Ways to reduce ground signal with PI detectors

[Wessex warrior]

Hi everyone, this looks like a fantastic forum.

I've recently built a DIY PI detector using Silverdog's excellent Surf PI PCB, thanks Silverdog!

All seems to be working well, but it is awkward that the ground is producing a signal whenever the coil is lowered to the ground, this is with an unshielded monocoil.

I was wondering what you all thought is the best way to reduce the ground signal. I have tried aluminium foil as a shield, but it reduces the detection depth a fair bit.

The more 'elegant' solution to me seems to use two coils, to cancel out the ground effect. I was thinking of putting a smaller coil inside the main coil, so you have a concentric arrangment, using this setup would it be possible to cancel out the ground signal?, if so how?

Perhaps using one coil for transmit, and the other for receive.

Many Thanks.

[Qiaozhi]

Hi everyone, this looks like a fantastic forum.

I've recently built a DIY PI detector using Silverdog's excellent Surf PI PCB, thanks Silverdog!

All seems to be working well, but it is awkward that the ground is producing a signal whenever the coil is lowered to the ground, this is with an unshielded monocoil.

I was wondering what you all thought is the best way to reduce the ground signal. I have tried aluminium foil as a shield, but it reduces the detection depth a fair bit.

The more 'elegant' solution to me seems to use two coils, to cancel out the ground effect. I was thinking of putting a smaller coil inside the main coil, so you have a concentric arrangment, using this setup would it be possible to cancel out the ground signal?, if so how?

Perhaps using one coil for transmit, and the other for receive.

Many Thanks.

If the detector is responding to ground, then you have the sample delay set too low. To test if this is really a ground detection problem, try waving a ferrite slug or a ferrite core in front of the coil. If there is no reaction, then it's not a ground problem. I assume with the name of "Wessex warrior" that you live in the UK, so it's quite likely that the ground you're testing on is iron-infested. In this case, it might be reacting to iron trash in the soil.

Winding aluminium foil around the coil will not improve ground elimination. It's purpose is to act as an electrostatic shield.

[Wessex warrior]

Thanks for the reply Qiaozhi,

Yes I do live in the UK, infact I live close to an area that was mined for iron ore, when you say "iron-infested" do you mean natural-occuring iron ore in the soil? (I am using the detector inland). I think it might well be being caused by this but shouldn't jump to conclusions. However, I have tried making the sample delay as long as possible (100k pot), and it still detects the ground. Do you think it is simply the iron-rich soil?

I have thought about building a coil setup like this to cancel ground: http://www.geotech1.com/pages/metdet...p/twinloop.pdf
but I was wondering if this could be done with a smaller coil inside another, rather than two large coils side by side.

Thanks to all.

[Carl-NC]

Winding aluminium foil around the coil will not improve ground elimination. It's purpose is to act as an electrostatic shield.

Depends on the ground. Shielding eliminates the response due to e.g. wet grass that alters capacitive effects in the windings. First task is to shield the coil. If it still responds to ground then the delay is set too low.

[Qiaozhi]

Thanks for the reply Qiaozhi,

Yes I do live in the UK, infact I live close to an area that was mined for iron ore, when you say "iron-infested" do you mean natural-occuring iron ore in the soil? (I am using the detector inland). I think it might well be being caused by this but shouldn't jump to conclusions. However, I have tried making the sample delay as long as possible (100k pot), and it still detects the ground. Do you think it is simply the iron-rich soil?

I have thought about building a coil setup like this to cancel ground: http://www.geotech1.com/pages/metdet...p/twinloop.pdf
but I was wondering if this could be done with a smaller coil inside another, rather than two large coils side by side.

Thanks to all.

As Carl rightly stated, you should shield the coil first to see if the effect goes away. The shield acts like one plate of a capacitor, with the ground acting as the other plate. If there is no shield, then the coil itself replaces the shield as one capacitor plate. In this case, any charge buildup between the coil and ground can produce a response. The problem is not so much that the charge is there, but that it changes as you move the coil.

In your case I suspect it's the naturally occurring iron ore in the soil that's causing the problem.
Please let us know what you find.

[Wessex warrior]

Thanks for the replies,

I tried waving a ferrite bead in front of the coil, the type that clips around cables, which was not detected, I am not sure what this indicates Qiaozhi? Thank you.

Unfortuantly it is impossible to shield this coil because it is now glued with epoxy to some plywood.

[Qiaozhi]

Thanks for the replies,

I tried waving a ferrite bead in front of the coil, the type that clips around cables, which was not detected, I am not sure what this indicates Qiaozhi? Thank you.

Unfortuantly it is impossible to shield this coil because it is now glued with epoxy to some plywood.

It indicates that the detector is not responding to targets with a very short time constant, as would happen if the ground was lightly mineralized. Try filling an ice cream container with some typical soil (with the naturally occurring iron ore) from the area where you detect. Then wave this in front of the coil and see if it responds.

[Wessex warrior]

Hi there,

Are you guys/ladies sure that coil shielding does nothing to reduce the ground signal? This article: http://www.geotech1.com/pages/metdet...s/FastCoil.pdf , seems to indicate that it does:

"If you choose to make a coil for delaysbelow about 30us, a shield must be used
to prevent the ground from being
detected when the coil is lowered."

Even with a longer delay, surely a shield would help in reducing the ground signal. Also, the issue of wet grass affecting the capacitance of the coil windings - surely this would only be an issue if the bare coil is exposed to the ground, and not if it is inside a casing?

Thanks again.

[maikl]

Shield would help in reducing the ground signal and wet grass, but not completely... necessary ground balance... with ground balance reduces the sensitivity... in fact, the depth of detection.