Coil detection theory

[Greggles]

Hi All, I have read a fair bit of VLF metal detection theory and some coil design theory, Carl has grouped together more coil designs than I have found else where.

However I do not very well understand some concepts.

From what I have read:

The DD design appears to be prefered by Minelab, that does appear to be significant.

Regardless of that.
If the receive "D" coil was instead 2 coils divided in half from the center of the D then would the capacitive / ground induction effectively be half?

If the Ground balance signal is halved but the target signal is the same (depending on the location in the halved "D" RX) then is the signal to noise ratio halved?

Please enlighten me Yoda.

Regards Greggles

[Carl-NC]

If the receive "D" coil was instead 2 coils divided in half from the center of the D then would the capacitive / ground induction effectively be half?

Not sure what you mean, a diagram would help.

[Greggles]

Something like this.

[Davor]

It should work as you expect, but please observe the phase reversal in, say, lower Rx coil. It shouldn't be a problem with 4 quadrant phase detectors, but the most nowadays MDs use 2 quadrant detection so the reversed Rx coil would be useless.

[Greggles]

Thank you for the reply.
This seems to be where I am losing the understanding of what takes place in the magnetic field.

Would the target effect on the magnetic field be considered a localised disruption that could be better detected with multiple smaller receive coils?

You would not need to have a super complicated detector as you could multiplex the receive coils through one at a time.

[Carl-NC]

If the 2 RX coils are out-of-phase, then you can't get induction balance. If they are in-phase, then the break (theoretically) does nothing if the coils are passively combined, but probably hurts sensitivity somewhat. In security walk-through detectors, the otherwise long, narrow RX coil is often broken up into smaller coils, but they are usually fed to separate RX circuits in order to get detection zones.

- Carl

[Davor]

If the 2 RX coils are out-of-phase, then you can't get induction balance.

I'd say such contraption is induction balanced regardless of the overlap, yet with phase reversal with target being closer to the reversed Rx coil.
I am considering an Rx with 4 quadrant detection to be able to utilise such coils.

Greggles, If you care to experiment, and you happen to have lots of wire at disposal, you could do the same with Tx coil as well and get a kind of "BB" coil. In such case phases will be OK, you'll have to balance the overlap, there will be a dead zone in the middle, and your sensitivity will be somewhat lower, but the E field and far field problems will be gone.

[Greggles]

More input

Thank you both Yoda's for helping me to learn more about what transpires with the induction.
I wasn't looking to build a coil like that to experiment, more to ask and try to understand the theory from a localized magnetic field point of view.

I can see now that the induction balance in what I proposed would be more of a problem than I had thought and so would not reduce the signal to noise ratio.

I am trying to visualize the different target effects on the magnetic field where we are looking for the change that tells us that it is a target and not the ground characteristics.
I have read that the target depth increases but the sensitivity reduces as the coil size increases, so I was thinking can the sensitivity be maintained if we have multiple smaller RX coils.

Most discussions seem to be quite vague about the finer points and more towards empirical information.

I am interested Davor in how you would design / position the segments in a 4 quadrant and what shape the RX and TX coils would be?

Many thanks for your enlignment.

Regards Greggles.

[Carl-NC]

I'd say such contraption is induction balanced regardless of the overlap

You're right, my mistake. Out-of-phase RX coils will give a dead spot right in the middle.

[Davor]

I am interested Davor in how you would design / position the segments in a 4 quadrant and what shape the RX and TX coils would be?

4 quadrant phase detector would be able to utilise phase reversal because some target, say a coin, would have a response that is always on a same diagonal. If it has a positive response in, say, first quadrant, its counter-phase will be in 3rd quadrant.

In a 2 quadrant detector a "ground balance" is a referent angle, and all positive response metal targets appear as positive, e.g 1st and 2nd quadrant. All of them.
Negative angles, appearing in 3rd and 4th quadrant are discarded.
Target responses are further discriminated by their angle against a "Disc." angle.

So a detector that discriminates targets as per their diagonal phase angle will keep ground balance angle, and discrimination angle, but will utilise the remaining 2 quadrants in criss cross fashion.

My idea of a coil to make the best use of such detector would comprise 4 coils in a "lucky clover" configuration, with Rx (and Tx) coils being set in counterphase on opposite sides of a diagonal. Such contraption has a very dead zone right in a middle so it is a perfect place to put electronics and make everything very intrinsic. Like for scuba diving kind of intrinsic. Or a hovercraft kind of intrinsic.