"travelling frequencies," because they can (theoretically) be
used throughout the US without concern for interference
from broadcast television. Legal limits of deviation (+
/_12
KHz) allow high quality audio transmission. Once again,
power is limited to 50 mw. Propagation characteristics are
good, and antenna length is more manageable at about
one-half meter for a quarter-wave type.
Unfortunately, the primary users in this band include
many business band and government operations such as
digital paging services, forestry, hydro-electric power
stations, and the Coast Guard. Since the secondary user
category is not restrictive, the potential for interference
from both primary and other secondary users is always
present. Also, general RF noise is still fairly strong in this
band. In addition, due to the limitation of available
frequency bandwidth, and the spacing of the prescribed
eight frequencies, it is only feasible to operate, at most, two
or three units simultaneously on travelling frequencies.
Finally, these frequencies are not generally legal outside of
the US and Canada.
The larger part of the high-band VHF region is 174-216
MHz. This band is designated by the FCC for use by
broadcasters and by commercial film/video producers ("Part
74"). The primary users of this band are VHF television
channels 7-13. Once again, high quality audio is possible
within legal deviation limits (+15 kHz). The 50 mw power
restriction is the same as for low-band, propagation losses
are still minimal, and acceptable quarter-wave antenna sizes
range down to less than one-half meter.
The possibility of interference from other secondary
users and general RF noise exists, but it is much less
likely than for low-band frequencies. In addition,
although this range includes powerful primary users
(television channels 7-13), there are ample frequencies
available (locally unused television channels) in almost
any part of the US.
Selection
and Operation
of Wireless Microphone Systems
CHAPTER 3
Wireless System Operation
23
UHF VS. VHF
Like the VHF region, the UHF region contains several
bands that are used for wireless microphone systems.
However, certain physical, regulatory, and economic
differences between VHF and UHF regions should be
noted here. The primary physical characteristic of UHF
radio waves is their much shorter wavelength (one-third to
two-thirds of a meter). The visible consequence of this is
the much shorter length of antennas for UHF wireless
microphone systems. Quarter-wave antennas in the UHF
range can be less than 10 cm.
There are other consequences of the shorter UHF
wavelength. One is reduced efficiency of radio wave
propagation both through the air and through other non-
metallic materials such as walls and human bodies. This
can result in potentially less range for a UHF signal
compared to a VHF signal of the same radiated power.
"Line-of-sight" operation is more important in the UHF
range. Another consequence is the increased amount of
radio wave reflections by smaller metal objects, resulting in
comparatively more frequent and more severe
interference due to multi-path (dropouts). However,
diversity receivers are very effective in the UHF band, and
the required antenna spacing is minimal. Finally, the
signal loss in coaxial antenna cables is greater in the UHF
range. Amplifiers and/or low-loss cable may be required
in UHF antenna systems.
While the regulations for users and for licensing are
essentially the same in the VHF and UHF bands (FCC Part 90),
regulations for the equipment allow two potential
differences. For FM signals in the UHF band, greater occupied
bandwidth is allowed. This effectively permits greater FM
deviation, for potentially greater audio dynamic range. In
addition, greater transmitter power is allowed (up to 250 mw).
Finally, the available radio spectrum for UHF wireless
microphone system use is eight times greater than for
high-band VHF. This allows for a much larger number of
systems to be operated simultaneously.
In practice, the effectively greater deviation limits of UHF
are not generally used because of the resulting reduction in
the number of simultaneous systems that may operated:
the corresponding increased occupied bandwidth of each
system uses up more of the available frequency range.
Also, use of increased transmitter power is rare due to the
resulting severely decreased battery life and to the
increased potential of mutual system interference. Even
with limited deviation and power, however, the capability for
an increased number of simultaneous systems is a
significant benefit in certain applications. This is especially
true since UHF systems can generally be used in
conjunction with VHF systems at the same location without
mutual interference.
The primary economic difference between VHF and
UHF operation is the relatively higher cost of UHF
equipment. Typically, it is more difficult and hence more
expensive to design and manufacture UHF devices. In
many ways this is a consequence of the behavior of high
frequency (short wavelength) radio signals. This cost
differential applies to antennas, cables, and other
accessories as well as to the basic transmitter and receiver.
Currently, though, economies of scale have reduced this
premium substantially so that it is now possible to produce
basic UHF systems at prices comparable to VHF. However,
advanced features and performance tend to remain in the
province of high-end UHF products.