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		Quote:			Posted by xavier
	Hmmm what do I do? if the coil is big resistance will go up if the coil is small resistance will go down this looks like a hit or miss situation how about I start off with a 230mm coil?

There's a long article on Geotech that describes making PI coils.

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Also, here's another on making air core coils. Written for multi-layer coils but applies to singles as well.

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Many people believe that the resistance in a coil is critical. It is not. All that does is determine the amount of power required to fully expand the field. It only affects the frequency if it affects the inductance. The total circuit Q (a measure of tuned circtuit efficiency) is affected but not the frequency. The reason people look at the resistance is down to the mistaken impression that a coil's inductance can be calculated from that value. For your use right now, it is irrelevant.

I recommend you go out and buy an RF, Ferrite Core choke of 350uh for testing and worry about your big coil later. Put that on the end of 10 inches of coax and you're in business. I believe that will also make a dandy pinpointer.

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		Quote:			Posted by Mudflap
	GD: Think you need coax for the loop? All you have is the mosfet, the coil and +12V. I didn't try to calculate the impedence at the detector operating frequency.

It's just habit with me. I have truckloads of it hanging around so I use it.


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Homefire, the resistance inside the AC components is not the same as in DC components. Adding a resistor outside is only for the safety of the FET and should be kept to an absolute minimum. The tuned circuit resistance affects the efficiency of the circuit or the Q. Inside the coil, the lower the resistance, the larger the resulting field. Adding resistance outside of the tuned circuit will add loading to it and nothing more.

This bugboo of resistance is a red herring when deciding what coil to use. I know you realize that the measurement of resistance has been offered because most of the DIY crowd have no idea what inductance and impedance are. They do understand resistance so we use the term resistance. As a technician, you already know the effect an incorrect inductance will have on the circuit. It simply won't work. Incorrect resistance has negligible effect on the tuning but heaps on the overall efficiency and, as a result, the power. An incorrect resistance will either give too much power (the field will still be active during the receiving cycle and indicate metal all the time) or too little making it very poor and finding treasure. But those are extremes and you have to add tons of resistance to really have a bad effect. A few ohms will make no difference at all and won't correct for improper inductance.

It's all a very delicate balance. Unfortunately, there is no circuit description for this design.

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Well thank you all for all your input and information 

Mudflap

I am busy making a coil mold (almost done) here in SA it is such a schlep to import stuff that I rather make it my self Example you sell me a detector I first have to have an invoice from you to get clearance from the (what ever department)  that then gives the bank clearance to pay your bank and this is BS if you ask me but thats how it is over here.

Regards Xavier

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Xavier

My calculations give me 36 turns of AWG 30 wire around a 5 inch coil form yield a 336uh coil. I don't care at all about the coil resistance. It's meaningless for this circuit.

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