Gary’s Wireless Handbook, Chapter 1: Batteries, Frequency, Intermod & Antennas
By Gary Stanfill
Colmar Systems

Gary Stanfill

While wireless microphone systems offer many advantages, it’s safe to say that not all users enjoy completely reliable and trouble-free operation. Fortunately, problems are largely avoidable, with correcting them going a long way toward ensuring trouble-free operation. Here are the several common wireless problems, and more will be addressed in subsequent chapters to be posted periodically on PSW:

Batteries. A frequent complaint is short battery life, and often the equipment is blamed. However, note that nearly all wireless transmitter failures reduce current drain rather than increase it.

When turned on, wireless transmitters draw power even if not being used, and the “mute” switch does not slow current drain. It should also be remembered that synthesized (frequency agile) UHF transmitters generally have only about half the battery life of similar crystal-controlled VHF units.

A constant supply of new, fresh batteries is a must. Batteries lose their capacity even if not being used, and heat and humidity accelerating the aging process. One suggestion is noting the date a new battery is installed, and its hours of use. This can be done on a tally sheet, or even noted on the battery casing with a Sharpie pen. Despite cost concerns, it’s better to toss a battery if there’s any doubt.

If budget permits, always start with fresh batteries and replace them when about 80 percent of expected life is used. If batteries must be reused, buy a good battery tester and test all batteries before they are installed in transmitters.

Name brand alkalines such as Duracell and Eveready are the best bet. While private label batteries are often nearly as good, their useful life can vary considerably from purchase to purchase. Make sure batteries are date coded by the manufacturer, and don’t accept any with expiration dates of less than three years. And never use zinc carbon or toy batteries; most can’t even properly power up a modern wireless transmitter.

Nearly all wireless manufacturers don’t recommend rechargeable batteries, for good reason. They provide much lower capacity than alkalines, and useful life is almost always shorter. This is particularly true of 9-volt units; most retail “9-volt” rechargeable batteries supply only 7.2 volts. The worst problem is that - inevitably - someone will forget to recharge them.

Frequency Planning. Wireless systems must share the radio spectrum with television stations and several other types of authorized users that can cause interference. The first step is to determine the TV channels in the area where the wireless systems will be used.

Both analog and digital channels must be considered. Future DTV transmissions should also be taken into account. Despite the inconvenience, wireless systems should not be used on occupied TV channels. Not only interference is almost certain, it is illegal.

Note that these determinations should be made before a system or systems are specified and purchased, particularly fixed frequency wireless systems. This goes a long way to solving the problem before it occurs. And usually, frequency changes in synthesized systems are easy to attain.

As new DTV transmitters fill up previously vacant TV channels, it will become increasingly difficult to find usable wireless frequencies. Frequency planning will be more important than ever.

Intermodulation. It’s a fact: wireless systems can experience severe interference even when operating on “vacant” frequencies. This interference is created by “intermodulation distortion”, basically two strong signals on other frequencies combining in the wireless receiver to create an interfering signal.

Generally, intermodulation (or “intermod”) is more common than direct on-frequency interference from other transmitters. It’s typically caused by other wireless systems on site, and sometimes in conjunction with local TV signals.

Even single systems can be affected by intermod, but the probability of grows in proportion to the square of the number of systems in simultaneous use plus the number of active analog TV channels present. By the time eight to ten wireless systems and six to eight TV channels are involved, finding usable frequencies can be quite challenging.

When intermod occur, one or more wireless frequencies have to change; there is generally no other practical solution. Once again, synthesized systems can be very helpful in this situation because they can search for “vacant” frequencies. But any frequency can potentially interact with any other, so changing one frequency can solve one problem and create another, or several others.

When changing frequencies or searching, it is absolutely critical that all RF systems of any type at the location be turned on and operating. As one good wireless frequency is found, that system must be left on, the next system tested, and so on, until all are operational. If more than ten systems are in use, a computer program to assist becomes more or less a necessity. And, some manufacturers offer help in selecting usable frequencies, provided the necessary information data is provided to them.

Shielding or Covering Antennas. A sizable volume of free space is required around an antenna in order for it to properly launch the radio wave; a sizeable volume of free space is required around an antenna. In particular, metal objects weaken and distort transmission, and reduce range. Performance of receiver antennas will also be compromised.

For bodypack transmitters, the antenna must be kept away from both the microphone cable and the bodypack case. Securing antennas to the transmitter case and tying antennas to microphone cables, are absolutely deadly to range.

Receiver antennas must extend away from the receiver case, and away from other antennas, equipment racks, other equipment, cabling and metallic objects. When rack-mounting multiple systems, it’s best to locate the receivers at the top of the rack and extend the antennas above and away from the rack and other gear. It should go with without saying… but - using rear-mount antennas inside a metal rack will almost always result in very poor reception.

For multiple receiver installations, the common practice of positioning front-mounted antennas in a “V” configuration, with the antennas all parallel, will also reduce range. This arrangement causes the antennas to function together somewhat like a TV antenna that’s pointed upwards; again, range will suffer. When antennas from different receivers touch, range will most certainly be compromised and interference becomes much more likely.

In our next chapter, to be posted on ProSoundWeb in the coming weeks, we’ll have a look more issues that can compromise performance, including clear RF path and more antenna issues.

A regular contributor to ProSoundWeb Install Sound, Gary Stanfill is principle consultant for Colmar Systems, based in Southern California. His company has provided engineering and marketing consulting services to wireless microphone manufacturers for several years. Stanfill was president and general manager of Vega, a leading manufacturer of wireless microphone systems, for a total of 18 years. He has over 30 years experience in audio, RF technology, communications and wireless systems. He can be reached at gjstanfill@earthlink.net

For more articles by Gary, including a look at the potential impact that DTV will have on wireless systems, click here.