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KAW
Section 5
Precautions & Cleaning
3. Poorly coordinated frequency set
A properly coordinated set of wireless frequencies must satisfy two criteria:
Frequencies must avoid local active TV channels
Frequencies must be mutually compatible
Television transmitters may operate at power levels up to one million watts while wireless
microphone systems typically have only 50 mW (fifty-one thousandths of one watt!) or less output
power. To combat broadcast television interference, avoid using frequencies of local active TV
channels.
How local is local? "Local" is generally considered to be up to 80 or 96 kilometres, depending on
the coverage area of the particular TV transmitter and on the location of the wireless microphone
system. The good news is that indoor setups are at less risk than outdoor setups because building
structures will usually strongly attenuate TV signals. Inside buildings of substantial construction,
it may be possible to ignore TV stations as close as 48 or 64 kilometres. Still, since the locations
and assignments of television stations are well known, it's pretty easy to choose relatively safe
wireless microphone system frequencies in a particular area.
To insure a mutually compatible set of frequencies once the local TV channels have been taken
into account, it is necessary to use one of two methods. The simpler method is to use the "Group"
and "Channel" frequencies that are already programmed into the wireless systems. By using
Channels that are all in the same Group, compatibility is guaranteed for small setups of like
equipment.
There is no such thing as "set and forget".
Even if your audio system doesn't move from place to place, the radio environment can change
unexpectedly. It's largely true that television stations remain constant, but if there are other
wireless systems in the frequency band - whether it's multiple systems in your own location or
interference from the coffeehouse down the street - your wireless frequencies may need to be
adjusted. What worked at sound check may not be failsafe when the show begins. And that's why
frequency coordination is so important.
4. Poor battery management
Even though transmitter battery life is a top concern with wireless mics, users continue to try and
cut operating costs by using inexpensive batteries. Most wireless manufacturers specify alkaline
or lithium single-use batteries because their output voltage is very stable over the life of the battery.
This is important because most transmitters will exhibit audible distortion or signal dropouts when
supplied with low voltage. Rechargeable batteries often seem like the ideal solution, but many
rechargeables provide about 20 percent less voltage than a single-use battery - even when they
are fully charged.
To combat battery problems, carefully compare the transmitter's voltage requirements with the
battery's output voltage over time to make sure that the battery will last through a full performance.
For AA applications, Ni-Mh batteries may last only a couple of hours.
Using rechargeable batteries is a great way to save money and landfills as long as you or someone
on your staff is able to effectively manage them. Remove batteries from transmitters after each
performance. This will keep you from using half-dead batteries the next time you need them and
will also prevent an accidental leak from damaging your transmitter if stored for an extended period
of time.
