Coil Design

It is relatively common for metal detector users to make their own coils, even though they purchase the rest of their detector from standard metal detector manufacturers.  For this reason, the design of coils is well covered in a number of metal detector sites and this site will not be covering it fully except for some of the main principles and some new ideas.  Once we have the links page completed we will have a number of links to pages that give more information on this.

Shielding

It is normal for metal detector coils to be shielded from external sources of Electromagnetic Radiation.  This shielding is typically some form of Faraday shield or Faraday Cage.  See

http://en.wikipedia.org/wiki/Faraday_cage

for a wikipedia description.

It should also be noted that the wires between the coils and the amplifiers and the wires between the amplifiers and the sound card also need to be shielded.  In this case, as well as or instead of shielding, balanced differential currents within a twisted pair may be able to be used so that interference is cancelled out.

One question that this design might look at is whether there is a better way of handling the external interference problem than using shielding.  An alternative to shielding from interference from outside radiation is to try to counteract the ambient electromagnetic noise by measuring it and reversing it out.

Magnetometers often utilise two transducers with one held above the user to measure the ambient magnetic field so that this can be subtracted from the values being measured by the transducer near the ground.  An additional coil at a height above the Transmit and Receive coils could be used in a similar way for a otherwise standard Induction Balance detector.

Litz Wire

Although not all metal detectors use litz wire for the coils, many do.  Litz wire consists of a number of insulated wire strands woven together in a carefully designed pattern, so that the overall magnetic field acts equally on all the strands and causes the total current to be distributed equally among them.

Litz wire  is used to mitigate the skin effect for frequencies of a few kilohertz to about one megahertz.  Litz wire is often used in the windings of high-frequency transformers, to increase their efficiency by mitigating both skin effect and, more importantly, proximity Effect.

Skin Effect

The skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core. That is, the electric current tends to flow at the "skin" of the conductor. The skin effect causes the effective resistance of the conductor to increase with the frequency of the current. Skin effect is due to eddy currents set up by the AC current.

Proximity Effect

In a conductor carrying current, if currents are flowing through one or more other nearby conductors, such as within a closely wound coil of wire, the distribution of current within the first conductor will be constrained to smaller regions. The resulting current crowding is termed the proximity effect.