User manual

915U-2 Wireless I/O Page 21
Page 21 User Manual
2.4 Antenna
The 915U-2 module will operate reliably over large distances. The distance that can be
reliably achieved will vary with each application and depend on the type and location of
antennas, the degree of radio interference, and obstructions (such as hills or trees) to
the radio path.
Typical reliable distances are detailed below, however longer distances can be
achieved if antennas are mounted in elevated locations – such as on a hill or on a radio
mast.
Using the 900 MHz Spread Spectrum radio the distances achievable will be:
 USA/Canada 15 miles - 6dB net gain antenna configuration permitted (4W EIRP)
 Australia/NZ 12 km - Unity gain antenna configuration (1W EIRP)
To achieve the maximum transmission distance, the antennas should be raised above
intermediate obstructions so the radio path is true “line of sight”. Because of the
curvature of the earth, the antennas will need to be elevated at least 15 feet (5 metres)
above ground for paths greater than 3 miles (5 km). The modules will operate reliably
with some obstruction of the radio path, although the reliable distance will be reduced.
Obstructions that are close to either antenna will have more of a blocking effect than
obstructions in the middle of the radio path. For example, a group of trees around the
antenna is a larger obstruction than a group of trees further away from the antenna.
The 915U-2 module provides a range of test features, including displaying the radio
signal strength. Line-of-sight paths are only necessary to obtain the maximum range.
Obstructions will reduce the range however, but may not prevent a reliable path. A
larger amount of obstruction can be tolerated for shorter distances. For very short
distances, it is possible to mount the antennas inside buildings. All radio paths require
testing to determine if they are reliable - refer section 5.6 “Network Statistics” Where it
is not possible to achieve reliable communications between two modules, then a third
module may be used to receive the message and re-transmit it. This module is referred
to as a repeater. This module may also have input/output (I/O) signals connected to it
and form part of the I/O network - refer to Chapter 4 Configuration of this manual.
An antenna should be connected to the module via 50 ohm coaxial cable (e.g. RG58,
RG213, Cellfoil, etc) terminated with a male SMA coaxial connector. The higher the
antenna is mounted, the greater the transmission range will be, however as the length
of coaxial cable increases so do cable losses. For use on unlicensed frequency
channels, there are several types of antennas suitable for use. It is important antennas
are chosen carefully to avoid contravening the maximum power limit on the unlicensed
channel - if in doubt refer to your distributor.
The net gain of an antenna/cable configuration is the gain of the antenna (in dBi) less
the loss in the coaxial cable (in dB).