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In the late 1920's, Dr. Gerhard Fisher, the founder of Fisher Research Laboratory was commissioned as a research engineer with the Federal Telegraph Co. and Western Air Express to develop airborne direction finding equipment. He was awarded some of the first patents issued in the field of airborne direction finding by means of radio. In the course of his work, he encountered some strange errors and once he solved these problems, he had the foresight to apply the solution to a completely unrelated field, that of metal and mineral detection. Now, an explanation of what a radio direction finder (RDF) is.

In any system making use of radio, there must be a signal source (or transmitter) and a receiver, operating at the same frequency. In the early days of RDF development, the transmitting tower of a local radio station provided an ideal transmitter. Its permanent location was precisely known and so was the frequency of its transmission. An aircraft approaching a city through fog or clouds could home in on a local radio transmitter by using a radio receiver equipped with a directional loop antenna.

To quickly examine these components: the transmitter signal is sent by cable to the antenna tower, which radiates energy in all directions. The radio station's program is beamed into the entire surrounding area. The loop antenna of the aircraft's receiver however, has the peculiarity of being very directional with respect to the transmitter.

The best or loudest reception is obtained when the plane of the receiver loop is parallel with a line toward the transmitting antenna. No signal at all is received when the loop is perpendicular to the line toward the transmitter. This No Signal position is very sharp and has always been used to obtain accurate bearings. Most table and transmitter radios have loop antennas and your reception will be very dependent on how you turn the radio. If you have a loop antenna on your receiver and this loop with its edge is pointing directly to the transmitter, you may also say it is parallel to a line toward the transmitter. You will then receive maximum signal. Turning the receiver 90 degrees results in a minimum signal or the zero induction position. Remember, this is the most important point in our basic theory.

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.