THE ABCs OF WIRELESS
Oct 1, 2001 12:00 PM, James Stoffo
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A majority of RF problems stem from a few factors, and you can avoid most RF headaches if you understand three or four general rules. Here's what I call the ABCs of wireless: Antenna, Battery and Coordination. And if you want to throw a “D” in there, it'd be Diversity. We've talked about antenna problems earlier in detail, so we'll begin with “B,” battery and power issues.
We've tested just about every battery you can buy, and found that certain manufacturers are consistent, and others are very inconsistent. With rechargeables, some develop memory, others don't, and certain ones hold the charge longer. So the “B” for battery means making sure that you have the proper batteries and changing them at the appropriate time, so the transmitter doesn't die while the talent is onstage. Fluctuating AC power can have a significant effect on receivers, which become much less sensitive as the line voltage drops. Luckily, on the shows that I do, the audio company provides some type of a voltage regulator. Most Mix readers won't have that luxury, but should at least know that receiver sensitivity is proportional to the AC voltage.
“C” is for frequency coordination. All types of devices radiate RF, not only TV broadcasters. Coordination basically revolves around ensuring that your receiver is on a frequency that's absolutely clear and is not interfered with by anything else. Find out what RF exists in your area, from television stations, radio stations, police fire and rescue, DTV, other wireless mics — even your own equipment. Many audio processing devices absolutely spew RF, and simply positioning an RF receiver too close to a mixing console or outboard rack can create interference to the point of making that system unusable. I once drove for four hours across the state to fix an interference problem on an RF system. Arriving there, I moved the receiver one foot to the left. The problems disappeared, and they haven't been back since.
“D” is for diversity — the proper use of diversity. Everybody seems to believe that having a diversity receiver means you won't have dropouts, but that's not true. On a 75-foot-wide stage, where both receiver antennae are located next to the RF rack, the stage will have a dropout point. For proper use of diversity, I generally have one antenna stage left, one antenna stage right. So there's one short cable run, one long run. On the longer one, I generally insert an RF line amplifier, to make up for the cable loss. Most systems use conventional polarized antennae: In this case, make sure that they're on different polarities — one vertical/one horizontal, or both at 45° — or something of that nature.
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