Advanced PI coils

[Aziz]

Hi all,

I'm almost finished with the most common and interesting coil types. The question is, whether I should take the loser figure-8 coil into account. But this figure-8 coil is causing a lot of work for nothing.

I have analysed the following 10 inch coils so far:
- Mono coil (reference)
- Mono + stacked AI coil (The Eric Foster's coil in this thread - see the coil picture in the earlier post)
- Ground loop (GL) coil (TX, RX separate, RX smaller)
- Ground loop (GL) + stacked AI coil
- Co-axial stacked AI IB coil
- Concentric co-planar IB coil
- Tophat AI IB coil
- DD IB coil

AI is referring to the anti-interference (AI) feature (RX+, RX- connected anti-phase in series).
IB is referring to the induction balance (IB) feature.
If IB is not mentioned, it's a plain pulse induction (PI) coil.

Well, I'm thinking of adding a few configurations as well (Davor-Sergey at least. BTW!, I have found a configuration, which doesn't require a compensation (CX) coil).

BTW, I can also answer the following question regarding the concentric co-planar IB coil:
What is better: Smaller RX/BX or larger RX/BX in regards to SNR?


One interesting thing could be measured however:
The AI EMI rejection ratio for different co-axial distances. So we know the gain margin for compensation of the target response losses (the break-even point) and the remaining gain to get into the beneficial region.
I think, I'll make new measurements with an ultra low-noise amplifier to figure out the rejection ratios.

Oh yes, a lot of calculated data needs to be imported into an excel table, which is also causing a lot of work.
Be patient please.
Well, I have almost two days before the doomsday! I should really hurry up now!


Cheers,
Aziz

[Davor]

So we learn it tomorrow? I was just about to go and buy some twisted pair wire for my stealth coil-in-a-bag anti interference, and good for roaming the beaches brand new coil based on a Davor-Sergey concept. We'll see what's on a menu tomorrow

[Aziz]

Thanks for the update.

Your comparison numbers between CC and mono confirm my own measurements.

Another update:

Can the mono coil be outperformed (regards to SNR)?
The answer is: Yes!

I have to make more coil simulations as a consequence of this finding.

Cheers,
Aziz

[Tinkerer]

Me too Tinkerer.
The results are very very interesting so far.

Did you know, that the standard concentric co-planar (CC) IB coil (RX, BX: 0.5 times diameter of TX) performs almost equally regards to signal-to-noise (SNR) criteria (EMI noise induction)?
And I can see the comparision to an equivalent DD coil. It does not perform better than CC IB coil.

Mono coil at depth (30"): 1.00 (reference)
CC at depth: 0.96 times of the mono coil response
DD at depth: 0.62 times of the mono coil response

You will be very surprised about the anti-interference (AI) coils, when there is more room for cranking up the gain to compensate the losses and can finally get into the magic beneficial region.


Cheers,
Aziz

Thanks for the update.

Your comparison numbers between CC and mono confirm my own measurements.

[Aziz]

I will wait for your results before building my new coil.

Me too Tinkerer.
The results are very very interesting so far.

Did you know, that the standard concentric co-planar (CC) IB coil (RX, BX: 0.5 times diameter of TX) performs almost equally regards to signal-to-noise (SNR) criteria (EMI noise induction)?
And I can see the comparision to an equivalent DD coil. It does not perform better than CC IB coil.

Mono coil at depth (30"): 1.00 (reference)
CC at depth: 0.96 times of the mono coil response
DD at depth: 0.62 times of the mono coil response

You will be very surprised about the anti-interference (AI) coils, when there is more room for cranking up the gain to compensate the losses and can finally get into the magic beneficial region.


Cheers,
Aziz

[Tinkerer]

Hi guys,

I'm getting more and more surprised about the calculated results. Normalizing the coil configurations to an equivalent RX EMI noise induction ( N(RX)*A(RX) = const) reveals where the real magic is.

I'm not finished yet. Stay tuned..
Estimated publishing date: The day before the end of days (tomorrow).


Cheers,
Aziz

I will wait for your results before building my new coil.

[Aziz]

Hi guys,

I'm getting more and more surprised about the calculated results. Normalizing the coil configurations to an equivalent RX EMI noise induction ( N(RX)*A(RX) = const) reveals where the real magic is.

I'm not finished yet. Stay tuned..
Estimated publishing date: The day before the end of days (tomorrow).


Cheers,
Aziz

[Duilio]

Hi guys!

I would like to make a good coil for my Surf PI machine. I've made one out of enameled copper wire but it seems to have too high capacitance, because the speaker beeps when I touch the coil. The ground is not detected, but wet grass and snow make this beep. The coil has no shield.
How about using UTP cable? How to connect its wires together? Does anyone have a good documentation of making one?
Or should I use PVC or teflon coated copper wire wound simply?
Which one would be better?

Thank you for your answers.

could affect: http://www.metdet.ru/korsina2.htm
I've tested this with more Surfmaster PI
feels a € 1 coin to 28cm

[andva]

Hi guys!

I would like to make a good coil for my Surf PI machine. I've made one out of enameled copper wire but it seems to have too high capacitance, because the speaker beeps when I touch the coil. The ground is not detected, but wet grass and snow make this beep. The coil has no shield.
How about using UTP cable? How to connect its wires together? Does anyone have a good documentation of making one?
Or should I use PVC or teflon coated copper wire wound simply?
Which one would be better?

Thank you for your answers.

[Aziz]

Regarding the optimum distance of RX+ and RX- in a co-axial stacked AI IB coil.
I have made it for 2 inch, 5 inch, 7.1 inch, 10 inch and 15 inch co-axial distances. There isn't an ultimate optimum distance. And the answer is complicated.

For detection distances 0" - 10", the 5 inch co-axial distance rules.
For detection distances 10" - 20", the 7.1 inch co-axial distance rules.
For detection distances above 30", the 10 inch co-axial distance rules.

The 2 inch version isn't going deep.

For a specific detection distance, there exist an optimum co-axial distance. But this optimum point changes with detection distance. We have to make a compromise.

A good compromise would be somewhere between 5 inch and 10 inch. That's the half diameter unit to full diameter unit co-axial distance. I personally tend to use half diameter unit or slightly more.

Now looking forward to see which coil is going to make the running (including the tophat-coil, concentric co-planar coil, etc. ).

Cheers,
Aziz

[Aziz]

Tinkerer,

Sounds like you're talking about a coplanar concentric coaxial coil. I don't think you can get very effective EMI cancelation and still get good sensitivity. I was addressing the stacked concentric coaxial coil, which is what Aziz is talking about. In that case, best EMI rejection is when both RX coils are tightly spaced (which would then be coplanar); as you spread the coils apart sensitivity will increase up to some point, then begin to decrease as it forces the target farther away from the TX coil. The optimum point can be calculated mathematically (Aziz, are up to the challenge?) or it can be determined empirically.

- Carl

Oh noooo!, I hate math!

Well, I'm doing some variations of distances. 2 inch, 5 inch, 10 inch (distance of RX+ and RX-, TX in the middle). One can probably see the location of the optimal point. If required, I can do more distance variations of course.

Yep, that's the crucial point. The more the distance, the more the EMI rejection is screwed up and the less the target response is cancelled. And the TX being farther away from the ground too.

I'll try to look for the optimal point.

Aziz

[Tinkerer]

Tinkerer,

Sounds like you're talking about a coplanar concentric coaxial coil. I don't think you can get very effective EMI cancelation and still get good sensitivity. I was addressing the stacked concentric coaxial coil, which is what Aziz is talking about. In that case, best EMI rejection is when both RX coils are tightly spaced (which would then be coplanar); as you spread the coils apart sensitivity will increase up to some point, then begin to decrease as it forces the target farther away from the TX coil. The optimum point can be calculated mathematically (Aziz, are up to the challenge?) or it can be determined empiracally.

- Carl

Right, my existing coil is a coplanar concentric coaxial coil.

As I am thinking of building another coil, stacked, concentric coaxial, I am trying to understand the difference. Difference in coil configuration and if a difference in signal processing needed.

[Carl-NC]

Tinkerer,

Sounds like you're talking about a coplanar concentric coaxial coil. I don't think you can get very effective EMI cancelation and still get good sensitivity. I was addressing the stacked concentric coaxial coil, which is what Aziz is talking about. In that case, best EMI rejection is when both RX coils are tightly spaced (which would then be coplanar); as you spread the coils apart sensitivity will increase up to some point, then begin to decrease as it forces the target farther away from the TX coil. The optimum point can be calculated mathematically (Aziz, are up to the challenge?) or it can be determined empirically.

- Carl

[Aziz]

Ok guys,

it seems, I have to do several coil configurations to show you the comparison. I need an EMI noise induction related normalizing reference. That will be the number of turns of RX * flux area of the RX coil. So all (individual) receive coils would induce the same EMI noise level or would have the same N(RX)*A(RX) product. That would be a fair comparison and we are limitted by EMI noise either.
In case of mono coil: TX = RX of course. TX will be normalized to total 300 µH. 10 inch mono coil as reference.
N(RX): number of turns count of RX coil
A(RX) = pi*R², R=radius of RX coil, A(RX)=the RX coil flux area
Anti-interference RX coil: Each RX coil part normalized to N(RX)*A(RX)

I hope to get some results before the end of days (dec-21-2012).

Let's start...
Aziz

[Tinkerer]

C&G (Tesoro predecessor) used to build coaxial coils, and Jack Gifford told me that he used RX=1/2 TX diameter coils to reduce ground pickup. It probably would also allow a tighter coil stack.



Ideally, zero, but zero is the worst for target detection. Empirical testing needed here.

- Carl

Thank you for the feedback.
I am a little confused. The coil configuration I have been using, is a concentric, coaxial IB. The TX is the outer coil. The RX is bundled with the Bucking coil at 1/2r of TX. Both Rx coils are wound in the same direction and center tapped, so the signal of one half is RX+ and the other half is RX-. RX+ and RX- enter the Instrumentation Amplifier. The - input is inverted and added to the +input. This subtracts the equal signed signals. Common mode signals are therefore nulled.

This coil configuration is very easy to balance, but, like all IB coils, once potted, it is final.
There is a tap between the TX and BU coils, that allows for fine re-adjustment of the balance, by bypassing some of the coil current, after the coil has been potted.

This coil configuration works fine in the very noisy lab. The residual is low. The sensitivity is excellent. The sensitivity field is similar to the field of a mono coil, but with the added FE discrimination.

The only problem is the cable. 6 legs. The TX cable is thick, so it does not twist equal as the thin RX cables. Careless cable leading, can cause capacitive coupling between the cables and unbalance the differential input.

If I now want to build a EMI rejecting coil, I need to wind the RX+ coil in opposite direction than the RX- coil. Adding the signals + and -, the result is null. Using the differential input INA, that inverts the -input, I have again the same result as above. (forget the BU for now) The only difference is the distance between the R+ and R-. The target signal will be reduced to the difference in amplitude between the 2 coils, caused by the difference in distance from the target.

EMI, coming from a large distance will be received by both coils equal = common mode.
The target signal, coming from, say 100cm distance will be seen at a higher amplitude by the nearer coil, say 10cm distance between the coils. Not 1/10th times higher amplitude, but the cube root of it.

Do I have that right?