Weidmuller WI-MOD-945-E: 900Mhz Wireless Ethernet & Device Server v2.16 - Manual (Page 14)

Weidmuller Wireless Ethernet Modem & Device Server WI-MOD-945-E User Manual
Page 14 www.weidmuller.com Rev 2.136
Note: When only one antenna is used, it must be connected to the TX/RX connector.
Antenna Diversity
There are two main reasons for using Antenna diversity; the first is to improve the reliability of a radio link that may be
affected by multipath signals. Often if radio signals are transmitted in built-up area the signal can get reflected off different
surfaces and when these signals are received they can cancel each other out due to slightly different time delays. Using
more than one antenna the radio is able to choose the best signal thus providing a more robust radio link.
The second reason to use antennas diversity is to increase the received radio signal into the receiver. All countries have
radio licensing regulations that can often limit on the amount of transmitted power and radiated power from the antenna.
In the US this is 630mW or 1000mW for the WI-MOD-945-E-H of transmit power and 4 watts EIRP (Effective Isotropic
Radiated Power) from the antenna. If a high gain antenna is used to try and improve the receive signal it will also increase
the transmit level and push it over the EIRP regulation limit.
Using Antenna diversity allows two antennas to be used, one for receive and the other for transmit/receive. The TX/RX
antenna has the normal restriction on gain to keep it below the regulation limit, however the receive antenna has no
regulatory limits as it does not radiate power so a higher gain antenna can be used to receive weaker signals.
See Section 3.9 - “Advanced Radio Configuration” for details of configuring Antenna Diversity
In North America the maximum allowable radiated power (EIRP) for a WI-MOD-945-E is 4 Watts, which is 8dB higher that
the modules transmit power of 630mW or 6dB higher that the transmit power of the WI-MOD-945-E-H. Therefore we are
able to increase the antenna gain as long as overall system gain (antenna Gain
– coax loss) does not go above 8dB for
the WI-MOD-945-E or 6dB for the WI-MOD-945-E-H.
Example

If using the WI-MOD-945-E with 10m (33ft) of Cellfoil coax cable (approximately3dB of loss) and an 8 dBi
collinear antenna would equate to approximately 5dB of gain, below the regulated 8dB limit.

If using the WI-MOD-945-E-H with 20m (66ft) of Cellfoil coax cable (approximately 6dB of loss) and a 10 dBi yagi
antenna would equate to approximately 4dB of gain, which is below the regulated 6dB limit.
Bench test and Demo System setup
Care must be taken with placement of antenna in relation to the radios and the other antennas. Strong radio signals can
saturate the receiver, hindering the overall radio communications.
When setting up a bench test/demo or a short range system the following considerations should be taken into account for
optimum radio performance and reduced signal saturation.

If using Demo Whip antennas (DG-900 and WH-900) it is recommended that only the Access Point be fitted with
an antenna.

If using Demo Whip antennas on each end, 20dB coax attenuator must be connected in-line with the coax cable.

If using Demo Whip antennas, modules and antennas must be kept a suitable distance from each other. Check
the receive signal strength
on the “Connectivity page” of the module and ensure the level is not greater than -
45dB
Demo Whip antennas should not be used in the final installation as the maximum performance of the modem cannot be
guaranteed.
If using a DG-900 antenna, it is better to keep the antennas at least 3ft (1m) away from the module so as to limit RF
saturation.
Plant and factory installations
Another application where antenna diversity may be needed is in industrial plants and factories installations which can
suffer from “multi-path fading” effects where multiple reflected radio signals adversely affect the signal strength. In a
“static” installation, where the radio path is not changing, moving an antenna to the position of maximum signal solves this