Active signal application requires the use of a signal transmitter designed to produce from battery power an a.c. voltage of known frequency and ‘Signature’, and the means of applying it to the target buried conductors. 

The rectangular coil in a signal transmitter fed with the output AC voltage sets up a magnetic field through the coil, returning through the earth below. A conductor AB lying parallel to the coil is linked by this field, and therefore has a voltage induced in it. If the conductor is oriented across the coil in position CD there will be no linkage and no voltage. Correct alignment of coil with target conductor is therefore essential, and the field will be concentrated in a narrow band below the coil. Laying the coil horizontal produces a much less localised field spread, useful for ‘blanket’ signal application, but there is no coupling to a line directly below the coil.

It should be noted that the signal current induced will depend upon how well grounded the line is, on the frequency used, and also on the absence or presence of insulated pipe joints. In general, a frequency of 8kHz or higher will be found effective for induction, but the higher the frequency the more easily will the signal be coupled into other adjacent lines. Where more than one line is linked by the transmitter field, the one with the best grounding will generally carry the strongest signal. Sometimes induction is the only way of applying an active signal to a line, but it is not as effective as connection or clamping. Lines may be adjacent and more than one line will receive the signal; it is not often possible to be certain about the identity of the line to which the signal has been applied and the signal is not as strong as when applied by the other techniques.