XR71 Operation Help

		Quote:			Posted by satdave
	Litz wire which is many twisted cores coated with Teflon has and still does been used by some manufactures, so can you please explain because that's knocking your theory.

Litz wire is not the same at all. The design is not for keeping audio from straying outside of the wire at all. It's made to shield a broad range of frequencies from coming in during a high powered transmission. I don't mean really high power, just a few microwatts of power but the key is that the signal purity is very high. It's not so efficient as a receiver but the high gain of modern amplifiers overcomes the limitations. The Litz wire helps a great deal because the signal coming back from a target is very consistent and that aids in target identification.

My 'theory,' as you refer to it, is repeated over and over in patents worldwide. Look up some of them and read the most modern of them.

Back to twisted pair audio wire and CAT5 cables. They're designed to keep audio frequencies inside of the twisted pair so the losses to phone and Internet communications are minimized. They also keep the audio frequency EMF inside when used as metal detector coils. Now here's a comment you won't hear anywhere else. The big boys (manufacturers) don't really care to educate you on this and they never use that very clever (but misguided) technique themselves. Any idea why they don't tell you that your method is a joke? Because they want you to build your toy coils and fail so you'll run to them for efficient machines that really work. How's that for a theory? Don't believe me though. Call any of them and ask about using that wire as a coil to their machines. Folks have been coming here, thumping their chests for years now telling us how they built a 1 meter coil using this method and those of us here with direct experience in the metal detector design arena laugh politely behind our hands as we read their stories of wonder.

Want to know why these coils don't work like on the Meteor Hunter shows? Because the detectors on those shows are modified to work with large coils for one thing. And those coils are not made from twisted pair wiring. They're actually made with copper wire, potted in place inside of PVC. They cost about $4,000 to manufacture and then the detector itself must be modified to increase the power that drives the coil. Using a stock transmitter section that normally drives an 8 inch coil to drive one as big as a refrigerator is a joke. The output power must be increased by at least twenty fold to energize that size coil. The power must increase by the square of the coil diameter ratio.

I know these things because I am an electronics engineer with decades of experience in metal detector design. Go back and read some of my posts here.

Posted on: September 26, 2012, 03:21:00 PM

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		Quote:			Posted by satdave
	What would opinions be on using a single solid core varnished wire interwound with a fine lace to create a air gap between windings to lower internal capacitance?, its something I'm going to try when time permits, just wondering your thoughts on the matter. I'm at school on this subject so please more input the better.

That's a very interesting idea and some manufacturers do add capacitors inside their coils to keep them near to the coil. Adding them at intervals is worth building a prototype to see how that configuration performs. Look into the use and development of a Smith chart for your experiments. It will help you determine the effectiveness of your design. I have a few coils that are not conventional myself which are still being developed and improved. They may never replace the standard configuration of today but it pleases me to see how they perform or don't perform in my lab.

One of my experimental coils has a very large (not physically large, just electrically large) transmitter coil and aligned within that is an array of receiver coils. There are twelve receiver coils all using the transmitter coil as a ground plane during the receive cycle and turned off (but used as the TX ground plane) during the transmit cycle. The results are really lousy as far as depth goes (10 inches on a quarter) but if I tilt the receiver coil array ever so slightly off of the transmitter coil plane, I can begin to form a 3D image from the return. The real issue is the speed (or lack of speed) in processing the images. Because of that, my sweep time for swinging the coil is abysmally slow and then trying to pinpoint the target is impossible. I mean really impossible.

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Thanks golddigger
The way you have put it to me it all makes sense, will do some more reading up and try what i suggested with single varnished wire and let you know all results, largest i have in stock is .33mm, or whats your opinion if i interwind two off .25mm with two off nylon strand, or should go and buy .5mm single and interwind with single nylon or would I get away with the .33mm?.
Or otherwise just try everything Ive got and take note of performance.
After talking to you here I'm thinking once I nail a good performing coil the minipulse 3 is going to be one very good detector.
Thanks again and to you Xavier, I'm glad that I mentioned it now, because I'm fed up with people feeding me bullshit when really they don't know what there on about.
By the way may sound dumb to you but where can I get a smith chart from? 

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satdaveuk

		Quote:			Posted by satdave
	By the way may sound dumb to you but where can I get a smith chart from?

A Smith chart is round graph paper graduated logarithmically. You can find blanks using Google. Here's a tutorial.

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Don't let the math scare you off. You have two ways of developing a chart. One is using theoretical mathematics and the other is empirical measurements based on your readings. As to Xavier and my comments about him, well, he spent months trying to wind the perfect coil and when he had his best result, he measured everything and plugged his data into a Smith chart only to find out that the results were 3db off. Well, that is an accidental discovery on his part to what some of us have been preaching here for years. In a long ago post here, I described a fast and easy way to measure field strength using an old relay coil and a computer sound card. Works great and costs next to nothing. I'll let Xavier tell you about it.

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		Quote:			Posted by 6666
	Searching on "relay' "sound card" "field strength"  etc just comes back with Database Error am I doing something wrong in the search ?

At the moment the search feature isn't working right and hasn't been for over a year now. Your best option is to look in the DIY boards for posts using the browse feature. There's lots to see there. Should keep you busy for months. See you after Christmas???

Posted on: September 28, 2012, 05:41:02 PM

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Alternatively, use Google but start your search term with thunting.com in the wording. Then add your search term. Like this WITH the quotation marks:
thunging.com "field strength" "GoldDigger1950"

The quotation marks ensure that both terms are found and that both are next to each other. Adding my name and/or Xavier (also in quotes) will help to narrow it down quickly. Bear in mind that we are both very prolific contributors to this forum so the results list will be huge.

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Thanks very much for the imformation, got plenty of old relays etc, sound cards, laptop from old, so no problem there, will have a fiddle.

Regards Dave

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satdaveuk