The twin aerial antenna and depth estimation

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The detection of a single buried conductor in a green field situation is simple and can easily be done using a single aerial instrument. However, buried pipes and cables are usually adjacent to other conductors and they are often situated under overhead power lines or other sources of electromagnetic interference. Results from using a single aerial instrument in these situations can become confusing or the interference may render the instrument ineffective.

Radiodetection technology and equipment is based on the twin-aerial, using two horizontal aerials about 400mm (16”) apart to detect the same signal, with the following benefits:

Narrow response for accurate location

We have seen how the receiver response rises and falls as a buried signal source is traversed. It is the change in response that enable the instrument user to recognise the location of the conductor below the point at which the response is maximum. If the signal is weak or the source deep, the rate of change will be reduced, and the maximum response point much less clearly defined.

With the Radiodetection twin aerial design each receives the same signal but from a different distance, with an appropriately different strength.

In the more simple-to-use locating tools the electronic circuit compares the aerial responses and only allows the amount by which Eb exceeds Et to produce a response to the user.

On more sophisticated tools and instruments the slopes to the response as well as its peak are produced for the user. These locators are usually provided with visual indication as well as the audio response of the more simple locating tools.

Interference rejection

A significant advantage of receiving the signal at two aerials is that their outputs can be compared and analysed. By comparing and rejecting all signals other than those which are stronger at the bottom aerial, the twin aerial instrument can be used to give good results in areas where interference makes a single aerial instrument ineffective.

Radio mode operation

The twin aerial system also makes it possible to locate conductors re-radiating a VLF radio signal. This radio energy penetrates the soil and is re-radiated by a buried line acting as an aerial. The twin aerial antenna rejects the atmospheric signal which is received at equal strength at each aerial and only accepts the weaker re-radiated signal which is received at greater strength by the bottom aerial.

Depth estimation

Just as two eyes enable us to judge distance, the twin aerials enable Radiodetection instruments fitted with visual displays to give depth measurement, once the conductor has been pinpointed. This is usually only applicable to active location, as the signal level should be at a constant level.

The receiver circuitry does the arithmetic to provide a depth reading on the instrument meter. Accuracy may be affected by signal distortion from a nearby tee, bend, or other local interference or adjacent cables. If necessary this quick simple estimate can be verified by triangulation.

Note that the same depth estimating facility is also available with sondes. However, the different shape of magnetic field produced by the sonde means that the receiver calibration is different

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