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to Eric, coil shielding

A

Anonymous

Guest
i am building a large pi coil, what is the best method to shield a coil? or is it really neccesarry. thanks,mark
 
I am ready to dabble in making a coil, but the material used to make a faraday shield is delicate.
Rather than re-invent the wheel, I really would like to know what and where the best over all shielding material is and gotten from.
I know ultra thin lead is probably best, stainless steel wire may be acceptable, metal paints are also candidates.
Thanks for any information
 
Hi Mark,
What size coil are you proposing to build and for what detector? PI's for large objects buried deep, where the pulse delay is 50uS or more, can get away without coil shielding. The main purpose of the shield is to prevent spurious signals from coil to ground capacitance. This effect becomes more pronounced at short delays, as are used when hunting for small coins and thin rings. Also, the effect is worse on a salt wet beach than in fresh water or dry sand. A secondary benefit of shielding is that it does reduce r.f. pickup on the coil from radio transmissions.
There are several ways to shield a coil and much depends on the method of construction as to which is best. I've used aluminium coated Mylar film, nickel loaded paint, lead foil and fine pitch ribbon cable, all of which are effective. Others have tried stainless steel wire and conductive plastic for the coil housing.
Eric.
 
I see in a old metal detector circuit the shielding with aquadag to minimize the interferences. This is the paint of TV screen.
 
... Is <IMG SRC="/forums/images/flag.jpg" BORDER=0 ALT="usa">ble my information?
Esteban
 
Hi Esteban,
No reason why Aquadag should not be OK. It's all a question of how much you put on. Using the nickel loaded paint from an aerosol spray, I find I can put on two or three coats without the search coil seeing it. This gives very adequate r.f. and capacitance screening. If Aquadag is more conductive, then using less or thinner coats will have the same effect. Aquadag is a water based graphite paint.
Eric.
 
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