The top bracket must be at least six brick courses below the apex of the gable-end. In this instance, the masthead amplifier has been mounted at the bottom of the pole so that it is accessible for maintenance. In very weak signal strength areas, it may be preferable to mount it higher up (say, 1M below the aerial) to eliminate the cable loss of 1dB or so, before the signal is amplified.
4. Troubleshooting
Digital errors are caused by the corruption of the signals relaying the data. The reasons for this fall into three main groups:
Poor or inadequate signals picked up by the aerial.
Distortion or interference caused by the reception system.
Interference from outside sources.
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It may be possible to reduce or eliminate the effects of weak signals by realigning or replacing the aerial.
Distortion due to the reception system is usually due to the incorrect setting of levels, which inherently affect the C/N ratio as well. The overall system performance is usually determined by the lowest signal level anywhere on the system, which must be within the correct window of operation. Check for faulty amplifiers, waterlogged cables and faulty connectors. In addition, too much signal at the input of an amplifier will result in «intermodulation» distortion.
Impulse Noise
One common cause of digital errors is impulse noise – interference of short duration that can cause the receiver to lock up and freeze. The display below shows a typical example where the C/N ratio is degraded by interference:
External interference can arise from many sources, examples of which are as follows:
DECT and TETRA relay equipment
Microwave transmitters.
Overhead power cables.
Motor vehicles