Wireless Mic FAQ

Wireless Microphones have always interested me. Since high school, I have been fascinated with how they work, and how to improve their performance. To that end, I have written a guide to wireless microphones based on my experience and on articles, list-serv posts, and books that I have read.

General Questions

What is a wireless microphone? What does it consist of?

A wireless microphone is a microphone that is connected to a sound system with a radio-frequency link, as opposed to a cable. It consists of two parts, a transmitter and a receiver. The transmitter is built into the microphone, and it sends out a radio signal with the audio data on it (just like an FM radio station). The receiver takes this signal and outputs it out on an XLR cable, which can be used just like a wired microphone. Only one microphone can be used with a single receiver.

What kinds of transmitters are there?

There are three major types of transmitters:

  • Handheld Transmitters: These transmitters are typically built into a slightly larger-than-normal handheld microphone. These types of mics are easy to hold and sing or speak into.
  • Bodypack Transmitters: These transmitters are typically small boxes, about the size of a deck of cards (some are a bit smaller) and either clip onto one’s belt or are hidden inside a costume. They are connected to a small microphone which is usually mounted on the face of an actor or on the lapel or tie of a speaker. These types of microphones are very useful for musical theatre in which the actor’s hands need to be free, or in presentation environments where the speaker needs to be able to walk around freely.
  • Plug-On Transmitters: These less-common transmitters have an XLR port on them and either plug directly into a wired vocal microphone, or can be hidden in a podium or other location to make a standard wired microphone wireless. These transmitters usually provide phantom power to the microphone attached to them as well.

What radio frequencies do wireless microphones operate on, and who else uses them?

Wireless microphones operate in the VHF and UHF part of the radio spectrum, typically in the ranges from 174-216 MHz and 470-698 MHz in the United States. Other countries allow microphones to operate as high as 900 MHz. Licensed television stations typically operate in and share this range.

Do I need a license to use a wireless microphone in the US?

No. As of January 15th, 2010, the Federal Communications Commission granted a waiver to Part 15 of their rules to allow wireless microphones to operate in the ‘core TV bands” (54-72 MHz, 76-88 MHz, 174-216 MHz, and 470-698 MHz less channel 37). These microphones may operate with an output power of up to 50 mW, though requests for exceptions will be granted if they are deemed necessary. THis waiver expires on June 12th, 2010.

The FCC is currently seeking comment on what final rules to develop with regard to wireless microphones in the US.

Additionally, there are eight frequencies, commonly referred to as the “Hydrological Frequencies” that microphones may be legally operated in without a license. To use a microphone in this band, the transmitter must output no more than 50 mW of RF power. The permitted frequencies are:

  • 169.4450
  • 171.0450
  • 169.5050
  • 171.1050
  • 170.2450
  • 171.8450
  • 170.3050
  • 171.9050

I’ve seen ads for 900 MHz wireless mics. These ads promise interference-free operation because they are not in the TV band. Should I buy these mics?

There are two types of 900 MHz wireless microphone systems, ones that operate in teh license-free ISM band from 902-928 MHz and ones that operate between 944-952 MHz.

If you are in the United States, absolutely do not by 950 MHz units. These microphones operate in that is commonly called the Aural STL band. This is the band where radio stations send the signal from their studio to the broadcasting tower to be broadcast. Even though wireless mics are low power, the STL receiver is on a tall mountain and using a very sensitive antenna – so the potential for interference is high. If you are caught using a mic here, the chance of being fined is very high. It would also be very poor engineering practice to operate here.

As far as ISM band mics go, they may work for you but they are generally not considered to be professional grade units.

What about 2.4 GHz wireless mics?

A couple companies are now making wireless microphones that operate at 2.4 GHz. While these mics may actually perform fairly well, the RF environment at 2.4 GHz is unpredictable at best, and hence wireless mics may be subject to random interference issues. Therefore, I do not recommend the use of 2.4 GHz wireless mics.

I have VHF Wireless Microphones. Will I have problems with them in the future?

Yes and No. While this band has not been removed from the TV service, the television allocations have been changed significantly with the transition to digital TV. This may have an impact on your operations of a new transmitter has come online on what was a clear channel. The good news is that broadcasters tend to be moving to UHF, and portable consumer TV Band devices will operate at UHF only, so VHF may end up proving to be a good bet.

You can see where new TV stations will be coming online at <http://www.rabbitears.info>.

Why are wireless microphones so expensive? Will El-Cheapo brand work for me instead of this expensive unit?

Two things make wireless microphone systems expensive: reliability and performance. Good wireless microphone systems have parts that are made of metal to resist the damage and abuse that they will suffer in their years of use. Good systems also use very good designs and components to ensure high-quality operation. Generally speaking, more expensive units are less prone to interference from TV stations and other microphones, have better sound quality (both in terms of frequency response and dynamic range), and sound more natural compared to less expensive systems.

Unless you’re using only one wireless microphone in the middle of Siberia (please check to ensure you are operating legally in Russia), it is highly recommended that you use a wireless microphone by a well-known manufacturer that costs over $500/channel (that is, for a transmitter and receiver pair – excluding the microphone element). This will ensure that you have quality equipment that will perform well.

“White Space” (TV Band) Devices and Interference

I’ve heard that the FCC is going to allow consumer broadband devices in the TV band? What’s the deal with that?

The FCC voted on November 4, 2008 to allow unlicensed devices to operate in the UHF television bands in the US. These devices will be required to ensure that they are operarting on a clear frequency by checking a database every day. THe devices will not be required to perform a spectrum sweep, however.

In every market, there will be two TV channels set aside for wireless microphone use, and they will be the first free TV channel above and below TV Channel 37. These channels are the safest bet for wireless mic use. In addition, the VHF TV channels will likely be fairly clear for wireless mic use.

How will these devices affect my microphones?

These devices will serve to raise the noise floor for wireless microphone systems. They may cause other interference as well, but this has not been shown yet.

How can I avoid these problems?

The best way to avoid interference is to purchase quality wireless microphone systems that have a receiver with very tight filtering. This means that the receiver very carefully picks out the signal it is trying to receive and discards other signals, some of which may be very close in frequency to the microphone’s operating frequency. Not only will this decrease the chance of interference, but it may also help improve range and overall performance. Also, use directional antennas with short coax cable runs, and avoid the use of inline preamplifiers in your wireless mic system.

700 MHz and Digital TV

I have heard that the 700 MHz part of the spectrum is going away. Is this true and how does it affect my microphones?

Effective June 12, 2010, it is illegal to operate microphones in the 700 MHz (698-806 MHz) band. See the FCC’s Consumer Advisory for more information on this.

Are all television stations going digital, and when will this happen?

Most television stations are now required be required to transmit in digital only, as of June 12, 2009. There are a few exceptions to this rule, however – translators and low power TV stations will remain analog for a few more years, as will Mexican and Canadian stations that serve viewers in the United States.

Planning your System

Is it necessary to plan out what frequencies/channels my microphones operate on?

Yes, it is. Due to the nature of of multiple radio frequency carriers traveling through electronic circuits, these carriers will mix with each other when going through any kind of amplified stage and create “phantom” signals. This phenomenon is called intermodulation. The phantom signals are called intermodulation products.

I just set my frequencies to be separeted by x MHz. Will that work OK?

Maybe, maybe not. While channel spacing is important, if you are using any kind of preamplifier or antenna distribution system, it is possible for these signals to mix with each other. Since they are all offset by the same distance, it is quite possible that all of the signal could recieve interference. It is highly reccomended that you use a computer program to plan out your frequences (see below).

What is intermodulation?

Simply put, intermodulation is the mixing of signals in (nonlinear) circuits such as receivers and amplifiers to create additional, undesired signals. Imagine two microphones, operating on frequencies A and B. Intermodulation products will occur on any frequency that meets this requirement: (±m*A) + (±n*B), where m and n are integers greater than zero. So, for example, an intermodulation product will occur on 2A-B. Because of this, it is necessary to ensure that no other microphones operate on that resulting frequency. The most common and strong products are those of the form 2*A-B and 2*B-A.

Additionally, in systems with more than two systems, products occur on any combination of frequencies: (±(x_1)*(F_1)) + (±(x_2)*(F_2) + … + (±(x_n)*(F_n)), where x_n is an integer greater than, and F_n is a frequency used by a wireless microphone. As you might imagine, this formula (and indeed the first one as well) predicts an infinite number of combinations. The good news is that the higher-order products (where x_n is larger than 5 or so) tend to be much, much lower in amplitude than the lower ones and therefore can be ignored.

I’m lost. How can I plan out my frequencies without taking calculus, linear algebra, differential equations, and quantum mechanics?

There are computer programs available which can very quickly compute a set of frequencies you can use for your systems. The most popular is a commercial program by Professional Wireless Systems called IAS. Sennheiser has a free program called SIFM that is slightly more difficult to use. Shure also makes a free program, available for Windows and Mac OS X, called Wireless Workbench. However, Wireless Workbench is limited to Shure systems only (as opposed to IAS and SIFM).

Other manufacturers have simple web-based applications to perform this task, but they do not take into account other systems that you may know about (such as IEM system and other wireless mics not made by that manufacturer). The above programs can take all of these factors into account.

Antennas, Multicouplers, and Coax Cable

What kind of antenna do I need for my system?

That depends. There are a number of types of antennas available for wireless microphone systems:

  • Omnidirectional Antennas: These are best for situations where there is not much noise near the frequencies being used by the microphone systems. Noise is often reduced by buildings, distance from television transmitters, and antenna location (lower is better). It is worth noticg that while these antennas pick up signals equally well from alld irections, they typically respond better to signals polarized the same direction as the antenna (for example, if the antenna is mouted vertically, the bodypack antenna should also be vertical).
  • Directional Paddles (Yagi’s and Log Periodic Dipole Arrays): These antennas are better suited for noise environments. By their nature, they are directional and receive well toward their front, reducing or nulling noise toward their rear. Some LPDA’s designed for wireless microphone use include preamplifiers as well. One downside to these antennas is that they inherently receive signals polarized one way better than signals polarized perpendicularly to the antenna. Directional antennas wil also give increased range as they have forward gain.
  • Helical Antennas: Helical antennas are directional antennas, but unlike LPDA’s and Yagis they are designed to receive signals of all polarizations equally well.

How many antennas do I need?

Almost all wireless microphone systems use two antennas for reception. This is called Diversity reception, and it helps to improve performance by reducing or eliminating dropouts.

How does diversity reception work?

There are many ways of implementing diversity reception, but they all work on the same principle. There is a chip inside the receiver which compares the two signals received by each antenna, and selects the one with the better signal. Some receivers do this directly at the antennas, while others do it after the audio has been recovered (meaning there are two demodulators in the receiver). Finally, some high end systems actually use two bodypacks and two receivers, operating on separate frequencies.

What kind of coax do I need to use?

Many people will tell you that you need very expensive 50-Ohm coaxial cable for your wireless microphones. However, this is not necessarily true. It has been shown by Jim Brown [1] that 75-Ohm CATV coaxial cable can perform just as well as 50-Ohm coaxial cable of a similar quality. Additionally, 75-Ohm cable is much more affordable. While it is always preferable to use the highest quality coax cable you can afford to, it is worth considering the use of 75-Ohm cable (preferably RG6 Quad Shield with a solid center conductor) if your budget is tight. Any double shielded, braid over foil, construction is the preference.

What is a Multicoupler, or Antenna Distribution System?

An antenna distribution system serves a number of functions. First, it amplifies the signal received by the antennas. It then splits it four ways (typically) to allow multiple receivers to share the same antennas. Finally, it often includes a power supply to reduce the number of power supplies in the wireless rack.

Do I need a multicoupler?

Multicouplers are generally a good idea for wireless microphone systems consisting of four or more units operating within the same frequency range. If you have two or more bandsplits (frequency ranges) in your system, you should have a separate multicoupler and pair of antennas for each, unless the multicoupler is designed for wideband operation.

References

Please note that I am not a lawyer, and that in no way should the advice given in this article be considered “legal” advice.

About the author:
Mike Benonis is a graduate of the Charles L. Brown Department of Electrical and Computer Engineering at the University of Virginia. Mike is currently attending Virginia Polytechnic Institute and State University as a graduate student in Electrical Engineering. In his free time, Mike works in the radio industry as a broadcast engineer.