Well, here it is. I promised I would dig through some old PMs to make available a test procedure that I use for testing transmitter coils for metal detectors of all kinds. It's easy, simple and might even be a free item if you happen to have the right parts laying around.
The first thing you need is an oscilloscope. Don't have one? Here's one that uses a PC sound card, which everyone probably has. The software is free and all you need is an old set of broken headphones for the wire and plug to turn it into a dandy tester for your coil.
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http://www.zeitnitz.de/Christian/scope_en
If you want to go a step further and add the function of a spectrum analyzer, use this link for a free audio range spectrum analyzer.
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http://www.sillanumsoft.org/
The second link may be useful to someone who is familiar with electronics but is probably way overkill for the beginner in DIY.
You now need only one other item of hardware and that is a coil. A special coil. Easily found in dozens of places. The coil you need comes from a solenoid or relay actuator. They are found in washing machines, pinball machines and hundreds of other sources. If you can't find one, a pencil with fine copper wire wound in a chunky, small area will do. Nothing about this is fussy except that it needs a lot of windings. At least 40 turns around a small form. More if you have patience but it really isn't at all critical. After you wind it, tape it securely and leave the two ends available. Clean the ends and solder that old headphone wire to it. One side of the coil will have to go to the tip (center conductor) of the stereo jack and the other to either channel. Attach the coil to the end of a 12 inch stick of wood or plastic and you're ready.
Those are all you need to get started. Simple and cheap. We will now refer to your handy little test pole as your coil probe.
To test your transmit coil, you need to start looking for its characteristics. You need to see how large your TX signal gets in various places around the coil. Put your detector coil somewhere away from all metal preferably suspended over air from a table or chair. Turn it on and make sure it works by passing a metal object in front of it. If all goes well, continue.
Using your coil probe hooked up to the input of your scope, wave it around the coil when it's working and see what you can pick up at varying the distances from the coil. Slowly wave that coil around and see what your return looks like. That will give you an idea of what your transmitter is doing. Yes, your detector will see it as a metal target but ignore the beeping. When you first start out, just wave it around and see what happens.
As you do that, you will get a mental image much like a fuzzy ball of energy around your coil. That is the field you are sensing. The signal on the scope will go from zero to some value as you move around. Test all sides including the top, bottom and edges. Move from very close to very far away. You'll be amazed at how well this works for getting a mental image of your coil's magnetic field.
If you wish, you can make a graphic representation of this using a Smith Chart. Look that up on Google and see if you're up to it. It's very, very technical but the result gives a three dimensional report in two dimensions. Sounds insane? It is.
You can also use the software above to save data points to use in a spreadsheet. It's beyond the scope of this message to explain those so don't ask.
I hope you find this useful and have a great deal of fun filling your head with the images of the field on your coils. It may also open your eyes to how knowing your coil's pattern can improve your searching methods.
Paul A, this topic is dedicated to you, mate. You asked for it. You got it.
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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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