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Offline GoldDigger1950Topic starter
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« on: March 15, 2013, 07:40:23 am »
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Recently, two other threads have had discussions about this complicated issue associated with improved performance. Here is a discussion I was going to post in another thread but began a new topic instead. Feel free to add discussion and ask questions here.

Quote:Posted by gambol1
Why do you wrap it in tin foil? That seems like it would completely shield the RF and change the electrical properties of the coil.??

You're quite right in your assumption but the thickness of the foil is the key. Here's why. Also, I might mention straight away that the complete wrap is only done on monocoils and not on dual coils. For them, only the upper half of the coil is shielded and even then, mostly only the TX side of the coil is done.

In a monocoil configuration, the TX and RX circuitry share the same coil over a period of time. The TX circuitry builds up a field, there is a short delay while the field collapses and then the RX circuitry comes on to sense the current condition of the field. Any residual field is treated as a metallic object. When you generate the TX field, it can penetrate the very thin foil quite easily with only a minimal degradation in field strength. We're talking paper thin foil here, not standard kitchen foil. That's way too thick. We're not trying to deflect the outgoing field at all. We are, however, trying to block any stray interference from incoming signals from happening. This shielding is called Faraday Shielding named after an early pioneer in RF and AF technology. The foil can eliminate incoming stray fields in this way.

First note that the field of a coil is only generated by a mismatched design of the outgoing TX signal. If you were to send the signal into a precisely tuned circuit that is designed to exactly match the TX frequency, you would get zero output from the TX coil. This is because a tuned circuit does not allow any voltage related energy to escape. Your static field is 100% voltage induced. On the other hand, it allows the unimpeded passage of amperage which is "sunk" back into the detector circuitry by a term called "snubbing." The current is absorbed by a very low value resister and that absorption is to the ground circuitry in your detector. This is the combined resistance of the MOSFET and an external high wattage (very low resistance) resistor. Tuning the TX coil off by 3-6db is how we get a big field in the TX coil. We don't care that we are opposing that voltage because we want it to escape in the form of the TX field. The circuit is detuned so the detector will work.

On the RX side, the coil and the internal circuitry must be very nearly a perfect match. The better the circuitry is tuned to the exact frequency of the receiver, the better the detector's ability to "see" the returned signal from metal objects. Metal objects which are in an electrostatic TX field will build up eddy currents from that field (Faraday predicted and proved this). If the objects are ferrous, the field is built up in a very organized way inside of the object, through and around it. If they are non-ferrous, the field is built up on the surface in a swarm of tiny fields which are additive. The size and shape of the object will determine the eddy current strength. They don't last long at all on non-ferrous items but the can build up somewhat on ferrous objects. That's why an iron or steel object often sounds much louder than a gold object and seems far bigger in size. The field can build up a little bit each time and remains there for a while even during the RX cycle. The repetitive TX signals will build it up a bit each time until the ferrous object seems bigger than it really is.

Now, back to the Faraday shield. The receiver can react very badly to stray energy (noise emitters) coming from automobiles passing by, street lighting, house lighting or even your cell phone and GPS. Your detector electronics can also emit stray radiation (electrical - not nuclear) which can cause erratic readings and false signals (Yes, this is where those come from). The Faraday shield rejects the miniscule fields from nearby noise emitters while allowing the tiny ones to get in from the metal objects in the ground. By comparison to stray noise, the eddy currents are huge and your receivers often have first stage gain circuits that amplify the signals by a factor of half a million times the incoming signal. Those ever so small returns are amplified by tha many times before they pass on the the next stage. The Faraday shielding cuts down on the local interference a great deal and still allows the tiny signals from real objects in the ground to get through.

A couple of things to keep on mind here are this. The shielding must be extremely thin. The foil does effect tuning because it is effectively a capacitor added to the circuit. That means you have a different number of windings in the coil if you choose to add Faraday shielding that if you didn't. This is probably what is giving Xavier such a hard time.

The end result of this can be increased performance despite the slightly diminished TX and RX performance. That's because we can predictably eliminate outside interference. Some coils use a type of wire called Litz wire - Faraday shielding is built in to that wire. Not FULL shielding like an antenna wire or an microphone wire but limited shielding offered by the very thin coating on the wire. To learn more about that, search Google for that kind of wire.

If you have kept up with all this, you deserve a medal. I'll call Washington, DC and ask Congress to cast one for those of you who managed to make it all the way to the end of this message. Don't hold your breath waiting for it.

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« Last Edit: March 15, 2013, 03:43:43 pm by GoldDigger1950 »
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Offline xavier
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« Reply #1 on: March 15, 2013, 01:26:48 pm »
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Posted on: March 15, 2013, 08:25:53 PM
Quote:Posted by xavier
If you have kept up with all this, you deserve a medal. I'll call Washington, DC and ask Congress to cast one for those of you who managed to make it all the way to the end of this message. Don't hold your breath waiting for it.

Better call them then mate LOL

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« Reply #2 on: March 15, 2013, 05:51:17 pm »
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hi GD that is good explanation for Far. shield i used alu. foil for cc coil and like you say it must be thin foil.
it eliminate interference both ways from the coil and to the coil.and Faraday is the one who invented the capacitor is it not so.
thanks great thread

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« Reply #3 on: March 15, 2013, 09:02:37 pm »
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Quote:Posted by kabier2
and Faraday is the one who invented the capacitor is it not so

 Actually a German scientist named Ewald Georg von Kleist invented the capacitor in November 1745. Several months later Pieter van Musschenbroek, a Dutch professor at the University of Leyden came up with a very similar device in the form of the Leyden jar. Faraday indeed invented ways to use the capacitor.

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Offline GoldDigger1950Topic starter
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« Reply #4 on: March 16, 2013, 06:31:19 am »
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Faraday was a scientist/engineer who did groundbreaking work in electro-magnetics; something we rely on heavily in our daily use of metal detectors. I won't be so bold as to say that without him we would nt have what we do because many scientists of his day were studying the same things and the same phenomena. He was merely the first to establish relationships between voltage, current, inductance, reactance and impedence. He further correlated them into a set of mathematical formulas which we still use to this day.

I am semi retired from electronics engineering. I spent decades as a contract engineer designing thiings for clients who had good or great ideas but lacked the ability to turn them into reality. I still do design work but only for myself. I also lecture periodically when invited. I enjoy contributing to these forums because most of you here are either eager to learn or eager to contribute to the discussions that go on. I enjoy the spirit of experimentation and the exchange of ideas that happen here. This is truly the way that advances in science are made.

Enjoy yourselves. I certainly am. Thank you for all of your comments here.

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« Reply #5 on: March 16, 2013, 01:18:56 pm »
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When I build a coil at a specific frequency on a former 200mm I will end up with an inductance of X I then take half the diameter of TX for the RX coil wind X turns again to the required frequency, but here's the problem. For the phase shift to be correct I need an inductance in the one coil (TX) to be 6mH and the other (RX) 6.5mH  now the way that I figure it out is that no mater what the coil diameter, I should always have the same percentage of change in inductance. (Respecting the 50% diameter of RX) The length of wire does not change. Now I followed to the T a document from the Geotech forum on a coplanar coil, the coil worked but when I used a graphite spray to shield it, it caused so much interference that the coil was rendered useless. Here is what I think, if I follow the instruction on the construction of the coil using frequency instead of inductance I will not get the correct phase shift I need to have a precise inductance and frequency for it to work properly and for my shield to be affective (probably due to the phase shift) and stop playing funny tricks on me (Not funny at all) Now I use my coil with no shielding and I'm having some fun while I think about this problem.  Undecided       

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Offline GoldDigger1950Topic starter
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« Reply #6 on: March 16, 2013, 01:24:45 pm »
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The worst part of all this is the timing and rest period in a PI machine. You can't see any of it with a scope because as one field is collapsing the next is building from the receiver. It gets hectic and everything begins to overlap. On the up side, all of your calculations are very true to reality because the internal resistance is only a factor for the overall power output.

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« Reply #7 on: March 16, 2013, 01:36:34 pm »
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So I would be right in saying that I could change the inductance without changing the frequency and by doing so I would have the correct phase shift? The way that I see this is that the RX coil being smaller that the TX coil it will have more tension on the wire (The coil will be tighter because of the number of turns and size) so one would need to find the correct tension to wind the two coils, perhaps 1.2 x the tension on TX than RX. This could only be done by trial and error but could be well worth my while.   

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« Reply #8 on: March 16, 2013, 03:06:30 pm »
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Ok, I know how to do it and I Know sorta how it works.

But I don't understand this.

In the USAF we had all our secure com in a Faraday Cage.

It kept our transmissions from going out and any beamed stuff from coming in and being modulated and reflected back out to a receiver.   Spook Stuff.

I seen you can tell every Key Stoke of a Computer from over 200ft away with the right Equipment.

I know limited Shielding can take up and dump the Ground Effects.

We are Using Magnetic and RF fields to do the job but at higher relative powers then in the Distant RF Realm.

Is this how we get away with it?

Are we just concerned with the Capcitive Effects of the Ground?






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Offline GoldDigger1950Topic starter
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« Reply #9 on: March 16, 2013, 03:39:25 pm »
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Xavier, your mathematics probably won't steer you wrong.

Homefire, a Faraday cage is not at all the same as a Faraday shield used in a metal detector. Not even close. If you dent that finely tuned cage, it degrades. It's made to shunt spurious radiaton going out not any stray fields coming it. Don't forget WE are dealing with an ELECTROSTATIC field, not an RF radiation.

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