Locating using active frequencies

Follow Avatar Anthony Caplin
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Active frequencies

Active frequencies are applied to a buried conductor using the transmitter. The transmitter can apply a signal using three methods:

Direct Connection

In Direct Connection, you connect the transmitter output directly to the utility. The transmitter will then apply a discrete signal which you can locate using the locator. This is the preferred method of applying a transmitter signal to a utility and in the majority of applications will apply a stronger signal to the utility, which may increase the locate distance.

To directly connect to a non-energized conductive utility:

  1. Switch the transmitter off.
  2. Connect the Direct Connection lead into the transmitter accessory socket.
  3. Clip the red connection lead to the utility ensuring that the area around the connection is clean and that a positive connection is achieved.
  4. Clip the black connection lead as far away as possible and at 90° to the ground stake or suitable ground point nearby ensuring that a positive connection is achieved.
  5. Switch the transmitter On.
  6. The display will show the Direct Connection lead connected icon.

direct connection lead.png

Direct Connection lead icon


warning icon.pngWARNING! Direct Connection to live conductors is POTENTIALLY LETHAL. Direct connections to live conductors should be attempted by fully qualified personnel only using the relevant products that allow connections to energized lines.

warning icon.pngWARNING! The transmitter is capable of outputting potentially lethal voltages. Take care when handling the terminals, connection leads and ground stake, notify other technicians working on the line of the hazard and guard exposed conductors to prevent accidental contact.

warning icon.pngWARNING!  Ensure the TX transmitter is switched OFF before making a connection and before disconnection of the direct connection lead to a service.



In this mode of operation, the transmitter is placed on the ground over or near the survey area. If a Direct Connection lead or signal clamp is not plugged into the transmitter, it will automatically go into induction mode.

In this mode, only frequencies applicable for induction mode will be made available as the frequency key.png key is pressed.

Once activated the transmitter will induce the signal indiscriminately to any nearby buried conductors.

Please note that these signals will also be airborne and it is advisable to keep the distance between the transmitter and locator at least 10m / 30’ – this distance may need to be increased, particularly if depth measurements are taken.

Signal clamp

An optional signal clamp can be connected to the transmitter and clamped around a cable or pipe to apply the transmitter signal. This method of applying the transmitter signal is particularly useful on insulated live wires and removes the need to disconnect the supply to the cable. Clamps are available up to 8.5” / 215mm in diameter.

warning icon.pngWARNING! Do not clamp around uninsulated live conductors.

warning icon.pngWARNING! Before applying or removing the clamp around a power cable, ensure that the clamp is connected to the transmitter at all times.


Choice of frequency for active location

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 active signal


This low frequency is most useful for line tracing and identification over long distances. It does not couple easily to unwanted lines however it is too low for induction, and it falls within the band of power frequency harmonic interference.



8kHz active signal


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



33kHz active signal


This higher frequency is easily applied by induction to most lines, so is very useful for initial search. It travels on small diameter line however it couples more easily to unwanted lines, and loses its strength over shorter distances than lower frequencies.



100kHz active signal


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


Selecting frequencies

It is important to select the correct or appropriate frequency for your particular application. For more information refer to the application note “The theory of buried cable and pipe location”, which is available as a free download from www.radiodetection.com

To select a frequency on the locator:

  1. Press the frequency key.png key to cycle through available frequencies.
  2. Alternatively, hold down the frequency key.png key and press the up arrow.png or down arrow.pngkeys to cycle up or down the range of frequencies.

If locating using an active frequency you must also set your transmitter to output the matching frequency.

You can change your transmitter’s output frequency manually using your transmitter’s keypad or automatically using iLOC (Bluetooth transmitters only).


To manually select a transmitter output frequency:

  1. Press the frequency key.pngkey to cycle through available frequencies.

NOTE: Some frequencies require that you connect an accessory, for example an A-Frame, before the frequency is available.


Inductive search

An inductive search procedure is a more certain technique for locating unknown lines. This type of search requires a transmitter and locator and two people. This type of search is referred to as a ‘two person sweep’. Before starting the sweep, define the area of search and the probable direction of lines crossing the area. Ensure the transmitter is switched on in induction mode.


inductive search.png

Inductive search


The first person operates the transmitter and the second person operates the locator. The transmitter induces a signal onto lines as it passes over them and the lines are then detected with the locator at a suitable distance away from the transmitter (around 15 meters / 50 feet – although this will depend upon the level of induction power used.)

Hold the transmitter with its length aligned with the assumed direction of any lines.

The second person holds the locator at the start of the area to be searched and with the locator antennas at right angles to the probable direction of the buried lines. Set the locator sensitivity level as high as possible without the locator picking up any airborne signals directly from the transmitter.

When the transmitter and locator are in line both operators start to move forward in parallel. The operator with the locator sweeps it backwards and forwards, keeping the locator vertical, as they proceed in parallel with the transmitter. This method allows for misalignment of the transmitter, locator and buried line.

The transmitter applies the strongest signal to the lines directly below it, which are then located with the locator. Move the transmitter from side to side to establish the highest signal which indicates that the transmitter is also directly above the line(s).

Mark the ground at the point of each Peak signal detected with the locator. Repeat the search along any other possible paths of lines. Once the positions of any lines have been marked, reverse positions, place the transmitter over and along each line in turn, and trace the line out of the search area.


Nulling Out

Sometimes it is possible for some utilities to be masked by other utilities and this can happen when one or more utilities are in close proximity to each other or when stronger signals may radiate.  In certain applications and congested areas the ‘Nulling’ technique allows operators to eliminate the induction signal directly beneath the transmitter but at the same time induces the transmitter signal onto other nearby utilities that previously have not been able to be located


nulling out.png

Nulling Out effect


Two persons Nulling Out technique:

  1. Place the transmitter close to the services you wish to trace (a parallel sweep may be used for this) and using the locator at a distance of approximately 10m/30’ from the transmitter, pinpoint the strongest signal.
  2. The sensitivity on the locator is adjusted so that the bar graph response is approximately 75%.
  3. Release the side support arm.
  4. With the locator over the centre of the signal the second operator should hold the transmitter at waist level, with the transmitter on its side – release support arm pointing down.
  5. The transmitter is then moved from side to side to find the ‘Null’ spot between two detectable signals; the transmitter should then be lowered towards the ground keeping it in the ‘Null’ spot.
  6. At ground level we are ideally looking for a ‘Null’ no wider than 50mm/2” (Sensitivity on the locator may need to be adjusted to achieve this).
  7. With the transmitter left on the ground in the ‘Null’ spot. The locator is used to check for additional  signals either side of the ‘Nulled’ signal.
  8. Should the ground be uneven the transmitter may be turned across the line of the service provided the release support arm remains facing downwards. (this will improve the stability of the unit).


One person Nulling Out technique


one person sweep.png

Nulling Out - Single person sweep


  1. Lay the transmitter on its side using the side support arm.
  2. Sweep the area around the transmitter with the receiver at least 10m/30’ from it.
  3. Reposition the transmitter at 5m/15’ intervals around the area and repeat step 2
  4. Stop when there is a response.
  5. Pinpoint and mark any lines.
  6. Trace the lines out of the area.