Air-coupling occurs when the transmitter’s signal travels through open air and is received directly by the receiver. This causes the receiver to sense the signal from the transmitter when what you want is the receiver to sense the signal from the utility line so you can trace it. When you’re close to the transmitter, this direct signal is much stronger than the signal coming from the utility you’re trying to locate and it may add confusion.

Air coupling distance can vary over a wide range and you must always carefully confirm that a real utility is detected and that an accurate depth measurement is being made. The air coupling distance can be large, greater than 70 feet (20m) if the induced utility is deep and poorly grounded at both ends. It can be short, perhaps 15 ft (5m) if the induced utility is very shallow and well grounded. Always confirm that air coupling does not distort your readings. Air coupling varies continuously and is not simply an off and on distortion. As you move away from the transmitter the distortion caused by air coupling continues to reduce until the signal from the induced utility dominates and air coupling effects become small.

To test for air-coupling using Multi Directional Antennas, tilt the locator about 45° toward the transmitter, with the lower antenna of the locator touching the ground. Then tilt it about 45° away from the transmitter. Observe the depth reading. If it changes significantly, air-coupling is occurring.

Another method to test for air-coupling is to stand in the area you want to locate and notice the indicated depth measurement on the locator with the lower antenna on the ground. Then, raise the locator vertically, about 18 inches (45 cm) and observe the change in depth indication. If the locator is reading on the conductor only, the depth will increase accordingly; if it is reading on the transmitter’s field (air-coupling) it will not change by 18 inches, but may change disproportionately.

Locator 1 is too close to the conductor and is receiving signal directly from the transmitter through the air, while locator 2 is receiving more or all of the signal from the target utility.