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| Antennas and Cables |
Antennas connected directly to wireless
receivers work well for most wireless applications. The whip
antennas supplied with most receivers are reasonably efficient,
convenient and inexpensive. There are situations, however, where
the antennas need to be at some distance from the receiver.
This need arises when receivers are mounted in metal racks and
cabinets, when the preferred receiver location does not have
good reception and in other similar circumstances. In these
cases, other types of antennas and RF cables will be required.
In many situations it is feasible to locate the receivers themselves
at the preferred antenna location and install longer cables
for the audio. This approach also works well for other types
of receivers and is often simpler and more effective than using
remote antennas. The main drawback is that the adjustments and
indicators on the receiver probably will not be readily accessible.
The use of remote antennas and coaxial RF cables can affect
performance in unexpected ways. Coaxial cables have substantial
loss at the frequencies used by wireless systems and can significantly
reduce operating range. Certain types of antennas require special
mounting arrangements and all antennas require clear space around
them in order to operated properly. Achieving good results with
remote antenna is not difficult, but does require a bit more
care than just using antennas on the receivers.
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| Using Remote Antennas |
A simple approach to remote antennas is
to merely remove the whips from the receivers and connect them
to lengths of RF cable. Unfortunately, this usually results
in terrible performance. Whip antennas need to be mounted on
a good-size metal surface to work properly. When they are attached
to the receiver, the receiver case (or internal circuit boards)
and the audio and power cables meet this need. The RF cable
does not, and operating range can drop to a small fraction of
what it should be.
Whips can be used as remote antennas if they are supplied with
the necessary metal mounting surface. For example, if holes
are drilled in the metal top of an equipment rack and RF "feedthru"
connectors installed, good performance is possible. The whips
are connected to the outside end of the RF connector and the
RF cable to the inside end. It is important that the outside
of the feedthru connectors be electrically connected to the
metal top of the rack. A piece of sheet metal may also be used
to mount the whip antennas. A size of roughly 36 inches by 36
inches (0.9 m by 0.9 m) is recommended for VHF frequencies,
while 12 inches by 12 inches (30 cm by 30 cm) is adequate for
UHF use.
There are other ways of mounting whip antennas so that they
work well for remote use. The important thing is that the outside
of the RF connectors on the cable and on the antenna have a
good electrical connection to a proper-sized metal object. It
is also important that the whips extend up and away from the
metal object and are not held parallel to the metal. Any approach
that is basically similar to the arrangement used on the wireless
receiver should work reasonably well.
Other types of remote antennas are available from antenna manufacturers.
Except for high-performance directional antennas with gain,
the performance is generally similar to ground planes. Directional
antennas are sometimes used for special applications such as
when operation at very long range is needed. Because such antennas
are inconveniently large at VHF frequencies, they are normally
only used outdoors or in very large spaces. UHF directional
antennas are more reasonable in size and are more common.
Whatever type of antenna is used, it is very important that
there be a clear path between the transmitter and the receiving
antenna. Metal objects block RF signals and cause dropouts and
noisy audio. For best results, there should be a clear path
at least 6 feet (2 m) in diameter between the transmitter and
receiving antennas at VHF; 2 feet (0.6 m) is adequate for UHF.
The RF path should be completely clear of scaffolds, equipment
cabinets, support structures, mesh screens, cable bundles and
other metal objects. Mounting the antenna high above the floor,
8 feet (2.5 m) or higher, will improve range and help keep the
path clear of obstructions.
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| Using Antenna Cables |
Only coaxial cables designed specifically
for RF should be used to connect remote antennas to wireless
receivers. Other types of cables, even those that appear similar,
will have extremely high losses and will usually reduce operating
range to a tiny fraction of what it should be. RF cables should
be in one continuous length; splices and mended spots will also
significantly affect range. For the same reason, only the correct
type of RF connectors should be used, and they must be carefully
and correctly installed.
Even the best RF cables have considerable loss at wireless microphone
frequencies. Since long cables will significantly reduce operating
range, especially at UHF, RF cables should be kept as short
as practical. The size and type of cable used should be matched
to the frequency range and the required cable length. When the
cables must be long, a higher-quality cable is required and
a larger size with lower losses should be selected. RF cable
losses are considerably higher at UHF frequencies as compared
to VHF frequencies, and cable length and cable type are much
more critical.
Even with the best cables, there is a maximum practical length
of RF cable that can be used. To assist in cable selection,
the table below shows the maximum recommended length of various
types of common RF cables for VHF and UHF at a 30% and 50% reduction
in range.
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VHF
Cable |
UHF
Cable |
| |
Length
for 30% Range Reduction |
Length
for 50% Range Reduction |
Length
for 30% Range Reduction |
Length
for 50% Range Reduction |
| |
ft |
m |
ft |
m |
ft |
m |
ft |
m |
| Lo-cost RG-58 |
33 |
10 |
65 |
20 |
15 |
4.5 |
30 |
9 |
| Quality RG-58 |
54 |
16 |
107 |
32 |
24 |
7 |
48 |
14 |
| Lo-cost RG-8 |
70 |
21 |
140 |
42 |
31 |
9.5 |
63 |
19 |
| Quality RG-8 |
110 |
33 |
220 |
66 |
48 |
14.5 |
96 |
29 |
Foam RG-213
(Belden 9913) |
165 |
50 |
330 |
100 |
75 |
23 |
150 |
46 |
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Notes:
1. RG-58 type cables are approximately 0.2 inch (5 mm) in
diameter; RG-213 type cables are approximately 0.41 inch (10.5
mm) in diameter.
2. The reduction of range listed in the table above is in
comparison to the results that would be obtained with the
same antenna connected directly to the receiver.
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At UHF frequencies and for some VHF applications,
it may be practical to overcome part or all of the cable loss
by using a directional antenna with gain. It might also be possible
to locate the antennas where they will receive more signal.
Using a higher mounting location, moving the antennas closer
to the transmitters or repositioning them to avoid objects that
block the RF signals will all be helpful.
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| Using Antenna Distribution Amplifier |
When remote antennas are needed for several
wireless receivers, the cost and complication of using separate
antennas and cables for each receiver can become impractical.
This is especially true when long cables are needed. The solution
to this problem is to use an ADA. An ADA allows connecting several
receivers (usually four) to one antenna, or two antennas for
diversity receivers.
There is a drawback to ADA's, however. Because of the amplifiers,
they are vulnerable to intermodulation problems when strong
RF signals are present. Care should be taken to prevent overloading
ADA's, especially if the antenna cables are short and the transmitters
are close to the antennas. In this case, too much signal can
cause almost as many problems as too little signal. |
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