CONCENTRIC SEARCH COIL DESIGN

[Qiaozhi]

Small RX coil is not efficient (fact). RX coil needs some flux coverage area to go deep enough.

This may be the difference between theory and practice. In the real world, Tesoro concentric coils (with their relatively small RX) give very good depth. I have not found any advantage with concentric coils where the RX is closer to the TX. The usual rule for concentric coils is to make the RX half the radius of the TX, but Tesoro have made their RX coils smaller than this, and it appears to work just fine.

[Aziz]

This may be the difference between theory and practice. In the real world, Tesoro concentric coils (with their relatively small RX) give very good depth. I have not found any advantage with concentric coils where the RX is closer to the TX. The usual rule for concentric coils is to make the RX half the radius of the TX, but Tesoro have made their RX coils smaller than this, and it appears to work just fine.

No! Small RX coil is not efficient.
All secondary caused magnetic fields from the target should go through (inside) the RX coil. Every magnetic field outside the RX coil isn't visible to the detector.

The "half radius rule" for the concentric coil isn't really a rule.
Perhaps nobody analysed (or calculated) the optimum radius for the inner coils.

But there is an optimum inner radius for a given concentric coplanar coil.

Best to test it with big targets at depth.

Aziz

[Tinkerer]

No! Small RX coil is not efficient.
All secondary caused magnetic fields from the target should go through (inside) the RX coil. Every magnetic field outside the RX coil isn't visible to the detector.

The "half radius rule" for the concentric coil isn't really a rule.
Perhaps nobody analysed (or calculated) the optimum radius for the inner coils.

But there is an optimum inner radius for a given concentric coplanar coil.

Best to test it with big targets at depth.

Aziz

Hi Aziz,

right now I have a 80cm coil, with a 48cm Bucking coil and a 11cm, 1000uH RX coil. The assembly is not properly balanced, because I just wound some pieces of wire that I had lying around.
However, moving the Bucking coil and the RX coil around inside the TX coil, I got some kind of balance, stretching the TC coil into an oval shape with the Bucking coil and the RX coil offset to one end of the oval.

This leaves a large part of the TX coil free of other coils. This area also is quite sensitive, but does not have FE discrimination. I wonder what influence the number of turns in the TX has in that TX only part of the coil in respect of the RX sensitivity.

The way I understand it, we do not receive the eddy currents with the RX coil. In an IB configuration, we measure the distortion caused by the eddy current magnetic field, within the Induction balanced field of the IB assembly.
The usual co-planar concentric IB coils are also quite sensitive to targets outside of the coil assembly.

Tinkerer

[ApBerg]

No! Small RX coil is not efficient.
All secondary caused magnetic fields from the target should go through (inside) the RX coil. Every magnetic field outside the RX coil isn't visible to the detector.

The "half radius rule" for the concentric coil isn't really a rule.
Perhaps nobody analysed (or calculated) the optimum radius for the inner coils.

But there is an optimum inner radius for a given concentric coplanar coil.

Best to test it with big targets at depth.

Aziz

Perhaps nobody analysed (or calculated) the optimum radius for the inner coils.

But there is an optimum inner radius for a given concentric coplanar coil.

Hello Aziz

This is right !


Some 15 years ago I have made many inner coils, the outside coils where the transmit and first receive coil 100% coupling e.g. transmit 100 turns, first receive coil 18 turns both clockwise and 22 cm wire 0.25mm
Then made some tests with the second receive coil(s), placed counter clockwise in the centre from the tr. and first rec. coil.
To large sec. receive coils give bad pinpoint property's there was 2 times detection from small coins...in one sweep.

.....I find your coil discussions very nice ! Thanks !

Best regards

Ap

[Aziz]

The way I understand it, we do not receive the eddy currents with the RX coil. In an IB configuration, we measure the distortion caused by the eddy current magnetic field, within the Induction balanced field of the IB assembly.
The usual co-planar concentric IB coils are also quite sensitive to targets outside of the coil assembly.

Hi Tinkerer,

this is only valid for the following condition:
If we place a ferrite (or any other magnetic material having no conductivity and hence no eddy current would exist), the balance is disturbed only due to attraction of magnetic field lines to the ferrite. This inbalance of the flux area of the RX coil is then detected. There are other materials, which repel the magnetic fields, but the effect is much much smaller. The ferromagnetic attraction is much stronger.

For the receive coil however, it is totally irrelevant, what happens outside of it's flux area. It only detects, what is happening inside of it.

We usually measure the eddy current response of the targets. These eddy currents cause a secondary magnetic field. The sec. magnetic field distorts the balance in the flux area of the receive coil, which we measure only the secondary magnetic field induction.

Aziz

[Aziz]

Look at the magnetic field lines below. It is a 0.5*R concentric coplanar coil, showing the magnetic field lines without the target response.
If you add all magnetic field vectors (vector addition), you get a zero magnetic field vector (0,0,0) (=balanced coil).

[Aziz]

Coil Comparisons

Hi all,

below is the comparison of the interesting coils:
Every coil 10 " outer diamater (round)
- Standard mono coil (used in PI's only)
- Standard Double-D IB coil
- Standard concentric co-planar IB coil (R=0.5, half the outer radius)
- Optimized concentric co-planar IB coil (R=0.8, bigger inner radius)
- Coaxial IB coil (80 mm distance between RX and TX/Bucking)

Don't look at the response voltages. This is irrelevant. Look how it compares to other coil types. The x-axis is the detection depth in inch (air). Pity, you can not see how the depth performance goes further. The coaxial coil is becoming better in the distance but not better than the mono coil. Note, the deeper going coils are having less near-distance sensitivity.

As mentioned earlier, not really worth to make all the effort.

Cheers,

Aziz

[DOOLEY]

cheers aziz , as you said before , mono coil apears to take some beating.

can i be a pain and ask another question ??

coulple of PI designs out there that use sepperate TX and RX coils ,

going to try and word this so it makes sense.

if i wind a TX coil to be 400uh

then wind an exact copy for the RX and it's 400uh

if i have them close together , will the inductance of either coil alter ??

thinking of taking a former of say 12 inches , then winding both TX and RX at the same time , with same number of turns to get between 300uh to 400uh,

will the coils interact to alter the inductance of either ??

or will the inductance of each coil remain the same ??

only asking as dont want to end up with a realy slow coil arangement.

[Aziz]

Hi Dooley,

in the quote, you will find the blue answers.

cheers aziz , as you said before , mono coil apears to take some beating.

can i be a pain and ask another question ??
sure

coulple of PI designs out there that use sepperate TX and RX coils ,

going to try and word this so it makes sense.

if i wind a TX coil to be 400uh

then wind an exact copy for the RX and it's 400uh

if i have them close together , will the inductance of either coil alter ??
No, if no induced current is flowing in the other coil. If a current is flowing, we have then a mutual inductance. RX coil usually senses voltage rather than current. The current is normaly ultra tiny (op-amp input current).

thinking of taking a former of say 12 inches , then winding both TX and RX at the same time , with same number of turns to get between 300uh to 400uh,

will the coils interact to alter the inductance of either ??

or will the inductance of each coil remain the same ??

The coils might be sligthly different due to variation of winding compactness. But generally, they should have quite same inductivity. Again, if an induced current (inductive, capacitive) is flowing in the other coil (RX), the mutual inductance will have an effect to the TX coil.

only asking as dont want to end up with a realy slow coil arangement.

Dooley, you should focus to the easy to build coils. The DD coil arrangement is very simple and you can not make much mistakes.

Aziz

[DOOLEY]

CHEERS AZIZ ,

was thinking of trying a PI on seperate TX and RX , between 300uH to 400uH each approx , wound together same diameter, with the TX between + and mosfet to - ,
and RX grounded to - and other end going to op-amp , then that way i can use "one" voltage rather than multiple voltages. simples (sorry, meercat quote)

as coil inductance seams to be directly linked to speed of decay , i didnt want to end up with a realy slow coil if somehow they interacted somehow resulting in a combined inductance.