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Since FM broadcast signals die out faster than AM broadcast signals along the electric line, this technique narrows the search area considerably. When the "hot zone" is located, taps should be momentarily disconnected while the operator listens for changes in the signal. If the noise stops, the tap should be reconnected and search should be continued from the tap. A decrease in the noise upon disconnect may mean that the transmission characteristics of the line have been affected by the removal of the tap, thereby altering the level of the signal. The noise, however, is not originating from the tap. After tap isolation has been effected, and the noise continues, each pole in the noise zone should be struck with a hammer and subjected to visual inspection. A hammer blow will not affect staple noise and some other types of interference. It will, however, show up poor connections.

16-8. Instrument requirements for checking electrical interference.

A receiver suitable for power systems should have sufficient sensitivity to detect a signal that would interfere with an automobile radio or a sensitive home receiver. It should be selective enough to isolate the noise in the broadcast band without interference from broadcast signals. It should be capable of tuning over the broadcast band in order to identify signals which are causing radio disturbances in the area. Another necessary instrument feature is the inclusion of short-wave reception up to 20 or 30 megahertz, for noise tracing at the higher frequencies as previously described.

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