Advanced PI coils

[Aziz]

I think I cracked the Davor-Sergey coil - solution is a Tx boosting coil. It would "homogenise" Tx field. So actually ppodg was right, only I did not think of the boosting variant at that time and I unjustly dismissed the idea.

@Aziz, could you be bothered to run a test as in the example above with:
RX+ N=50, RX- N=100
TX: 300 µH at Rx+ radius, Tx boost 19 µH at Rx- radius, 1A coil current, f=10 kHz

Perhaps it would be interesting to see Tx field as well

That would work.
I'll make some configurations.
TX: 1 unit diameter
RX+: 0.8..0.9 unit diameter
RX-: RX+/sqrt(2) unit diameter
BX: same as RX- but with minimal distance gap
Aziz

[Davor]

Yes, a minimum gap between Tx-es and Rx-es would do for capacitance reasons. Guess 0.8 is far too much for 19uH boost.

[Aziz]

Congratulations

Congratulations guys,

you finally have a fully concentric-coplanar anti-interference coil with compact coil size and a hole in the center region, if you want to look through it. Do you see the benefit of collaboration now?

Rough induction balance can be achieved by varying the distance gap of RX+ to TX or RX- to CX (compensation coil). Notice, that the CX has the same current/winding direction as TX, so it doesn't counteract the TX coil. RX- being always the half flux surface area or 1/sqrt(2) times the size of RX+. Fiddling with additional single turn CX wire loop nearby the CX/RX- arrangement allows a fine balance tune.
But you have to crank up the gain of the receiver. A substantial amount of target response will be cancelled by the RX- coil.

If you take for instance 0.9*TX for the RX+ size (or smaller; RX-/CX accordingly smaller) you don't need much turns count for the CX coil. You can make the RX+, RX-/CX more smaller of course.

I get an approx. turns count relation for the IB condition of 7/16 (CX/TX) for the full size RX+ coil. Or 29 µH / 233 µH, approx. of course as I calculate with simplified single turn coil models for speeding up the calculations.
Don't forget: RX- 2x turns count of RX+ and opposite winding direction.

Here is a cross-section view of the magnetic field strength (1 A, 300 µH TX/CX total):


You can interchange the positions of RX+ and TX (accordingly: RX- and CX). So you don't need to make it exactly same as shown above.


Cheers,
Aziz

[Davor]

Yesss!

I'm not afraid of cranking up the gain now that the interferences will be cancelled at the source. Same goes to the ground response. If compared with any other IB configuration, this one leaves a bigger chunk of space underneath the inner Rx coil, the one with positive metal response, and unlike the solution with a bucking coil, this one has more homogeneous Tx field in a centre region compared to the rim, so this is win-win all over. And with a bigger hole in a middle to look through.

I'm happy

[Aziz]

Yesss!

I'm not afraid of cranking up the gain now that the interferences will be cancelled at the source. Same goes to the ground response. If compared with any other IB configuration, this one leaves a bigger chunk of space underneath the inner Rx coil, the one with positive metal response, and unlike the solution with a bucking coil, this one has more homogeneous Tx field in a centre region compared to the rim, so this is win-win all over. And with a bigger hole in a middle to look through.

I'm happy

Yes sure. It would be interesting to know, how this coil compares to a equivalent size concentric co-planar coil. So we can see the required additional receiver gain and see the depth performance comparison. I'll make a comparison, when I have more time.

Cranking up the gain isn't a problem since we can use 1nV/sqrt(Hz) amplifiers. The only problem is the different RX specs as the anti-interference condition suffers a bit (different resistance of RX+/RX-, different parasitic capacitances, etc.) . But it shouldn't be as much critical as the AI feature would suffice I think.

Cheers,
Aziz

[Aziz]

Sorry guys!

I'm so sorry guys. But the last coil does not go deep compared to a equivalent concentric co-planar coil (CC).

The equivalent CC-coil:
TX total: 300 µH, 10" diameter,
RX: 100 turns at 1/sqrt(2) size of TX (7.1" diameter),
BX/RX same distance gap

It offers a better pin-pointing feature in the near coil region (no gain compensation required up to ~3 inch distance).
at 10 inch distance: Gain x 6.5 (~ +16 dB) to be equivalent to a concentric co-planar coil.
at 20 inch distance: Gain x 34 (~ +30 dB) to be equivalent to a concentric co-planar coil.
and it gets worse at more detection distances of course.

If you can get a EMI rejection ratio of 40 dB, there is some improvement possible. No doubt, it will work to some detection distance well enough.

Aziz

[Davor]

No worries, I expected that much. I'm most interested in obtaining a coil which will work well on difficult terrains, such as sea shore and red clays. Being a differential coil, this one will cancel ground as well. With suppressed interferences I can compensate losses (to some degree) with additional gain, and it all fits the bill very nicely.

I'm happy.

[Aziz]

Hi all,

let's think further now:
RX- being half diameter and four times turn count of RX+

Anyone want to guess or comment on this idea?
(But don't run into the patent office - Ok? HAHAHA)

Cheers,
Aziz

PS:
Oooops!!! F§$k!, I have obviously killed my previous post by editing it. Sorry for this stupid action by me. I will see, whether I can reconstruct it. You know, I'm saving my posts to argue against the patent trolls.

------------ Ok, reconstructed now -----------------
Yep, it definitely deserves a go. The ground effect rejection to some degree is very valuable of course.
I'll try this coil other time.
Ok, I'm happy too.

Aziz
-----------

[Aziz]

Last proposal:
TX: 1 unit diameter
RX+: 1 unit diameter, N turns count
RX-: 0.5 unit diameter, 4*N turn count
CX: 0.5 unit diameter, decreased turns count compared to 1/sqrt(2) unit diameter

A better depth & pin-pointing performance compared to 1/sqrt(2) unit diameter CC AI IB coil.

The difficulty is to match RX+ and RX- for best EMI rejection.
Aziz

[ppodg]

There is only one problem: ground compensation will never be perfect with nonidentical rx coils and will strongly depend upon the height above the ground ; DDO coil is much better, though its central phase reversal (or 'dip' if rectified) feature seems very unpleasant - but, after rectification, good for pinpointing.

[Sergey_P]

DDO coil is much better

http://www.md4u.ru/viewtopic.php?f=94&t=8150

[Davor]

For this very reason I'm about to make a 4 quadrant adapter. In the meantime it is good to know that there are some other possible solutions for 2 quadrant detectors as they are right now.

[sido]

Hi all.

To adapt the coils presented so far for Vlf detectors do i still need to use a capacitor for the TX frequency and only TX. Where as RX obviously has been taken care of in its anti interference configuration and therefore does not require a capacitor.
Am i correct?

To build such coils as presented above, can they be used on existing VLF and PI detectors and how do you go about it? Do you build according to original coil specifications?

The reason i ask is i want to build a very large VLF coil and would love to have the ground and noise cancelling capability of the coil designs presented.

Just not sure how you go about adapting it to existing detectors?
Great work

Thanks Sid

[ppodg]

http://www.md4u.ru/viewtopic.php?f=94&t=8150
[ATTACH]22364[/ATTACH]

If +/- parallel to ground

[Davor]

Hi sido,

in all these cases the coils have some inductance, even if wound in counterphase. It is EMI and ground response that is cancelled with these arrangements. The great grandfather of them all is a DDO coil which is a true differential coil, and it has a phase reversed under one of the D-s. This particular coil is otherwise perfectly balanced, has perfect ground suppression, and perfect EMI suppression. Only thing missing is a receiver that can use it properly.

So, for all these coils it is business as usual, capacitors and all. The only way of destroying most of the inductance is winding a bifilar coil and connecting it in counterphase - which is exactly the case with centre tapped coil and it's ability to divert a common mode noise to the ground.

These are all IB coils. Unfortunately all of these coils except Davor-Sergey configuration are reversing phase at odd places.

These IB coils can be used for PI in case Rx is connected to a separate coil, and if they are wound to inductance as specified for your rig. In case you insist on a monocoil, you can try figure 8 coil which cancels EMI, but nothing else. It will not go as deep as a simple mono, but may come handy in industrial areas with strong EMI. It will not create any problems with phases because it is both Tx and Rx reversing the phase simultaneously, but for that very reason there is no ground signal suppression. Only EMI.