Wireless Mic Applications - Prior to Installation

Site Survey

Once you have arrived at a wireless system that you are comfortable with, be it a microphone, auditory assistance, or intercom system a few things should be done to prevent you from having to make service calls to an unhappy customer. First, a good site survey prior to ordering the new system will help to ensure that you will not encounter interference from a local source. One very good tool for this is either a frequency scanner, or a spectrum analyzer. Since the spectrum analyzer can be a very expensive investment, a simple programmable scanner such as a Uniden BC-175 is often a valuable tool in determining frequencies to avoid. The scanner should be set for the “priority mode” to search for any recurring frequencies in the range desired. This type of scanner sells for $220 and will scan between 148-174 MHz. Above 174 simply check the local TV channels to determine what is unused.

FCC Rules

For most VHF wireless microphone applications you must use one of the eight “traveling” frequencies between 169 and 172 MHz. The only exceptions are for licensed broadcasters or individuals connected with the motion-picture film and video industries. It is necessary to apply for a license for the traveling group as well. It is advisable to handle all these details in advance of turning the system over to your customer before they read the manual and become intimidated of the FCC requirements. Transmitters in this band are licensed under part 74 of the FCC rules, while the receiver is covered under Part 15.

Multiple System Installations

One very commonly asked question is if more than one transmitter can share a common receiver. The answer is no! In the theoretical sense, a conventional receiver is incapable of discriminating between two identical RF signals operating at the same time. The “capture effect” will force the receiver to lock-on to the signal with the greatest strength. If the two signals are nearly equal in strength, they will simply beat against each other resulting in some audio mixed with heterodyning and noise at the receiver. Multiple transmitters can be used with the same receiver provided only one is turned-on at a time. This is an economical way have more than one person use the system, but can be disastrous if the individuals are not careful to insure that only one transmitter is on at a time. The preferred way to handle multiple systems is to use a discrete system and frequency for each microphone desired.

Multiple Frequency Systems

As explained earlier, the only effective way to use multiple transmitters is to have discrete frequencies for each system. Although this may sound very simple, it really is a very difficult and often time consuming process. Many people have made the mistake of assuming that a minimum frequency separation between two transmitters is all that is necessary to properly set-up any number of frequencies. In reality this could not be farther from the truth.

Successful multi-system installations require a great deal more than just different frequencies all placed together. When multiple transmitters operate in the same environment, the multiple RF transmissions interact with each other mathematically creating numerous harmonics that combine with the original frequency inside the receiver. These extra harmonics are referred to as intermodulation distortion, or more commonly “intermod.” Intermod is generally responsible for many wireless microphone “interference” complaints ranging from noisy audio, to outright on-channel interference. A simple selection of different frequencies spaced apart from each other is not enough, because the harmonic relationship between frequencies is not related to channel spacing, but rather mathematical combinations.

Receivers are also not immune from this phenomenon because their internal local oscillators can also inte ract when operated in close proximity to each other as in equipment racks. Receiver by-products are usually referred to as local oscillator harmonics.

When transmitters combine to form by-products other than the original carrier frequency, it is important to remember that these combinations occur inside the receiver, so a good receiver design can have a dramatic effect on the extent of interference experienced.

The system contractor does not normally have to calculate these products since most quality wireless manufacturers have either charted many combinations or are able to use a computer program to handle the tedious calculations. In the example shown, only four frequencies were used, and the intermod was only calculated to the 5th order. Even with only 5th order calculations, there were still 25 combinations that needed to be done in order to plot all of the harmful harmonics generated by only four frequencies.

The important point to remember is that frequency selection should be a very serious process, and guesswork must be eliminated from the selection or you will end up with major problems. If you have chosen a quality product, and installed the system according to all the manufacturer's directions, you can be successful 99% of the time. If you do encounter a problem, work with the manufacturer and give them all pertinent details to help to isolate the difficulty. Don’t overlook the obvious such as batteries, and other non-related equipment in the building such as lighting control systems. Simple replacement of the unit without determining the cause is usually a good way to insure that your customer will have further problems down the road. Use the resources available to you like the Sales Representative or Factory personnel to help you design the best system for the budget and geographical area.

This paper is intended to give the user a basic level of understanding of wireless microphone technology. For advanced topics and a more in-depth discussion of many of the topics discussed here, read The Amateur Radio Relay League Handbook. Copies of the ARRL Handbook can be purchased directly from ARRL, 225 Main Street. Newington, CT 06111, 203-666-1541

Jeff Peters
Telex Communications, Inc. 1995

WIRELESS MICROPHONES

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