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Recently, on one of the newsgroups, a fellow detectorist asked the question, can somebody build their own coil?

Hmmmm. Why not? What´s inside a coil? What do we hope to accomplish with the experiment? Who wants to do some "actual work" in building and reporting results?

I guess the first step is to ask, what is the coil, and what function does it perform in the detection process.

If we use a two-way radio as an analogy, the coil acts like both the speaker and the microphone. Interestingly enough, a speaker and some types of microphones incorporate a `coil´ in their construction. So the coil takes on the function of a transducer.

In a B.F.O. (Beat Frequency Oscillator) detector, the coil is part of the Oscillator. If you have never used a BFO detector, you may not totally understand how they acted. If you have ever seen an electronic theremin, it may help.

What the heck is a theremin? It is a very weird instrument that when you wave your hand past it, it changes frequency of oscillation. Almost like an analog motion detector tied to a synthesizer. ala Pink Floyd? With a BFO detector, the target is the disrupter that altered the frequency of oscillation. The theremin was invented by a Russian scientist Dr. Theremin, and was often used in old sci-fi movies to give that “alien” sound.

Anyway, the key element here is that the coil exhibits an inductive quality. If you are an audiophile, 8 ohm speaker means something to you. You may not know exactly what, but it is a common starting point. If you had an ohmmeter, and put it across the terminals on that 8 ohm speaker, you would get a reading close to a dead short. Something less than .2 ohms. Where does the 8 ohms come from??????? It is the resistance that the source sees, generally at 1000 Hz. This is known as Inductive Reactance. Generally expressed as Xl.

The `inductor´ is the L part of an L-C circuit that makes up an oscillator. Another component is C which stands for capacitor. Now here is the magic. When the Inductive reactance is equal to the Capacitive Reactance, there is Resonance. In other words, if we had a Capacitor that had a capacitive reactance of 8 ohms at 1000 Hz, and we connected those two components together, we just created a resonant circuit. If we had a way of stimulating that circuit (electrical charge) and it was a perfect lossless world, we would hear a very pleasant 1000 Hz tone for infinity.

Getting back to reality, most of our detectors operate near 17,000 Hz. So we have one of our parameters to look at. We either have to find out the Inductance or the Inductive Reactance.

The formula is XL = 2 TT F L

Frequency is F, L is inductance in Henrys, TT is 3.14159

Once we determine the Inductance, it´s another formula to wind a coil to achieve that value. I´ll be darned, but wouldn´t you guess that part of that formula includes the coil diameter. So if you are into using that old hula hoop as a coil form, this might get interesting. e-mail me if you are in for the ride.

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