Choice of Frequency for Active Location

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The choice of signal frequency is an important factor for effective tracing and identification of buried lines, and there is no single frequency that covers all conditions. For simple instruments to be used by relatively non-technical personnel, there is no option but to make a compromise, and choose a single frequency high enough to give good performance in the induction mode, but not so high that it will couple too easily into unwanted lines. Active signals between 8kHz and 33kHz are commonly used for these applications. For more comprehensive equipment for problem-solving by technically competent technicians, a range of frequencies may be provided. Typical examples of these and reasons for their use are illustrated below.


512Hz

This low frequency is most useful for line tracing and identification over long distances. It does not couple easily to unwanted lines.

but

It is too low for induction, and it falls within the band of power frequency harmonic interference.


8kHz

This medium frequency is the most useful general-purpose signal, high enough for induction, outside the power frequency interference band, and with limited coupling to wanted lines.

but

It may not be high enough to impose a strong signal on small diameter line like telecom cables.


33KHz

This higher frequency is easily applied by induction to most lines, so is very useful for initial search. It travels on small diameter lines.

but

It couples more easily to unwanted lines, and loses its strength over shorter distances than lower frequencies.


100kHz and Over

 

This very high frequency range deals with the difficult cases – induction onto small diameter lines in dry sandy soil, and short lengths of cable. It is very easy to apply by induction

but

It couples very easily to unwanted lines, and does not travel far.

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