User's Manual

Installation
RFI-148 & RFI-900 High Output Power Paging Transmitters User Manual Page 8 of 134
2. Installation
2.1 General Considerations
There are a number of rules to observe when installing a paging transmitter.
Antenna selection is vital to a good RF link. Different antennas are required depending on the application.
Please contact your antenna manufacturer or STI Engineering for correct antenna selection.
Antenna placement has a significant impact on RF link performance. In general, higher antenna placement
results in a better communication link. A vantage point should be chosen to clear the propagation ellipsoid.
An unobstructed, line-of-sight link will always perform better than a cluttered or obstructed link.
Obstructions, such as walls and poles, will distort the antenna radiation pattern and VSWR, resulting in less
efficient transmission and reception.
Antennas in close proximity are potential sources of mutual interference. A transmitter can cause overload of
a nearby receiver, if due precautions are not taken in antenna location. Moreover, transmitters in close
proximity may cause intermodulation. Slight adjustments in antenna placement may help solving interference
problems.
All items of radio equipment, such as antennas, are sources of RF radiation. They should thus be placed away
from electrical equipment, such as computers, telephones or answering machines.
Serial cable runs between radio modem and attached terminal equipment (eg RTU or PC) should be kept as
small as possible. A maximum cable capacitance of 2,400 pF is recommended for transfer rates up to 19.2
kbit/s. If a non-shielded, 30 pF / foot cable is used, the maximum length should be limited to 80 feet
(approximately 24m). For higher interface speeds, the length of the serial cable should be shortened.
Long serial cables should also be avoided in areas with frequent lightning activity or static electricity build-
up. Nearby lightning strikes or high levels of static electricity may lead to interface failure.
The Ethernet cable from the RFI-148/900250 to the Ethernet switch must be less than 10 metres long.
STI Engineering supplies a range of external data interface converters for applications requiring long cable
runs.
2.2 External Antennas
Long antenna feed lines cause RF loss, both in transmission and reception levels, and degrade link
performance. When long cable runs are required use a suitable low-loss cable.
As an example, RG58 (tinned-copper braid) will exhibit a loss of 7.1 dB / 30 m at 148 MHz – 174 MHz,
whereas RG58 CellFoil will exhibit 3 dB less (4.2 dB / 30 m).
Antennas should not be located within close reach of people, due to radiation hazard. Exposure guidelines
should be followed at all times.