Microwave Data Systems EL805, Руководство по установке и эксплуатации

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MDS 05-2708A01, Rev. B MDS TransNET 900 I&O Guide 11
Invisible place holder
Figure 9. Typical Yagi antenna (mounted to mast)
Feedlines
The choice of feedline used with the antenna should be carefully considered.
Poor-quality coaxial cables should be avoided, as they will degrade system
performance for both transmission and reception. The cable should be kept as
short as possible to minimize signal loss.
For cable runs of less than 20 feet (6 meters), or for short range transmission,
an inexpensive type such as Type RG-8A/U may be acceptable. Otherwise,
we recommend using a low-loss cable type suited for 900 MHz, such as
Times Microwave LMR 400
®
or Andrew Heliax
®
.
Table 2 lists several types of feedlines and indicates the signal losses (in dB)
that result when using various lengths of each cable at 900 MHz. The choice
of cable will depend on the required length, cost considerations, and the
amount of signal loss that can be tolerated.
Table 2. Length vs. loss in coaxial cables at 900 MHz
Cable Type
10 Feet
(3.05 Meters)
50 Feet
(15.24 Meters)
100 Feet
(30.48 Meters)
300 Feet
(91.44 Meters)
LMR 400 0.39 dB 1.95 dB 3.9 dB 11.7 dB
(not recommended)
1/2 inch
HELIAX 0.23 dB 1.15 dB 2.29 dB 6.87 dB
7/8 inch
HELIAX 0.13 dB 0.64 dB 1.28 dB 3.84 dB
1 1/4 inch
HELIAX 0.10 dB 0.48 dB 0.95 dB 2.85 dB
1 5/8 inch
HELIAX 0.08 dB 0.40 dB 0.80 dB 2.4 dB