GE MDS iNET 900, Справочное руководство

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130 iNET Series Reference Manual 05-2806A01, Rev. J
• Suitable entrances for antenna, interface or other required
cabling
• Antenna location that provides as unobstructed a transmission
path as possible in the direction of the associated station
(
s
)
These requirements can be quickly determined in most cases. A possible
exception is the last item—verifying that an unobstructed transmission
path exists. Radio signals travel primarily by line-of-sight, and obstruc-
tions between the sending and receiving stations will affect system per-
formance. If you are not familiar with the effects of terrain and other
obstructions on radio transmission, the discussion below will provide
helpful background.
4.1.3 Equipment Grounding
To minimize the chance of damage to the transceiver and connected
equipment, a safety ground (NEC Class 2 compliant) is recommended
which bonds the antenna system, transceiver, power supply, and con-
nected data equipment to a
single-point ground, keeping all ground
leads as short as possible.
Normally, the transceiver is adequately grounded if the supplied flat
mounting brackets are used to mount the radio to a well-grounded metal
surface. If the transceiver is not mounted to a grounded surface, it is rec-
ommended that a safety ground wire be attached to one of the mounting
brackets or a screw on the transceiver’s case.
The use of a lightning protector is recommended where the antenna
cable enters the building; Bond the protector to the tower ground, if pos-
sible.
4.1.4 Terrain and Signal Strength
While the license-free 900 MHz band offers many advantages for data
transmission services, signal propagation is affected by attenuation from
obstructions such as terrain, foliage or buildings in the transmission
path.
A line-of-sight transmission path between the central transceiver and its
associated remote site
(
s
)
is highly desirable and provides the most reli-
able communications link.
Much depends on the minimum signal strength that can be tolerated in
a given system. Although the exact figure will differ from one system to
another, a Received Signal Strength Indication (RSSI) of –77 dBm (–80
dBm for iNET-II) or stronger will provide acceptable performance in
many systems. While the equipment will work at lower-strength signals,
signals stronger than – 77 dBm provide a “fade margin” of 15 dB to
account for variations in signal strength that may occur from
time-to-time. RSSI can be measured with a terminal connected to the