905U Wireless I/O Module User Manual
105S Serial I/O Module
Page 74 © January 2011
The radio path is reliable if the transmitted signal is 10dB above the noise level, or better than –
98dBm. For example, if the noise level is –115dBm, then the minimum level for reliability is –
98dBm. If the noise level is –100dBm, then you need –90dBm for a reliable path. If the laptop
displays a scale measurement instead of a numerical measurement, then the transmitted signal
should be at least 3 divisions, and at least 2 divisions above the noise level.
If the weather is poor during the test, then the transmitted signal needs to be 5dB above noise, or 1
division. It is best not to do radio tests during poor weather.
Record these measurements for comparison later during commissioning or if the system has
problems later.
If the radio path test is not successful:
1. Increasing the height of the antenna at either module, or at both modules can significantly improve
the result. Sometimes moving the antenna to the side helps, if there is an obvious obstruction in the
radio path.
2. Change one or both antennas to a higher gain if regulations allow.
3. Use a shorter coaxial cable between the antenna and the 905U.(this may involve moving 905U
nearer to antenna mounting), or use a different coaxial cable with lower loss.
4. If a reliable radio path is not possible because of distance or path obstructions, you will need to
consider using a repeater module. The ideal repeater is another module in the system, in a good
location to act as a repeater. If this is not the case, you need to consider installing a module to act
specifically as a repeater.
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