Copyright 2011 by Bruce Bartlett
Chffffft! That’s the sound of a wireless-mic dropout. The microphone makes a burst of noise or cuts out. That happens because the mic’s radio signal was lost, either by a weak transmitted signal, a weak received signal, or interference with other radio waves.
Fortunately, there are many things you can do to prevent dropouts, and we’ll cover them here.
First, remember that a wireless-mic system has two parts: a transmitter connected to a microphone on stage, and a receiver that picks up the transmitted signal and feeds audio to your mixer. The transmitter has an antenna and the receiver has one or two antennas. Each wireless mic must have its own separate receiver, and each mic/receiver pair must be set to the same channel.
To minimize dropouts, try the suggestions in this checklist.
- Use fresh batteries. Change them every 4 hours of use. Check battery voltage with a digital voltmeter and replace 9-volt batteries at about 8.5 volts or slightly lower.
- Check that “static” sounds aren’t just clothing noise.
- Put belt packs in condoms or sandwich bags to keep off sweat.
- Make sure that each mic is on its own channel, not shared with other channels. If two mics must share the same channel, turn one mic off.
- If two performers will be working close together, assign them frequencies that are as far apart as possible to prevent interference.
- Avoid channels 52 to 69 (700-800 MHz band) because those channels are no longer allocated for wireless mics.
- If a body-pack transmitter has a separate antenna wire, keep it taut with a rubber band and safety pin clipped to clothing. Do not loop the body-pack antenna cable over itself or over the mic cable. Keep the antenna and mic cables separated. Try to keep the antenna wire vertical.
- If the body-pack transmitter uses the mic cable as the antenna wire, keep it as straight as possible. Do not coil or bunch up the mic cable.
- Sweat absorbs RF energy. You might put heatshrink tubing or surgical tubing over the transmitter antenna. Try to arrange the transmitter antenna so that it’s not tight against the body.
First, note that receiver antennas can be connected directly to the back panel of the receiver, or they can be remote. A remote antenna (or two for diversity operation) is useful when you have several receivers in a rack that is far from the stage, and you want to put their common antenna closer to the actors to pick up a strong signal.
- Keep a clear line-of-sight between the wireless mics and their receivers. To do that, you might need to mount the receivers up high on a platform.
- Separate diversity antennas at least 1/4 wavelength. Angle apart the two antennas on the back panel of a diversity receiver.
- Suppose your sound system uses a pair of remote antennas that feed all the receivers. There should be an RF signal splitter (RF distribution amp) connected between the antennas and the receivers. Make sure it’s turned on.
You want to see if the remote antenna system is working. To find out, turn on a wireless mic and check the RF signal level on its receiver. Stand next to the receiver, then take the wireless mic all the way to onstage. Does the RF level drop significantly?
If so, either (1) repair the distribution amp, (2) install a new amp, or (3) connect antennas or stiff wires on the back of each receiver. Use 16” wires for VHF or 3”-6” wires for UHF. On diversity receivers, angle the two antennas apart. If several receivers are rack mounted, you might remove them from the rack and separate them; otherwise the antennas might interfere with each other’s reception.
Avoid using limp wires as antennas. Instead, use stiff wires, whip antennas or rubber duckies. Such antennas are half-wave or quarter-wave omnidirectional types.
A half-wave omnidirectional antenna
- If back-panel antennas are not working well, keep those antennas away from metal racks and metallic surfaces. Place the receivers and their antennas closer to the actors: in front of the balcony, hidden in the set, or in the wings. Place each receiver to get the best line-of-site for each actor. Remove obstacles between the transmitters and receivers. Mount antennas high to see over obstacles.
- Avoid long antenna cables. Keep the receiver antenna path as short as possible.
- Consider using one or two remote ground-plane antennas, which look like tripods. Separate them a few feet. Do not connect the antenna signals in parallel to several receivers. Instead, connect them to an antenna signal splitter (RF distribution amp) that feeds several receivers.
A ground-plane antenna
- If the distance from the antenna to the stage is more than 50 feet, consider using a remote UHF directional antenna (or two for diversity operation). Some have a built-in RF amplifier which reduces interference over long cable runs. It typically provides 10 dB of gain. If the antenna needs to feed several receivers, connect it to an RF distribution amp.
A directional UHF antenna
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Audio Engineering Society and Syn Aud Con member Bruce Bartlett is a recording engineer, live sound engineer, microphone engineer (www.bartlettaudio.com) and audio journalist. His latest books are “Practical Recording Techniques 6th Edition” and “Recording Music On Location.”