In recent days, and in fact seemingly forever, I have received and been answering questions about coils for this or that machine. Sometimes, a really interesting question comes up which fascinates me and stimulates me to thinking deeply about coils for metal detectors. There are a few questions that continue to come up and I will attempt here to answer most of them in a general way. For those who want to build a coil of your own, dive in. For the rest, either read and enjoy or find another topic. Seriously. No arguments will be tolerated. This is NOT a two way dialog. It is instructional. I will answer one question with each new topic here on this forum.
Question: How do I build a 1m by 1m coil for my detector?
Answer: Wind the coil using a semi rigid, light weight form such as PVC pipe. To further answer this question and define how it may or may not perform is the crux of a genuine answer rather than what I just wrote, which from an engineering standpoint is just plain silly. Yes, you CAN build a coil of that size. Will it perform to what you see on TV in the meteorite men? Absolutely not. The reason is that there are issues of impedance matching, capacitive and inductive interacting, resistance that cannot possibly be met with additional wire, additional power and, lastly, the timing of all internal pulses in your machine.
To properly use a coil of that size, a designer must, and I do mean MUST, design the metal detector around the coil and not try to add one to an existing machine. You will be able to get a return on a metal object but you can NEVER get optimum performance unless you match all parameters of the coil to the machine. It is impossible from a physics standpoint to do so no matter how hard you try or how much you believe in your ability to do it. You just have to design a different metal detector that will work with your new coil. That's all there is to it. People here discuss results that are so poor that my 12 inch coil can outperform what they are seeing. Yet they continue to believe that they can squeak out another 10cm or so. A coil of that size should have a theoretical range of over 10m on a door sized object. More on a buried car. The DIY experimenters are happy with just under 2m. As I said, there IS a way to do it but NOT by adding a coil to your detector. You MUST design a new machine to match the coil. Modifying your machine is not possible.
One last point is this. Using phone wire or CAT5/CAT6 wire to wire your coil by cross connecting the wires into one long wire is absolutely the WORST thing you can do to wind a coil. Why is that? Because those wires are twisted pair wires for impedance matching and compliance to FCC requirements to not allow stray signals to leave the wire. See that last bit? It is DESIGNED TO KEEP THE SIGNALS FROM LEAVING THE WIRE. To a coil, that's like trying to find a coin from inside of your trunk. The twisted pair wire is wound to keep audio frequency signals inside of the cable. Who would ever believe that this obstacle can be overcome?
If you really want to build a huge coil, build it using single strand, enamel or lacquer coated copper wire in the open air. Bind the wire securely and give it several coats of epoxy to hold it all together. Then, slit open your PVC and slip it inside. Fill that with builder's foam and let it cure. And there you have it. Giant coil, ready to go.
My intention here is NOT to discourage you from building a large coil. Far from that. I hope you succeed but I just want to point out that you can't get optimum performance for a huge coil without matching the detector you are using to the coil. You will get SOME results, but not the best results.
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« Last Edit: May 08, 2012, 07:18:24 am by GoldDigger1950 »
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It's all about that moment when metal that hasn't seen the light of day for generations frees itself from the soil and presents itself to me.
Let's Talk Treasure!
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