2-16
INSTALLATION
MDS 05-2415A01, Rev. A
NOTE
A Yagi antenna can be oriented for either horizontal or vertical
polarization. All systems using a gain type omni-directional
antenna at the master station employ vertical polarization of the
signal; therefore the remote antenna must also be vertically
oriented, with its elements perpendicular to the earth’s surface.
If the antenna is mounted with its elements parallel to the
ground (horizontal polarization), the received signal strength
can be reduced by 20 dB or more.
Feedline Selection
Choice of correct feedline (the coaxial cable that connects the radio to the antenna) for the
particular circumstances of each installation is very important; poor cables can seriously
degrade system performance and low loss cables, while superior, can be quite expensive.
For example, 100 feet (30.48 meters) of RG-58A/U cable (commonly used for low frequency
operation) has an insertion loss of 13 dB at 450 MHz. A 5 watt transmitter operating into
such a feedline would produce only 250 milliwatts at the antenna; a similar loss in receiver
sensitivity would result and no amount of gain at the receiver can recover the signal lost in
the feedline. On the other hand, a 100 foot (30.48 meters) length of 1 5/8 inch HELIAX
has a
loss of 0.55 dB at the same frequency, but its cost is many times greater than RG-58A/U.
In any point-to-multipoint system there will be a distribution of remote stations, with some
closer to the master station than others. For the close-in units, feedline loss is not as
important, and 6 dB or more of loss can be acceptable. For the furthest-out remote units, each
decibel of loss directly affects bit error rate and the amount of time the system misses polls
due to fading. Here, it is good practice to keep feedline losses well under 3 dB, with a target
loss of only 1 dB.
NOTE
For each 3 dB of feedline loss, half the transmitter power is lost
and twice the receiver signal is needed to produce the same bit
error rate.
RG-8 or RG-214/U cable is widely available and inexpensive. It is suitable for close-in
remote units or for those installations with short feedlines. For longer feedlines and lower
losses, HELIAX® or similar cable is a good choice. Table 2-6 shows the maximum length of
various types of cable that can be used to give 1, 3, 6 or 9 dB feedline loss.
Table 2-6. Signal Loss versus Cable Length and Type
CABLE TYPE
LENGTH IN FEET (AND METERS)
TO PRODUCE INDICATED LOSS AT 450 MHz
1 dB
3 dB
6 dB
9 dB
RG-8
18 (5.49)
54 (16.46)
109 (33.22)
163 (49.68)
1/2 inch Foam HELIAX
66 (20.12)
199 (60.66)
397 (121.01)
596 (181.66)
7/8 inch Foam HELIAX
120 (36.58)
260 (79.25)
719 (219.15)
1079 (328.88)
1-5/8 inch Foam HELIAX
194 (59.13)
582 (177.39)
1165 (355.09)
1747 (532.49)
Содержание MDS 4310
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