Sierra Wireless DART 200 CDPD Modem - Manual (Page 27)

DART 200 CDPD Modem User’s Guide 2 Installation and Setup
PN1197-00 Revision 1.0 2-3
Antenna considerations
The DART 200 uses standard mobile cellular radio signals and any
standard cellular antenna of good quality with a maximum gain of 3 dB
(FCC requirement) will function properly subject to the following
guidelines:
• Location
Locate the antenna as far away from personnel as possible to
minimize signal blocking. For optimum reception, in fixed location
applications position the antenna above the height of personnel and
nearby equipment or structures. If used inside locate the antenna as
close to a window as possible. In mobile applications, locate the
antenna outside and away from or above any portion of the vehicle
body that can block the RF signals. To determine the actual signal
strength, attach a PC with an ASCII terminal emulator, such as
Procomm, or Kermit, to view the Receive Signal Strength Indication
(RSSI) value in S-Register 102.
• Antenna cable
Select an antenna cable with a low loss, high quality, 50 ohm,
coaxial cable with the appropriate connectors. The cable can be any
length, but lengths greater than 12 feet increase cable loss and offset
the antenna’s nominal gain. If longer length cables are required, use
a heavier wire gauge to reduce the dB loss/ft and to minimize the
effect of the cable loss on antenna gain.
• Ground plane
For installations where a good antenna ground plane (metal surface)
is not available, use a non-ground plane type of antenna to help
maximize signal reception
• Proximity to other antennas
In general, do not locate the DART 200 and its antenna closer than
five feet to another antenna; in certain cases, even more separation is
required. In many vehicular applications, there are high-power
two-way voice transmitters used and usually the antenna mounting
locations are not five feet apart. The effect of the interference from
the two-way transmitters varies from slowing down response times
to blocking modem transmission.
In this situation, separate the antennas as far as possible and then do
a test with the voice system also being used. If the DART 200 works
satisfactorily you do not have an interference problem. If it does not
work properly, then use filtering on the two-way output, the DART
200 input, or both. The calculation to determine the required filtering
is not trivial and usually requires an RF engineer. However, if you do
not have the option to do a test first, this analytical approach is your
only option.
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NOTE:
Snub-nosed (rubber
duck) antennas,
although they may work
in some areas, are NOT
recommended.
Operating the unit
without an antenna
does not damage the
modem but can cause
unpredictable results.
CAUTION:
For outdoor fixed
installations ground the
antenna cable using an
antenna discharge unit
to prevent damage to
the modem and the
attached equipment.