Specifications

interfere with each other if those frequencies are not
carefully chosen. The rules for frequency selection are
complex enough that computer programs are used
to calculate compatible systems. Again, the wireless
microphone manufacturer should be consulted for any
multiple system installation.
The selection of a wireless microphone system
includes several steps, some of which are similar to wired
microphone selection. It should be remembered that while
wireless microphones cannot ultimately be as consistent
and reliable as wired microphones, the performance of
present systems can be very good, allowing excellent
results to be obtained. Following these steps will help
select the best system(s) for your application.
First, define the application. In a meeting facility
system, this may be a wireless lavalier microphone for a
presenter and a wireless handheld microphone for
audience questions.
Second, choose the microphone type. The applica-
tion will usually determine which microphone type is
required: a lavalier or clip-on type attached to clothing,
or a head-worn type, both for hands-free use; a handheld
type for when the microphone must be passed around to
different users. Most handheld and headworn types are
unidirectional, while lavaliers are usually omnidirectional.
Unidirectional lavaliers are available, for use when
feedback or high ambient noise is a problem.
Third, choose the transmitter type. Again, the
application will specify the choice. All but the handheld
type will use some kind of body-pack transmitter. Some
body-pack transmitters, especially those with a multi-use
input connector, use a separate antenna wire while others
use the permanently attached microphone cable as the
antenna. A mute or audio on-off switch is desirable to avoid
turning off the transmitter power when the microphone is
not needed. Handheld types may have external or internal
antennas. Transmitter batteries may be one of several types
and their relative availability should be considered. Also,
power consumption of transmitters varies so be aware of
expected battery life.
Fourth, choose the receiver type. The basic choice
here is diversity vs. non-diversity. For reasons mentioned
in the receiver section above, diversity receivers are
recommended for all but the most budget-conscious
applications. Non-diversity types will work well in most
situations, but the extra insurance and extra features of the
diversity receiver are often worth the slightly higher cost.
Other features of the receiver, such as headphone outputs,
balanced outputs, different indicators, and potential for
battery power may be desirable.
Fifth, determine the number of systems to be used.
This should take into account future additions to the
system. Choosing a system that can only accommodate a
few frequencies may be a future limitation. It must also
take into account existing wireless systems with which the
new equipment must work.
Sixth, consult the manufacturer or a knowledgeable
professional about frequency selection to integrate the
planned number of systems. This must be done for any
multiple system installation and should be done for even
single systems to avoid potential interference problems.
Once the wireless system(s) choice is made, good
installation and proper use are necessary for satisfactory
performance. Antenna selection and placement are very
important aspects of installation. There are a few general
rules about antennas to keep in mind.
First, maintain line-of-sight between the transmitter
and receiver antennas if possible. Avoid human bodies,
metal, or other dense materials between the two.
Second, keep the distance from transmitter to
receiver as short as possible. It is much better to have
the receiver near the transmitter and run the audio
signal from the receiver through a long cable than to
transmit over long distances or use long antenna
cables. The maximum legal signal strength of VHF
systems is only fifty one-thousandths of a watt (0.050 W)!
This is a very tiny, tiny signal.
Third, use the proper receiver antenna. A 1/4-wave
antenna (about 17 inches long for high-band VHF) can be
used if it is mounted directly to the receiver. If the antenna
is to be located at a distance from the receiver, which will
be necessary if the receiver is mounted inside a metal
enclosure or at a great distance from the transmitter, a
1/2-wave or other high gain antenna should be used.
Fourth, mount antennas vertically and away from
large metal objects. This applies to receiver and to
transmitter antennas. Do not coil or fold up trailing wire
antennas, or microphone cable antennas, on body-pack
transmitters.
Fifth, use the proper antenna cable for remotely
locating receiver antennas. The correct impedance is
usually 50 ohms. Employ the minimum length necessary.
Use low-loss cable for longer cable runs.
Sixth, mount multiple antennas at least 1/4
wavelength (about 17 inches) apart. Use an amplified
antenna distribution system (sometimes called an “active”
antenna splitter) to minimize the number of antennas and
to reduce interference problems with multiple receivers.
This allows one antenna or one antenna pair (for a diversity
system) to be used with multiple receivers.
25
MEETING FACILITIES
Audio Systems Guide for