Sheilding VLF TR Coils

[Mike]

I've been reading posts on the forum, various patents, and Dave Emery's Coplanar Coil Construction article with a view to building my own VLF TR coils. But I'm getting confused.

Years ago when I first was interested in DIY metal detector building, the coil windings were sheilded with aluminium foil to reduce capacitive coupling effects between the coil windings and the ground. My interest in building metal detecters has recently returned, but now the sheild seems to be refered to as an electrostatic sheild. There is a Whites patent, for example, which specifically states that the sheild is there to dissipate static charges created in the search head by the head brushing against grass etc. The "state of the art" now seems to be to create a sheild that is electrically conductive, but that doesn't allow eddy currents, which are conflicting requirements.

I'd be very grateful if someone could enlighten me on the following (for VLF TR concentric and 2D coils):

1. what is the *primary* purpose of the sheild - static dissipation or capacitive coupling to ground mitigation?
2. is there an optimum resistivity that optimises static dissipation/capacitive coupling reduction and eddy current inhibition?
3. how significant is the reduction in performance between wrapping the coils in aluminium foil "the old way", and using a significantly higher resistivity coating? For example, is the reduction in depth actually noticable?

Regarding point 2., I've seen discussion on the forum of various sprays and paints, both commercial and home made. They typically have resistivities measured in single figure ohms per square. However, substances aimed at dissipating electrostatic chages tend to have much higher resistivities, more like thousands to billions of ohms per square. You can buy floor paints and even spray polishes for ESD protection. They would have extreemly low eddy currents yet easily dissipate electrostatic charges. Would these be as good as the graphite or nickle sprays?

Any comments would be welcomed.

Mike

[Charles (Upstate NY)]

It is my understanding that modern day VLF coil shields reduce capacitive coupling. They absolutely dissipate static charges and 100% coverage is required for this reason. The VLF coils are quite sensitive, at least the Minelab Explorer coils are. You can brush just a few blades of grass on an unshielded coil and get a false signal. These painted on shields are not effective against EMI/RFI however.

For the Minelab Explorer anything more conductive than the carbon black based shielding paints are too conductive. Electro-Dag measures about 75ohms per inch. It will not pass a continuity test even with the probes nearly touching. But with a thin drain wire they shield the coils nicely.

Nickel based shielding paint is too conductive. It will pass a continuity test and if you wave a shielded sample over an Explorer coil the Explorer will pick it up. Therefore if your coil flexes even a little, bumping it along the ground as you swing it, the coil will detect the shielding and false. The reduction in depth is also noticable.

For the Explorer the optimum seems to be about 75-100ohms per inch. 1-10 ohms per inch is too conductive. Note with the carbon black type shielding paints you don't have to worry about leaving a gap in the shielding.

An Explorer coil wrapped in foil would not work very well. You would see the coil is severely loaded on the scope. There was a noticable reduction in depth with nickel based paints, foil would be worse.

There's probably a happy medium, floor paints and such are probably not going to be conductive enough. Keep in mind that you have to wrap drain wires around the coil. A gap of about 2 inches with no drain wire is enough to cause the coil to false in that area. The higher the resistence, the more drain wire you would need.

My advice, save a lot of time and effort and just go with a carbon black/graphite based sheilding paint. I use Electro-dag but its nasty stuff, I think the solvent is MEK. The plus side is it dries really fast. I hear there are some water based graphite paints out there now which I will probably try after I use up this electro-dag.

Picking the shielding paint isn't the hard part, how to apply the shielding is what is difficult. You need a minimum spacing between the shielding and the winding. I suggest you look at this thread regarding a cheap and quick way to do this.

http://thunting.com/geotech/forums/s...d.php4?t=11164

Charles

[Mike]

Thank You

Charles,

Thanks for that very useful reply. It seems that as detectors have got more sensitive, sheilding has become more critical. No more aluminium foil sheilds, well not on VLF TR detectors at any rate.

Regarding the drain wire, is this the electrical connection to the sheild? Do you run a bare wire around the whole of the coils circumference (though not completing a whole turn), then paint over it? Is the wire spiral wraped around each coil? Wouldn't this wire effect the coil balance?

So many questions.

Regards,

Mike

P.S. Thanks also for your "Building an Explorer Coil" on detectorgear/coil building forum.

[Charles (Upstate NY)]

On Explorer coils I use a very fine bare wire for the drain, something in the 40 guage range but have seen larger guage drain wires on factory coils. I spiral wrap a lead around the left side, right side, and a third up the middle. This does not effect the balance of the coil. These drain wires attach to the transmit coil ground wire. I do leave about a 1 inch space where the ground wires come together at the tip of the coil. You don't need 1,600 turns of drain wire, maybe one loop per inch. Then as you guessed, you apply the shielding paint over this.

Charles

[Mike]

Thanks

Thanks for the clarification. I think that's answered everything about sheilding I could imagine

Mike