Microphones 559all devices are low powered so there will never be any
grossly high-powered RF interference potential. The
2.4 GHz band provides a viable alternative to traditional
UHF bands, and as long as line of sight between trans-
mitters and receivers is monitored users can easily get a
100 meter range. Another benefit of 2.4 GHz is that it
can simplify wireless inventory for traveling shows. The
same wireless frequencies are accepted worldwide, so
there is no need to adhere to the country-specific
frequency rules that severely complicate the situation
for international tours. The same applies within the
United States—the same frequencies work in all areas.
Currently wireless microphones are licensed on
several frequencies, the most common being:
The VHF bands are seldom used anymore and can
only be found on old equipment. The low band is in the
noisiest radio spectrum and, because the wavelength is
about 20 ft (6 m) it requires a long antenna (5 ft or
1.5 m). The VHF low band is susceptible to skip, which
can be defined as external signals from a long distance
away bouncing off the ionosphere back to earth,
creating interference.
The VHF high band is more favorable than the low
band. The ¼-wavelength antenna is only about 17 in
(43 cm) long and requires little space. The VHF band
has some penetration through buildings that can be
advantageous and disadvantageous. It is advantageous
in being able to communicate between rooms and
around surfaces. It is disadvantageous in that transmis-
sion is not controlled (security), and outside noise
sources can reach the receiver.
Most often the frequencies between 174 MHz and
216 MHz are used in the VHF band, corresponding to
television channels 7 to 13. The VHF high band is free
of citizens band and business radio interference, and
any commercial broadcast stations that might cause
interference are scheduled so you know where they are
and can avoid them. Inherent immunity to noise is built
in because it uses FM modulation. Better VHF
high-band receivers have adequate selectivity to reject
nearby commercial television or FM broadcast signals.
If operating the microphone or intercom on an unused
television channel—for instance Channel 7—protection
might be required against a local television station on
Channel 8. Another problem could be caused by an FM
radio station. If a multi–thousand watt FM station is
broadcasting near a 50 mW wireless microphone, even
a well-suppressed second harmonic can have an RF
field strength comparable to the microphone or
intercom signal because the second harmonic of FM
88 MHz is 176 MHz, which is in the middle of televi-
sion Channel 7. The second harmonic of FM 107 MHz
is 214 MHz, which is in the middle of Channel 13.
Thus, if a VHF wireless system is to be utilized fully,
especially with several microphones or intercoms on
adjacent frequencies, the wireless receiver must have a
very selective front end.
One television channel occupies a 6 MHz wide
segment of the VHF band. Channel 7, for example,
covers from 174–180 MHz. A wireless intercom occu-
pies about 0.2 MHz (200 kHz). By FCC Part 74 alloca-
tion, up to 24 discrete VHF high-band microphones
and/or intercoms can be operated in the space of a
single television channel. In order to use multiple
systems on adjacent frequencies, the wireless micro-
phone/intercom receivers must be very selective and
have an excellent capture ratio (see Section 16.10.1.3).
On a practical basis, this means using narrow-deviation
FM (approximately 12 kHz modulation). Wide-devia-
tion systems (75 kHz modulation or more) can easily
cause interference on adjacent microphone/intercom
frequencies; such systems also require wide bandwidth
receivers that are more apt to be plagued by interference
from adjacent frequencies. The trade-off between
narrowband FM and wideband FM favor wideband for
better overall frequency response, lower distortion, and
inherently better SNR versus maximum possible chan-
nels within an unused TV channel for equal freedom
from interference (max. 6). Poorly designed FM
receivers, are also subject to desensing. Desensing
means the wireless microphone/intercom receiver is
muted because another microphone, intercom, televi-
sion station, or FM station (second harmonic) is trans-
mitting in close proximity; this limits the effective range
of the microphone or intercom.
The UHF band equipment is the band of choice and
is the only one used by manufacturers today. The wave-
length is less than 3 ft (1 m) so the antennas are only
9 in (23 cm). The range is not as good as VHF, because
it can sneak through small openings and can reflect off
surfaces more readily.
All of the professional systems now are in the
following UHF bands:- A band 710–722 MHz.
VHF low band (AM and FM) 25 to 50 MHz
72 to 76 MHz
FM broadcast (FM) 88 to 108 MHz
VHF high band (FM) 150 to 216 MHz
UHF (FM) 470 to 746 MHz
902 to 952 MHz