CPL Assist
The only place I have seen an ADF rotating loop aerial on an aircraft is at the aircraft museum. I think it was on the underbelly of a Vickers Viscount aircraft. Regardless of its rarity, it is still the ADF system that is referred to in lectures when explaining the basic principles of the ADF, and its theory forms the basis of the ADF theory questions in the CAA exam.

I presume the reason for this is that; though modern ADF systems use a different methodology, the basic principles are the same, and the principles are easier to understand when explained using the simple form of the rotating loop.

The ADF Rotating Loop Aerial

The loop aerial takes the form of a circular loop of tubing. The diameter of the tubing is about 10 mm and the diameter of the loop is about 300 mm so it is quite a substantial piece of equipment. Wire strands are wound inside the tube to give the ideal aerial length. The loop is attached to the underside of the aircraft in a position that allows it to rotate about its vertical axis. The rotation mechanism and gearing is contained in a pod, which attaches to the aircraft.

The Incoming signal from the NDB is vertically polarized. As a result, the only parts of the loop that will efficiently pick up the signal are those that tend towards being vertical. In the case of this loop, it would be the two curved sides left and right. The top and bottom of the loop, being in the horizontal plane will not efficiently pick up any signal. Even though the sides of the loop are curved, they will evaluate to two vertical component aerials, as seen on the diagram. As a result the loop is said to be mathematically rectangular.

It is the position of these two aerials in relation to the incoming wave that causes the direction-indicating signal to flow in the loop. If the two aerials are in line with the NDB, then each aerial will detect a different phase of the incoming wave, the resultant imbalance of which, will cause a flow of current in the loop. If the two aerials are at right angles to the incoming signal then they will, at all times, be detecting the same phase of the same wave cycle. The currents induced in the two aerials will work against each other in the loop, thereby causing no current flow in the loop.

It is this changing current flow which, when detected, can be caused to drive the pointer of an indicator calibrated to show the direction to the beacon.

The theory as described above is not the end of the matter. The loop aerial working in this way will generate two maximum flow positions and two zero flow, or null positions. The loop aerial is said to be ambiguous. The ambiguity is removed by the inclusion of a further aerial called the sense aerial.

Removing the Ambiguity of the ADF Loop Arial

Let’s look a polar diagram of the loop current flow: In this plan diagram, points a and b represent the two vertical aerials which the loop resolves into. E, F and G are three possible positions of the NDB beacon. For this diagram, we move the beacon around the aerials and measure the current flow in the loop. The current flow value, with the max represented by one unit, is plotted from the diagram center; o towards the NDB.

If the NDB is in position G, the two aerials are in-line with the beacon. This position gives the greatest phase difference and therefore the maximum current flow, the value of which is represented by the distance o – d. When the beacon is in position F, the current flow in the loop is slightly less, represented by the distance o – c. When the NDB is at E, there is no current-flow in the loop.

This can be seen in the diagram at o and is said to be a null position . If we continued to rotate the position of the NDB, we would see the current increase to a maximum on the left and decrease again to zero as the NDB passed the bottom of the diagram. The two positions at which the current falls to zero represent the two ambiguous nulls.