I did not know that...

[John in NC]

This is a really interesting post. I tried this experiment also. You do lose some of the sensitivity because of the two RX coils. It has interesting possibilities though.

[simonbaker]

This is a really interesting post. I tried this experiment also. You do lose some of the sensitivity because of the two RX coils. It has interesting possibilities though.

I'm continuing to test this configuration. I did a quick test hooking up the three unshielded coils to a TGSL. My goal was to see if the noise level at the outputs of the final amplifier stage (LM308 outputs) was less with the 3 coils than with the normal 2-coil head.

First, I have a very noisy workshop. And the noise seems to ebb and flow markedly. One minute the noise seems low, the next minute orders of magnitude higher -- at least as measured at the LM308 outputs.

Results -- the noise with the 3 coils was not dramatically lower as I expected -- so I started investigating and then became embroiled in the usual weird science that happens with real circuits.

I found that when I short (jumper) the RX leads, the noise dropped, but there was still some significant noise. Is my jumper picking up noise!!!?

Then I shorted the inputs of the LM358 preamp -- just on the other side of the 5K resistors from the RX coil -- and the noise flatlines completely; disappears.

Anyway, I'm in the middle of trying to find out where the noise really enters the circuit. My assumption was that the RX coil is the dominent noise source (which is why the 3-coil design should help), but I need to investigate further. It is very tricky because o-scope probes pick up noise and/or influence the circuit (I've seen that before), etc., so have to proceed carefully.

The depth of my 3 unshielded coils was about 15 cm to 20 cm with 1 euro in my noisy workshop. I also tested dfbowers TGSL in same location -- it was also around 15 to 20 cm. The 3 coil head of couse had more beeps if you swept the coin across both RX coils.

I still feel the 3 coil head is probably not suitable for normal hunting, but interesting to experiment with and see what can be learned.

-SB

[bklein]

I just came across video/website this while researching wireless power transfer - and then recalled this thread. Maybe you'll find it interesting...
http://www.vk2zay.net/article/253

Seems to be a pretty smart guy. Wonder how he keeps such young looking hands....
He won a couple entries in the recent 555 design contest (for you electronics geeks).

Barry

[ivconic]

.....Wonder how he keeps such young looking hands....

Maybe because he never lifted nothing harder than a spoon!?
...
However; video is more than educative and i watched it with interest, thanks for posting it.
...
Simon that's really nice effort.
That's exactly the way how to learn and establish good knowledge - by making experiments such as you done.

[ApBerg]

I just came across video/website this while researching wireless power transfer - and then recalled this thread. Maybe you'll find it interesting...
http://www.vk2zay.net/article/253

Seems to be a pretty smart guy. Wonder how he keeps such young looking hands....
He won a couple entries in the recent 555 design contest (for you electronics geeks).

Barry

Hello Barry,

Thanks for link, very nice !

Best regards.

Ap

[Monolith]

Induction Balanced coils

If balancing 2 coils can be problematic, how easy will it be to balance 3 coils. And it is not just the building of the coils, it is to maintain the balance in the field with black sand, red clay and other highly mineralized soils.

Induction balanced coils are not difficult to build. Simple mathematical formulas produce accurate results.
However, Murphy's law is preponderant with IB coils. A tiny shift at the moment of casting and the coil looses most of it's sensitivity.
In the field of course, highly mineralized soils really upset the balance. Ironstone makes an IB coil useless.

To remedy this situation, a method or circuit needs to be devised that makes it possible to re-balance the coil in the field.

I do not know of any such circuit or method to be used on any commercial IB coil detector.

Does anybody know of such a method or circuit?

This would make it possible to use IB coil detectors in places where they do not function or function with very reduced sensitivity.

It would be a great step forward.

Monolith

[bklein]

To remedy this situation, a method or circuit needs to be devised that makes it possible to re-balance the coil in the field.

I do not know of any such circuit or method to be used on any commercial IB coil detector.

Does anybody know of such a method or circuit?

This would make it possible to use IB coil detectors in places where they do not function or function with very reduced sensitivity.

It would be a great step forward.

Monolith

I'll jump in here and risk exhibiting my minimal knowledge when it comes to coils - I'm wondering if you could use a tapped coil design where turns would be switched in and out much like some ham antenna tuners (original ones had wipers that ran across the turns, later came relays, now I'm thinking fets or some other solid state solution...). You could even have a selection of tweaking coil windings to select from when it comes down to very minute adjustments. A company called Microbridge makes a permanently adjustable resistor thingy called a rejustor. Resistance range kinda high perhaps but maybe something can be designed, as per this app note:
http://www.mbridgetech.com/pdfs/MB-A...ki_Coil-AN.pdf
I won a Wii in their design ideas contest when they were starting up a few years ago...

[simonbaker]

I just came across video/website this while researching wireless power transfer - and then recalled this thread. Maybe you'll find it interesting...
http://www.vk2zay.net/article/253

Seems to be a pretty smart guy. Wonder how he keeps such young looking hands....
He won a couple entries in the recent 555 design contest (for you electronics geeks).

Barry

Thanks Barry for this great reference. This guy worked the equations I'm too slow(=lazy) to do; right on the money. Gives some excellent insights. Also explains why all the kids in his neighborhood are born with six fingers...

I was particularly interested in the "proximity effect" which increases the apparent resistance of coils with frequency -- that was what I was trying to get at when talking about the skin effect with coils -- it is a similar principle I believe. Eventually I want to do some accurate measurements. I found my "7 ohm" coils seemed to act like about 30 ohm coils at 14.5 kHz, which may explain why heavier wire doesn't buy you as much as you'd like.

-SB

[simonbaker]

If balancing 2 coils can be problematic, how easy will it be to balance 3 coils. And it is not just the building of the coils, it is to maintain the balance in the field with black sand, red clay and other highly mineralized soils.

Induction balanced coils are not difficult to build. Simple mathematical formulas produce accurate results.
However, Murphy's law is preponderant with IB coils. A tiny shift at the moment of casting and the coil looses most of it's sensitivity.
In the field of course, highly mineralized soils really upset the balance. Ironstone makes an IB coil useless.

To remedy this situation, a method or circuit needs to be devised that makes it possible to re-balance the coil in the field.

I do not know of any such circuit or method to be used on any commercial IB coil detector.

Does anybody know of such a method or circuit?

This would make it possible to use IB coil detectors in places where they do not function or function with very reduced sensitivity.

It would be a great step forward.

Monolith

The balancing 3 coils seems to be about the same as balancing 2 coils. You can fix the RX coils, then just move the TX coil to get the null. What it all means is another question!

There are a number of ways to "electrically balance" two coils, meaning cancel the null signal. But I think it is not the same as "magnetically" balancing them, although it may be useful also in its own way. One of the important things about magnetically balancing is that the coils are independent from an impedance standpoint.

On this website someone posted a photo of, I think, a Heathkit metal detector search head which had an adjustment screw for rebalancing the coils.

A practical possibility for us I think may be to take a small piece of ferrite and move it across your coil head, and glue it down when the best null happens. You could probably even make a rotating piece with ferrite so you could rebalance as often as you like -- but I would think the main use would be to trim the balance after your potting hardens.

I still have not finished investigating the whole business of null with respect to TGSL design -- I still believe may not be as critical as most people think, but I'm keeping an open mind and will continue to experiment.

-SB

[ivconic]

Maybe is related to this, maybe not, but i stomped to this article:

http://waterpoweredcar.com/teslascar.html

It is suspected much earlier that Nikola Tesla already discovered the method how to transfer energy (more efficient than just few mA) via air, from transmitter to "receiver".
It seems again is all up to inductive relations... or maybe not?
However, certain CIA look like structure (or whoever else at that time (who will remember all those idiotic organizations that appeared so far!)) successfully covered that discovery and somehow pushed Tesla to shut up about that.
At some period strong control over Tesla and his work was managed through J.P. Morgan's influence, money and investments. Tesla was genius, but up to some point pretty naive, believing that world is nice place to be, and that all the people could understand ideas which will improve humans survival. He couldn't anticipate nor dream about certain interests and plans that were present even at that early time in capitalistic surroundings.
So... discovery was covered and hided.
One century after, nowdays, we still can not solve the puzzle of transferring larger amount of energy at large distance without huge loses.
Don't tell me this is just another conspiracy theory - because it is not.
"Conspiracy theory" is the 2-word term - established and frequently used just by those "structures" we all are well aware about, yet we can do nothing against those...
...at least not yet.
Back to topic, yes there is much more to discover again and again.
Maybe one day we will have to reinvent the wheel again, because someone from those "structures" may decide that wheel is not possible!?