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Coil Shielding

A

Anonymous

Guest
I posted this a few days ago on the GeoTech forum in response to questions on whether shielding affects the sensitivity, and the type of material to use.
Shielding is essential for PI detectors if they are to have good sensitivity for small metal targets. Careful choice of the material for the shield, and also the positioning of the shield with respect to the coil, will result in no loss of sensitivity, in fact I can
 
If making a coil that has multiple windings such as a DD or Concentric and using lead or copper tape for shielding is it necesary to shield all the windings?
Thanks
HH
Beachcomber
 
Hi Beachcomber,
I would shield each winding separately and take each shield connection back to the electronics for grounding.
Eric.
 
Hi Eric
What is the reason to connect all ground shields of the individual coils at the electronic side together (star grounding)? For example you have a coplanar search coil with RX,TX & Bucking coil you need three individual ground connection through the search coil cable right? Would it be not enough to connect the three ground shields at the search coil arrangement (star grounding) and than run only one wire through the cable and connect it at the electronic?
Chris
 
Hi Chris,
Only two ground connections are needed, even if you use a bucking coil. The bucking coil is best placed in series with the ground end of the TX winding, so that ground serves for both coils. A star connection at the circuit board is what I have found best. Spurious responses seem to occur if I ground both coils at the coil end. The Minelab PI's that I have seen also ground at the electronics when using a DD coil. The cable is like two coax's in one.
Eric.
 
Hi Sid,
The shorter delay that you use, the more essential shielding becomes. You are right in that it does add to the total self capacitance of the coil circuit, and if by winding the coil in a special way, the intrinsic coil capacitance is reduced, then so much the better. In my experience, shielding is necessary below 20uS delay on a wet beach, and below 10uS you soon get to the point where even waving your hand in front of the coil gives a response. My ultra-fast PI that samples at 1uS would be quite unusable if the coil were not shielded, although I have to use a nickel paint shield on that one, as lead is much too conductive. Summing up, although having a shield does increase the coil capacitance, as long as the circuit can tolerate this, then the benefits far outweigh the disadvantages, particularly at short delays.
Also, the shield reduces spurious emissions and improves r.f. immunity so that the relative EU directives can be met. FCC regulations in the USA also have similar limits. The fact that most detectors are used in wide open spaces and are not likely to interfere with anything, (except maybe another detector) does not unfortunately make them a special case.
Eric.
 
Just my two cents worth. Eric is dead right. The grounds need to be on the circuit board side and connected together on the board.
The reason for this is that the current flowing in the transmit leads is very high and causes a voltage drop along the cable between the circuit board and the coil. If you now join the receive coils ground to the ground return of the transmitter then you have placed a small voltage on the receivers input which is equal to the current flowing in the transmit lead times the resistance of the transmitters ground return wire (shield). Although this will be a very small voltage many PI detectors amplify the receive coils input by as much as one thousand times. It is easy to see how this might cause severe instability.
 
Hi Eric, wow, what can I say more. One microsecond of delay is marvelous result. Is it a way leading to true PI discrimination? Sid
 
This is an awkward way to ground the shielding. I sent the shield to physical ground by using the old spike shoes, Cu strips, and a cable that vee's, velcro'ed to my back, to my right arm, and use an 1/8th audio plug to get on my circuit. I get consistent 4" - 5" absolutes and make a
'wired fashion' statement. A 14' could be right beside me and I do not get 'hit'.
Try it. Now everyone will initiate this idea. Long live Torvald.
 
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