Line of
Sight: Both 900MHz and 2.4GHz frequency
bands require line of sight (LOS) for outdoor applications
or distances greater than 500m. Once a clean LOS is
available, the two sites can be literally miles and
miles away. In some cases links have been successfully
established over distances as great as 45km.
Antenna and Cable Selection
and Installation: Depending on each
application, antenna requirements vary. For distances
greater than 1.5km, a high gain Omni or Yagi antenna
may be required on one or both ends. For cables less
than 10ft (3m) in length, signal loss can be ignored
as it is negligible. In most circumstances, however,
cables will be far longer than 3m. Low loss cable should
be used in such situations; and path loss must be calculated
to determine the signal strength. For antenna installation,
the rule of thumb is “the higher the better”:
the higher it is placed, the better an antenna will
perform. All antennas come with mounting hardware that
may include pipe clamps or U bolts. These can easily
be used to mount the antenna to an existing pipe or
wood pole. With all outdoor antennas, lightning protection
is absolutely essential. A lightning arrestor/protector
is connected in series with coaxial cable and grounded
to the building. The installation of pipe or wood poles
must be done by professionals.
Extremely
Important: Our tests have proven that
the performance of any directional antenna depends on
the physical orientation i.e. the perpendicular (vertical)
or horizontal polarization, varying from one site to
another. We recommend testing with both perpendicular
and horizontal polarization setups. In the case of directional
antennas, the higher the gain is, the narrower the beam
will be. To find the exact direction of the antennas
go through the following steps:
| Set up the link with H-polarized
antenna at both ends. |
| Record the BER for 5 minutes
then turn the antenna slightly to one direction
and record the BER for 5 minutes. Do the same
at four or five different angles. |
| Tighten the screws at the
angle with minimum BER. |
| Repeat the process at the
other end. |
In applications that require a link over
distances greater than a few kilometers, a path loss
analysis must be done before the equipment is actually
installed. The following parameters and formulas can
be used:
| Loss = 32.44 + 20 log f
+ 20 log d
where f = frequency in MHz
and d = distance in km |
| System Gain = Po (Tx power)
+ Gt (Tx antenna gain) + Gr (Rx antenna gain)
- Mf (fade margin) - Ms (system margin) - Pr (minimum
receive signal level) |
| Use 10 dB for Mf and 15dB
for Ms (standard)
Example: Using a 500mW Smartamp (27dBm), 24dB
antennas and -80 (dBm) as receiver sensitivity
we get:
System gain = 27 + 24 + 24 -10 - 15 - (-80) =
130dB
using in formula (1) we get d = 95.5km |
| Assuming that the entire
system gain covers the maximum path loss, we get:
Loss=130dB, 20logf=67.60, d=antilog 1.5 or 32km. |
In the event that the connection between
the radio and the antenna is an RF cable, the cable
loss will further reduce the system gain.
Coping with Undesired
Interferences: Most of Teletronics’
modems and voice equipment offer channel selection options.
If used correctly, most interference can be eliminated.
In some instances, you might be better off using 2.4GHz
equipment than 900MHz equipment. If possible, select
an antenna with minimum beam width: smaller beam widths
provide high gain while reducing your link’s vulnerability
to interference.
Data Cables:
RS232 cable is available almost everywhere. For V.35
interface consult with your sales person first. For
category 5 cables, make sure whether you need straight
through or crossover cable.
Power: If
the available power is other than 110V AC, please ask
for other alternatives.
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