PI monocoil shielding and ground contact questions

[joop]

I have bought a can of graphite spray, which I want to use to shield a pulse induction monocoil. The coil will probably be a spiral type, to achieve a low capacitance. Here's my question: do I need to leave a gap in the shield? Maybe use a small strip of mask tape before spraying and then remove it?

Second question: what's the best way to make contact between the sprayed shield and the coaxial cable. Maybe a plastic screw will do?

Thanks.

[Carl-NC]

Unless you spray it thick with low resistance, a gap is not necessary. To connect, lay down a piece of bus wire with ~5cm contacting the plastic, put a couple of dabs of fast epoxy to hold it, then spray over the wire so it is embedded in the paint. You will need 10-20cm of extra length to connect to the coax.

[joop]

Thanks Carl, very helpful

[Tinkerer]

I have bought a can of graphite spray, which I want to use to shield a pulse induction monocoil. The coil will probably be a spiral type, to achieve a low capacitance. Here's my question: do I need to leave a gap in the shield? Maybe use a small strip of mask tape before spraying and then remove it?

Second question: what's the best way to make contact between the sprayed shield and the coaxial cable. Maybe a plastic screw will do?

Thanks.

[ATTACH]23691[/ATTACH]

The spiral wound coil has a relative large surface area. To keep the capacitance between the coil winding and the shield low, use a spacer of about 5 to 8mm between the winding and the shield.

Tinkerer

[joop]

Okay that makes sense, thanks Tinkerer!

[WM6]

For visual guys:



Instead of using rivets, you soldering wire at about tiny 2x2cm Cu metal sheet, than glue at inner side of coil housing and then spraying by graphite.

[joop]

Thanks WM6

[golfnut]

Run the ground wire orthogonal to your coil turns.

If it the ground wire were to overlay the coil turns, Tx and Rx signal would be coupled to gnd - albeit by a very small amount - but we dont want to loose any.


Use really fine wire here, thick wire that can be detected by a detector supports eddy currents and this should be avoided.

S

[joop]

Thanks Golfnut. I think I have enough information to build the coil. Will post some pictures here later.

[darrenk]

Thanks Golfnut. I think I have enough information to build the coil. Will post some pictures here later.

pictures would be good when you are finished joop, and results of any tests

[andva]

Hi all!
I made a SurfXX PI metal detector. The coil was made of UTP cable, at first without shielding. The machine worked well when I used it in dry weather. My problem is that the detector goes crazy when it operates over wet ground. It gives false signals.
So I decided to shield the coil. I wrapped the coil in aluminium foil, then the foil was connected to the coil negative lead (GND) through a copper wire. The shield has a gap, approximately 1cm. But the detector doesn't work. It detects the foil. When I turn the Treshold higher, the detector responses to metal objects, but only about 1/5 of the distance without shielding. I tried to change the damping resistors value, but that had no effect. What should be the problem?
Thank you for your help.

[andva]

I have an other idea: how about making a coil out of alarm cable? It is similar to the UTP, but it has a shield. Has anyone tried this?

[joop]

So I decided to shield the coil. I wrapped the coil in aluminium foil, then the foil was connected to the coil negative lead (GND) through a copper wire. The shield has a gap, approximately 1cm. But the detector doesn't work.

Please make a picture of the coil and show us how you made the shield.

[joop]

I have an other idea: how about making a coil out of alarm cable? It is similar to the UTP, but it has a shield. Has anyone tried this?

It should work, but I wonder if you will have enough windings to achieve the correct inductance.

[andva]

I couldn't take pictures of my coil but I made a "professional" drawing of it. I hope you will see what you are interested in.
I made some modifications on the shielding, you can see the latest conditions on the drawing. I have experimented the appropriate number of turns, and if I remember well, it was good between 23-27 turns.
The detector is sensitive enough without the shield, it has about 20cm sensitivity for small coins. Works stable on dry ground, only problem is the wet soil.
I tried to change the damping resistor's value and the impulse length but no success. I guess the aluminium foil should be a wrong material or it is too thick.