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These multiple paths result in differing levels, arrival times
and phase relationships between the radio waves. The net
received signal strength at any location is the sum of the
direct and reflected waves. These waves can
reinforce or interfere with each other depending on their
relative amplitude and phase. The result is substantial
variation in average signal strength throughout an area.
This creates the possibility of degradation or loss of the
radio signal at certain points in space, even when the
transmitter is at a relatively short distance from the
receiver. Cancellation of the signal can occur when the
direct and indirect waves are similar in amplitude and
opposite in phase. (See Figure 2-19.)
The audible effects of such signal strength variation
range from a slight swishing sound ("noise-up"), to severe
noises ("hits"), to complete loss of audio ("dropout"). Similar
effects are sometimes noted in automobile radio reception
in areas with many tall buildings. The "size" of a dropout
region is related to wavelength: in the VHF range (long
wavelength) dropout areas are larger but farther apart, while
in the UHF range (short wavelength) they are smaller but
closer together. For this reason, multi-path effects tend to be
more severe in the UHF range. These effects are
unpredictable, uncomfortable, and ultimately unavoidable
with single-antenna (non-diversity) receivers.
RECEIVER:
DIVERSITY TECHNIQUES
Diversity refers to the general principle of using
multiple (usually two) antennas to take advantage of the
very low probability of simultaneous dropouts at two
different antenna locations. "Different" means that the
signals are statistically independent at each location. This
is also sometimes called "space diversity," referring to the
space between the antennas.
For radio waves, this "de-correlation" is a function of
wavelength: a separation of one wavelength results in nearly
complete de-correlation. In most cases, at least one-quarter
wavelength separation between antennas is necessary for
significant diversity effect: about 40 cm for VHF systems and
about 10 cm for UHF systems. Some increased benefit may
be had by greater separation, up to one wavelength.
Separation beyond one wavelength does not significantly
improve diversity performance, but larger areas may be
covered due to more favorable antenna placement.
There are a number of diversity techniques that have
had some degree of success. The term "true" diversity has
come to imply those systems which have two receiver
sections, but technically, any system which samples the
radio field at two (or more) different locations, and can
"intelligently" select or combine the resulting signals is a
true diversity system.
The simplest technique, called "passive antenna
combining" utilizes a single receiver with a passive combina-
tion of two or three antennas. Antennas combined in this
manner create an "array," which is essentially a single
antenna with fixed directional characteristic. In its most
effective form (three antennas, each at right angles to the
other two) it can avoid complete dropouts, but with a
reduction of maximum range. This is because the array
output will almost always be less than the output of a
single antenna at the optimum location. If only two
antennas are used, dropouts can still occur in the event of
an out-of-phase condition between them. Cost is relatively
low but setup of multiple antennas can be somewhat
cumbersome. This is not a "true" diversity design.
(See Figure 2-20.)
A true diversity variation of this technique is "antenna
phase diversity." It also employs two antennas and a
single receiver but provides an active combining circuit for
the two antennas. This circuit can switch the phase of one
antenna relative to the other, eliminating the possibility of
phase cancellation between them. However, switching
noise is possible as well as other audible effects if
switching is incorrect. Range is sometimes greater with
favorable antenna combinations. Cost is relatively low.
Setup requires somewhat greater antenna spacing for
best results. (See Figure 2-21.)
Selection
and Operation
of W
ireless Microphone Systems
16
C
H A P T E R
2
Basic Radio Systems
Figure 2-18: multipath
Figure 2-19: signal level at two antennas with multipath