Shielded Loop Antennas

The loop antennas discussed thus far in this chapter have all been unshielded types. Unshielded loops work well under most circumstances, but in some cases their pattern is distorted by interaction with the ground and nearby structures (trees, buildings, etc.). In my own tests, trips to a nearby field proved necessary to measure the depth of the null because of interaction with the aluminum siding on my house. Figure 15-8 shows two situations. In Fig. 15-8A we see the pattern of the normal “free space” loop, i.e., a perfect figure-8 pattern. When the loop interacts with the nearby environment, however, the pattern distorts. In Fig. 15-8B we see some filling of the notch for a moderately distorted pattern. Some interactions are so severe that the pattern is distorted beyond all recognition.

 

The solution to the problem is to reduce interaction by shielding the loop, as in Fig. 15-9. Loop antennas operate on the magnetic component of the electromagnetic wave, so the loop can be shielded against voltage signals and electrostatic interactions. In order to prevent harming the ability to pick up the magnetic field, a gap is left in the shield at one point. There are several ways to shield a loop. You can, for example, wrap the loop in adhesive-backed copper-foil tape. Alternatively, you can wrap the loop in aluminum foil and hold it together with tape. Another method is to insert the loop inside a copper or aluminum tubing frame. Or—the list seems endless.