz) In 1982, the FCC adopted this range of frequencies for use by low power hearing assistance systems. In general, the range tends to be clear with the exception of some urban areas where old style Police/Fire call boxes or an occasional commercial paging service may still be using these frequencies. While these frequencies are clear from atmospheric problems, they are not legal for use as “wireless microphones” and so use is limited to hearing assistance systems and auditory trainers.

Commercial FM Band (88-108 MHz)

This range is most commonly used for commercial FM radio broadcasting. It has also been used for years by very low power “Mr. Microphone” transmitters. This range is technically legal for use by a low power wireless microphone, but Commercial stations operate at such extreme power levels (often 100,000 watts) that low power transmitters are usually swamped by the stronger signal. This generally makes use of this range of frequencies for wireless microphones very impractical.


VHF High Band (150-174 MHz)

This has been the most popular range for wireless systems for the past 10 years. The lower end 150-160 tends to be used by Police/Fire, and business. The 169-172 range is most commonly referred to as the “Traveling Band” and is used primarily for wireless microphones. In recent years this range has become very congested due to the sheer number of users, and in many areas these frequencies may be too congested to get reliable performance. Since wireless microphones operate with 50 mW or less, systems operating in this range are commonly affected by high power hydrological transmitters and radio frequency interference generated by computers and lighting control systems. In spite of this, the 169-172 MHz “traveling frequencies” are the only legally licensable VHF frequencies available to a non-broadcaster user. The other option is to look at the range from 150-168 MHz for possible open frequencies. Use in the entire range is subject to FCC rules part 90.

VHF Broadcast Band (174-216 MHz)

This range contains the upper Television broadcast band (channels 7-13) and is reserved for licensed broadcasters, and individuals connected with the TV & motion picture production. Operation here is usually clean provided there are no local TV stations using the frequency. The FCC does not assign adjacent channel operation, meaning that in the upper VHF range (7-13) that if a channel 7 is present, there will not be a channel 8, and so forth. Cable TV does not interfere with wireless microphone operation as it is a closed circuit system, and not broadcasted into the environment. There are usually open upper VHF TV frequencies in any given city, but use is governed by the FCC rules under part 74.


UHF Low (450-469) MHz

This range of frequencies is set aside for commercial users such as business communication. Use is governed by FCC rules part 90, and is on a strict license basis. Higher power outputs are allowed in this range which can complicate the use of a low power device such as a wireless microphone.

UHF Mid (470-806) MHz

This range of frequencies comprises the balance of the television broadcast band. The same conditions on adjacent channel operation apply to these frequencies just as the upper VHF broadcast band. This range of frequencies represents the newest area for wireless microphones because of the vast range (336 MHz UHF versus 42 MHz VHF) and there is very little usage by comparison with VHF. The FCC (U.S.) and DOC (Canada) rules allow wider transmission bandwidth and far greater output power, which results in superior audio quality, and longer operating range. The greater total bandwidth also allows for more simultaneous operating channels than the VHF range.

UHF High (902-928) MHz

This is the new “Junk Band” where operation is license free. Like the 49 MHz band, this range is typically not suitable for wireless microphones because of numerous consumer products, such as cordless telephones, toys, etc. that use these frequencies. In addition, the 902-928 band allows for operation of transmitters up to 50 watts of output power. High power transmitters in nearby locations tend to desense the receiver and make low power wireless microphone systems unusable and subject to serious performance problems. For this reason, serious wireless microphone manufacturers have shied away from this band of frequencies.

UHF High (947-952 MHz)

The STL (studio transmitter link) band is a relatively small (5 MHz) range, and is a set aside for high power data transmitters for broadcast use. This range is so small, and currently used by high power devices, that it is not a reliable band for wireless microphone systems.


CONTINUE: WIRELESS MICROPHONES

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