Test Loop Fabrication

Occasionally, it may be helpful to have a convenient means of testing the operation of a vehicle detector without having to make connections to an actual inductive loop. This document provides a series of guidelines which should prove useful in fabricating such a test loop.

A practical sized test loop can be constructed using a 2” Schedule 40 PVC coupling that can be purchased at most hardware or home and garden supply stores (see Figure 1).

Drill two small holes in the coupling. Make certain that the holes are aligned as shown in Figure 2. Feed the end of a length of insulated 24 AWG wire through the lower hole until three or four inches of wire is inside the coupling. Tie a knot in the wire (inside the coupling) to keep the wire from pulling out of the hole. Begin wrapping the wire around the coupling. Make certain to keep all wraps tight and avoid overlapping the wraps. When all wraps are completed, feed the other end of the wire through the upper hole.

To ensure a long, trouble-free test loop life, it is very important to keep the wire wraps from becoming loose and possibly crossing each other. Clear nail polish can be used to achieve this result. Simply coat the wire wraps with a liberal amount of clear nail polish and allow it to dry. An alternative method of ensuring that the loop wire wraps remain in the proper position is to firmly wrap the wire with electrical tape. Whichever method is used, make sure that the wire wraps are not disturbed during the process of securing them.

The ends of the test loop wires can be terminated in one of two ways. The easiest method of termination is to drill two small holes near the bottom of the coupling (below the loop wraps) and feed each end of the loop wire through either hole (see Figure 3). Trim the loop wire ends to a convenient length, strip back a short length of insulation to allow connection to the test loop wire ends, and tin the exposed wire ends with solder. A second method of termination is to use Banana jacks. Once again, drill two appropriately sized holes near the bottom of the coupling (below the loop wraps), mount the Banana jacks, and terminate each end of the loop wire at either Banana jack (see Figure 4).

The actual inductance of the test loop depends on the number of wraps (turns) of wire wound when constructing the loop. The following table lists approximate inductance values for various numbers of turns of wire wound around a 2” Schedule 40 PVC Coupling.

Approximate Test Loop Inductance