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Tips on using wireless microphones
Always start with fresh batteries in the transmitter and receiver.
Most radio mics use 9v alkaline batteries. A new battery reads approximately 9.3
volts, and will gradually wear down to 5 or 6 volts. However, most radio mics
begin to function poorly just below 8 volts. So plan on changing batteries every
four or five hours, depending on your particular make and model. Experience will
dictate how long you can safely go on a single set of batteries.
Good antenna placement is crucial for optimum wireless mic performance.
Give careful thought to the line of travel between the transmitter and the
receiver. Avoid transmitting through obvious obstacles such as solid metal
objects (inlcuding lighting stands, chairbacks, furniture). Avoid proximity of
electronics, including computers, televisions sets, neon signs, and any lights
that have ballasts. Human bodies also absorb RF energy, so take that into
consideration as well.
The most convenient site to mount the receiver (antenna) may be on the camcorder
or sitting on your soundcart, but it might not be the most efficient in terms of
signal path!
Camcorders are notorious for blocking the RF signal as well as for emitting
electromagnetic interference of their own. If your receivers humm when attached
to the camera, try moving them around the camera to find the "clean" spot!
Connect your audio with good, balanced, XLR audio cables. Even if your receiver
output wire terminates in XLR, some of those output adapters may be unshielded
or unbalanced and allow electronic noise to penetrate on the radio mic side of
the XLR connector. Try using a short mic cable to go between the radio mic's own
XLR and the camera.
A similar problem with humm can occur when you plug a receiver directly into an
AC powered mixing panel. A lot of panels emitt a slight magnetic field, so use a
short mic cable to achieve at least a foot of distance.
Improve the line of sight between antennas by positioning the receiver either
higher up and/or closer to the transmitter. Use a non-metallic pole (such as a
broomstick) to hold the receiver 10 to 12 feet high, so it can look down cleanly
onto talent rather than having to look through a lot of set obstacles.
Consider placing the receiver closer to the set, or even hiding it within the
set. Run a long mic cable back to your camera or soundcart.
Some brands of radio mics allow you to remote the receiver antennas. For
instance, the Audio Technica ATW-u100 series utilize common BNC connections. A
short length (under 20 feet) of BNC to BNC antenna cable can be used to separate
the antennas from the receiver itself. Note that antenna cable (RF cable) is 50
ohm, and is DIFFERENT from BNC to BNC video cable (75 ohm).
When rigging talent, be careful not to allow the mic cable to cross over the
antenna wire. The antenna wire on the transmitter should be kept somewhat
straight; avoid letting the wire just droop over itself.
It is perfectly okay to invert the transmitter so as to allow the antenna wire
to hang straight down. There is no problem with the mic cable looping over
itself, or even being balled up with a rubber band to eliminate excess length.
A trick for maintaining the "posture" of the antenna wire is to fasten a thin
rubber band to the end of it, and then use a safety pin on the other end of the
rubber band to attach it to wardrobe. Leave a little bit of slack. The rubber
band will stretch to adjust for body movement; and it should be thin enough to
break if extreme tugging should threaten to yank out the antenna wire.
The vast majority of wireless mics sold these days operate in the UHF spectrum,
same as UHF television. Although the marketing types talk about their brands
offering up to hundreds of operating channels, it is important to note that what
they really mean are hundreds of sub-channels within just a few actual
television channels.
As more and more local television stations begin broadcasting in HDTV, there
will be fewer unused UHF television channels left open for our radio mics. Do
not be alarmed if you discover that one to two thirds of your "hundreds of
channels" are beset with interference or very short range, since this is common
in some cities.
The simplest way to scan for usable "channels" is to turn on your receiver while
keeping the transmitter off. Watch the signal strength (or antenna) indicators
on the receiver while listening carefully with headphones. If there is no
visible activity AND the headphone feed is completely clean, then go ahead and
use that channel. If you see or hear anything, then select another channel some
numbers away, and check again.
One final note about using wireless mics: try NOT to use them unless absolutely
necessary. Because of all the variables involved, there is always a chance of
losing part of the dialogue. When feasible, consider running mic cables to
talent; save the wireless for shots when cable-free mobility is essential. You
don't need radio mics to interview someone seated right in front of you.
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