Discovering Metal Detection

This zero induction effect at aright angle is in very simple terms, the basic operating principle of an RDF. If the location of the transmitter is known, your direction becomes known when you obtain the zero induction effect from your receiver loop. For example, if a plane was flying into an airport through fog, he would set his receiver loop antenna at 90 degrees angle from the transmitter which would give him a zero signal. If he flew off course he would get a signal which would tell him he had to correct his direction until he received a zero reading. Some peculiar effects were observed during early research with airborne RDFs. Using the same radio tower as a transmitter, different pilots flying in separate planes at varying times would always encounter bearing errors in certain areas even though they were on proper compass reading as established before or afterwards with their RDF. Research revealed that these same areas over which interferences occurred were either highly mineralized or had major metallic surface features such as the large steel roof of a factory. Theses errors may be explained as follows:

If the signal emitted from the transmitter (radio tower) has sufficient strength, some energy will be introduced or induced into any substance which has good conductive properties. It is easy to imagine that a strong signal, such as transmitted from a high power radio station, will readily be induced in good conductors such as metallic ore body or a sheet metal roof, and set up strong current flow which in turn will rebroadcast the original signal at much reduced strength. This re-broadcast signal, even through weaker, is enough to be picked up by sensitive receiver. Remember that your receiver loop must have in aright angle to the transmitter to obtain the zero induction effect by which you navigate, however, with the presence of a metal object like the metallic roof you now have a second transmitter which unlike your radio tower is no longer at this all-important right angle relationship. This explains the error. With radio direction finding you can compensate for this effect, but Dr. Fisher recognized quickly that this effect could be utilized for underground metal detection.