User manual

Chapter Six Troubleshooting
man_905-105_2.9 Page 59
Signal
This option provides for testing the radio path between two 905U units for a suitable reliability margin.
Although a pair of units may communicate successfully, radio communication may be affected by a
range of influences, including atmospheric conditions, changing landscape, degradation of antennas or
co-axial cable, low battery voltage etc. “Fade margin” is an indication of how far a radio path can
deteriorate before communication becomes unreliable.
When using the Signal feature, the current received radio signal level is displayed in negative dBm
(dBm is relative to 1mW of RF power). A display of 100 means –100dBm. This means that a stronger
signal will have a lower measured value.
To check the radio path between two units, select the signal option at the local unit. The display will
initially show the background noise of the radio band. Determine the approximate average of the noise
level. The remote unit may then be set up for tone reversals (refer 1 above). Determine the
approximate average of the received signal strength. It is normal for the measured values to continually
change - the radios are continually changing frequency. Calculate the best average for both the noise
and signal. For a reliable radio path, the signal strength must be at least 10dB lower than the noise
level, or 95, whichever is less.
For example, if the noise level is 120, then the radio signal must be 95 or lower for a reliable path. If
the noise level is 100, then the radio signal must be 90 or lower.
A simpler method when remote units are not easily accessible is to cause a transmission from the local
unit to the remote unit (by setting a digital input which maps to the remote unit, for example). The
meter will latch the received signal from the remote unit for half a second, allowing the received level
to be read.
If any obstructions in the radio path are likely to change, then this should be allowed for. For example,
if the radio test is done during winter and the radio path is through trees without leaves, then another
10dB of margin should be allowed for to cover summer conditions when the trees have leaves.
When using directional antennas (i.e. YAGI antennas) this feature may be used to peak the received
signal level. Set-up the remote unit to transmit tone reversals as described above, and observe the signal
indication while adjusting the orientation of the antenna. A peak in signal level indicates optimum
orientation of the antenna.