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Faraday shield

A

Anonymous

Guest
I am involved in the redesign of some walkthrough metal detectors. At this stage I am only improving the electronics but would like to fix up the coils possibly as well. I don't know much about Faraday shields other than they are used to reduce the effects of capacitance. Are there common techniques used for walkthroughs?? The current method in use is I think an overwinding. I have wondered about a conductive "plate" similar to use in the SD series coils. More/ all info gratefully accepted.
Thanks heaps in advance.
 
Alan,
There is a Forum that is specifically set up for metal detector designing as this is a Pulse Induction only Forum. The guy that runs it is an Electronics Engineer with Analog Devices and is very helpfull plus you have various E.E.'s who are designers for well known metal detector companys on hand as well. Check it out and good luck!
Randy Seden
 
Hi Randy,
Many walkthrough detectors are pulse induction, so this could be relevant to this forum.
Eric.
 
Hi Alan,
Are the walkthrough detectors you are working on, PI based?
Eric.
 
The current design is not pulse induction but it is the method that I plan on experimenting with in the near future.
 
Hi Alan,
Faraday shields have two purposes. One is to prevent capacitance changes when a partially conductive and grounded object passes near the coil system, The second is to shield the coil from external rf interference.
A human body will cause capacitance change, which affects tuning, or damping in the case of a PI. A grounded shield is a must on a walkthrough if any real sensitivity is to be achieved.
Many places, such as airports, where walkthroughs are used, have high levels of rf interference and Faraday shielding helps to minimise this. No shielding on a metal detector can be 100% effective as it has to be transparent to the wanted signals that the detector is emitting and receiving. Two types that I use are nickel loaded paint and lead foil tape.
Eric.
 
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