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The Importance of Frequency Selection

Wireless microphones have moved from a place of luxury to practically a necessity in church settings and they have their advantages. However, with the increasing popularity of wireless technology, frequency selection continues to get more complicated.
In this article, you will learn about the frequency bands, frequency selection and a new wave of wireless technology that eases the frequency-selection process.

Primary Frequency Bands for Wireless Microphones
Wireless microphones operate in three frequency bands. The lowest, VHF, stands for Very High Frequency. It starts at about 25 MHz and extends to 216 MHz. UHF (Ultra High Frequency) ranges from 450 MHz to 955MHz. The most recent band, 2.4 GHz, was recently allocated by the FCC (Federal Communications Commission).

The lowest is the Lo band VHF, which is 25-50 MHz and 72-76 MHz. The Lo bands have a definite cost advantage, but are also very susceptible to interference from cordless telephones, toys, and garage door openers. This band is not currently used by major manufacturers for wireless microphones.

Presently, the most used band is the Hi band VHF, which is 169-216 MHz. Systems in this frequency band are affordable and reliable and the better systems can operate many wireless microphone systems at one time without interference (if the frequencies are selected properly). However, this band is becoming congested in many areas. Frequency coordination with other nearby users is important if you want to add an additional wireless microphone and avoid interference problems.

Traditionally, the highest frequency band has been the UHF band, primarily between 450-955 MHz. The UHF band provides very low interference. The equipment available on these frequencies is of high quality and reliability and many systems can operate at the same time without interfering with each other. But, in the past, it has also been very expensive, generally limiting their use to professional theatre or broadcast applications. Fortunately, these systems have now become much more affordable and there are now UHF systems in almost every price bracket.

The newest band, 2.4 GHz, was recently allocated by the FCC and is available for unlicensed devices such as wireless microphone systems.

Frequency Selection
It is a fairly easy process to select a frequency for a single wireless system. The major consideration is other wireless systems located within a mile. As wireless microphones become even more popular, this becomes a more difficult task. There are few churches that don’t have at least one system.

When a second system is added, there must be at least 400 kHz between the two frequencies. A complex equation is used to discover which frequencies are compatible.

As more systems are added, the math becomes increasingly complex. With the addition of a third system, another phenomenon called intermodulation (IM) must be considered. Basically, IM occurs when transmitters are in close proximity. The signals combine to form a new, different frequency, which can be picked up by a receiver on a corresponding frequency.

Help in selecting a frequency, without having to learn the in-depth equations and theories, can be obtained from a systems contractor or wireless microphone manufacturer. Shure’s web site, www.shure.com, and Audio-Technica’s at www.audio-technica.com both provide frequency resources. All of the wireless manufacturers have staff who can help in selecting the proper new frequency. When you call them, have a list that includes: 1. Your church’s present models and their frequencies. 2. Nearby frequencies that are being used. 3. A list of local TV stations. Don’t forget the new digital TV channels.

With the advent of digital TV, many TV channels have been assigned new digital channels in the UHF band. These could be or will be operating at the same frequencies as some existing UHF wireless microphones.
Wireless mics are mostly unlicensed, so any TV channels have priority. If you are experiencing interference from a digital station on a fixed frequency wireless microphone system, you are, unfortunately, out of luck. Your only choice is to send the unit to a service station and have the frequency changed.

New Technology
Many new UHF systems are multi-channel. If you have interference, it is simple to change the frequency of your system. Figure 1 shows a rack of wireless microphones. The top two units have over 1400 selectable
frequencies in their frequency band.

Many systems also have the ability to find the frequencies with the least amount of interference. Just push a button on the receiver and it scans the available frequencies and picks one. Manually tune the transmitter to the same frequency and you’re ready to go.

Figure 2 shows a system that can automatically tune each receiver in multiple systems. Just push a button and the first receiver scans the band to find the least interference. Then, the other receivers are set automatically to provide interference-free operation between themselves. All the operator needs to do is adjust the transmitters to the auto-selected frequencies.

In the past, it has been said that no UHF wireless microphone could operate legally around the world. However, one manufacturer can now boast that their product is the perfect solution for international traveling ministry groups. Recently available for purchase, their True Mobility (shown in Figure 3) series microphone operates in the 2.4 GHz band, even higher than the UHF frequency band. But it still remains the user’s responsibility to know the local frequency operating rules.

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