Country Home Products Wireless Microphone Systems - Manual (Page 19)

varies in a logarithmic progression so that at any given
frequency one or more dipoles are active while the others are
functioning as reflecting or directing elements, depending
on their size and location relative to the active element(s).
The longer the boom and the greater the number of
elements the greater is the bandwidth and the directivity.
Helical antennas are highly directional and also broadband.
Although these directional antennas are somewhat
large (3-5 ft. wide for VHF) and may be mechanically
cumber-some to mount, they can provide increased range
and greater rejection of interfering sources for certain
applications. It should also be noted here that these
antennas should be oriented with the transverse elements
in the vertical direction rather than the horizontal direction
(as would be used for television reception), again because
the transmitting antennas are usually also vertical.
ANTENNA CABLE
An important but often overlooked component of many
wireless microphone systems is the antenna cable.
Applications in which the receiver is located away from the
transmitter vicinity and/or within metal racks will require the use
of remote antennas and connecting cables. Compared to
audio frequency signals, the nature of radio frequency signal
propagation in cables is such that significant losses can occur
in relatively short lengths of cable. The loss is a function of the
cable type and the frequency of the signal. Figures 2-28 and
2-29 give some approximate losses for various commonly
used antenna cables at different radio frequencies. It may be
noted from this chart that these cables have a "characteristic"
impedance, typically 50 ohms. Ideally, for minimum signal loss
in antenna systems, all components should have the same
impedance: that is the antennas, cables, connectors and the
inputs of the receivers. In practice, the actual losses due to
impedance mismatches in wireless receiver antenna systems
are negligible compared to the losses due to antenna cable
length. Obviously, the benefits of even a high gain antenna
can be quickly lost using the wrong cable or too long a cable.
In general, antenna cable lengths should be kept as short as
possible. Antenna amplifiers can be used to compensate for
losses in long cable runs. (See Figure 2-33.)
In addition, the construction of the cable should be
considered: coaxial cables with a solid center conductor and
stiff insulator/shield are most suitable for permanent installation,
while cables with stranded conductors and flexible insulator/
shield should be used for portable applications which require
repeated setups. Finally, the number of connections in the
antenna signal path should be kept to a minimum.
ANTENNA DISTRIBUTION
The last component found in larger wireless receiver
systems is some form of antenna signal distribution. It is often
desirable to reduce the total number of antennas in multiple
systems by distributing the signal from one set of antennas to
several receivers. This is usually done to simplify system setup,
but can also improve performance by reducing certain types of
interference as will be seen later. There are two general types of
antenna distribution available: passive and active. Passive
antenna splitting is accomplished with simple in-line devices that
provide RF impedance matching for minimum loss. Still, a
single passive split results in about a 3 dB loss, which may
translate into some loss of range. (See Figure 2-31.) Multiple
passive splits are impractical due to excessive signal loss.
To allow coupling of antenna signals to more receivers
and to overcome the loss of passive splitters, active antenna
distribution amplifiers are used. These are also known as "active
antenna splitters" or "antenna multi-couplers." These devices
provide enough amplification to make up for splitter loss, they
Selection
and Operation
of W
ireless Microphone Systems
20
C
H A P T E R
2
Basic Radio Systems
Figure 2-27: examples of remote receiver antennas
Figure 2-29: coaxial antenna cable loss
at VHF and UHF frequencies
Figure 2-28: comparison of coaxial cable types
1/2 wave
(with amplifier)
log
periodic
helical