Coil improvements

[Tinkerer]

All sorts of coils have been made for PIMD's.
Are there still improvements possible?

I believe there are many improvements possible.
Attached are some pictures.

1mH_1Mhz_coil and close-up of the winding, is a coil winding, still on the winding form. The coil has a diameter of 27 cm.
Inductance is 1mH.
Self resonant frequency is 1Mhz. I hope this coil will give a broad RX sensitivity field.

Coil Focus are representing RX sensitivity fields.
To the left, a deep going sensitivity field that is funnel shaped. It goes deep, but the sensitivity focus at the deep end is very narrow.
To the right, a broad sensitivity field that does not go as deep but will have much better coverage.

I have not found much information in the public domain about focusing the detection sensitivity fields of PI coils.

Any information would be welcome.

Tinkerer

[Aziz]

Hi Tinkerer,

there is still improvement to squeeze out:

1. Coil capacitance and effectivity:
Coils making not too much wider. Instead, one could wind in the height direction too. So some distance ring could be used for this. The result will be a square cross-section coil. The wire intercapacitance can be kept low to make PI coils faster.

2. Shielding:
a) Shielding should not be directly made on the coil wires. Some distance 1-2 mm should be used, to lower the shield capacitance.

b) Using low eddy-current shielding:
High impedance (high resistance) coil shielding can be used. Such as with graphite shielding.

or
c) Using very thin enamelled wire wound across the wires. This wire goes from the coax shielding (soldered to shielding) to half of the coil. Another wire is wound on the other half. They are not connected together - they are only connected on one point of the coax shielding. The windings could be spaced on every 1-2 mm. It depends on the wave length, which EM fields you want to screen out. The closer the windings, the higher EM's could be screened out. But the closer the windings, the more capacitance it will be.
c) is probably not used yet. But this gives much improvements on PI coils and one should make some tests on it.

Regards,
Aziz

[Aziz]

Following image illustrates the very low eddy current shielding ( option c):

I haven't built such a coil but it is an idea to check out.
Aziz

[KingJL]

Tinkerer,
Any technical info on the coils that is pictured? I am wondering whether there is any value to making the RX winding of a concentric using that methodology. I know they make super sensitve radio Rx antennae with that type of winding.

BTW, I made a 9 1/2" x 4 1/2" eliptical figure 8 coil (as in attached diagram). I am pleasantly surprised by the results. Every bit as deep (maybe even deeper) than my 11" DD, and much much quieter. Windings were: Tx 7.125" 23 turns 298 uH, RX 9.54" 27 turns 575 uH. I think I may make a 15" x 4" rectangular version of a figure 8. I really like that coil!

[Tinkerer]

Coil improvement

Hi Tinkerer,

there is still improvement to squeeze out:

1. Coil capacitance and effectivity:
Coils making not too much wider. Instead, one could wind in the height direction too. So some distance ring could be used for this. The result will be a square cross-section coil. The wire intercapacitance can be kept low to make PI coils faster.

2. Shielding:
a) Shielding should not be directly made on the coil wires. Some distance 1-2 mm should be used, to lower the shield capacitance.

b) Using low eddy-current shielding:
High impedance (high resistance) coil shielding can be used. Such as with graphite shielding.

or
c) Using very thin enamelled wire wound across the wires. This wire goes from the coax shielding (soldered to shielding) to half of the coil. Another wire is wound on the other half. They are not connected together - they are only connected on one point of the coax shielding. The windings could be spaced on every 1-2 mm. It depends on the wave length, which EM fields you want to screen out. The closer the windings, the higher EM's could be screened out. But the closer the windings, the more capacitance it will be.
c) is probably not used yet. But this gives much improvements on PI coils and one should make some tests on it.

Regards,
Aziz

Hi Aziz,
Thanks for the reply.
1)Coil capacitance
How can we reduce inter-wiring capacitance?
Thin wire gives less capacitance.
Insulation of the wires reduces capacitance, best is air, Teflon is second, Polypropylene is good.
Too much insulation reduces the k factor, that is the inductance coupling effect between the wires, that means that more turns of wire will be needed to achieve the same inductance.
Where lies the sweet spot, where the inter wire capacitance is lowest and the k factor is highest?
2)Shielding
a)+ b) I have been using a spacer of 8mm Polypropylene/air board between the coil winding and the graphite composition shield. The results are good.
Again, there is a compromise to be found between distance and capacitance.
Air is the best spacer, therefore foam material is ideal for securing the coil within the housing to avoid any movement. Glass micro-balloon filled Epoxy resin would be easy to implement and should give good results.
b)Hmmm, the way I see it, the spiral wrapping wire would act like a capacitively coupled inductor.

Tinkerer

[bbsailor]

More Coil Improvements Exercise

Try the wire mesh shield called Scotch24. The specifications are at this link:http://multimedia.mmm.com/mws/mediaw...666WTUCOrrrrQ-

This mesh is made from AWG36 sizes wire. It has some air space so that the area is about 1/2 to 1/3 that of a solid shield. This stuff is not detected at very short delays.

For those doing experiments, try this exercise. It will be very educational.

1. Measure the coil self resonance without the shield but tightly bound together to obtain the final inductance. Deduce the capacitance to cause this self resonance.

2. Add a shield and measure the coil-to-shield capacitance with a capacitance meter.

3. Measure the new coil self resonance with the shield added and connected to one coil lead. Deduce the capacitance to cause this self resonance.

4. Compare the deduced capacitance is step 1 to the capacitance in step 3.

Questions.
1. How much capacitance did the shield add to the coil to lower it's self resonance?
2. What percentage is the added capacitance in question 1 to the total measured coil-to-shield capacitance in step 2?

Doing these measurement with thin coil bundles (about 4mm cross section) as well thicker coil bundles (8mm + cross section) will provide some interesting results and conclusions about what makes faster coils (lower capacitance).

bbsailor

[Aziz]

The method c) is not inductively coupled to TX coil. It is only inductively coupled to any possible external magnetic field.
Reason: flux area is perpendicular to magnetic field generated from TX coil.

A spacer like the shielding should be used before winding the super shield. An insulated thin wire core with thick insulation would be fine to keep the inter-wire distance constant.

Anybody interested on the super shielding?
Aziz

[Tinkerer]

Info on coils

Tinkerer,
Any technical info on the coils that is pictured? I am wondering whether there is any value to making the RX winding of a concentric using that methodology. I know they make super sensitve radio Rx antennae with that type of winding.

BTW, I made a 9 1/2" x 4 1/2" eliptical figure 8 coil (as in attached diagram). I am pleasantly surprised by the results. Every bit as deep (maybe even deeper) than my 11" DD, and much much quieter. Windings were: Tx 7.125" 23 turns 298 uH, RX 9.54" 27 turns 575 uH. I think I may make a 15" x 4" rectangular version of a figure 8. I really like that coil!

Hi KingJL,
your elliptical coil looks very interesting. Could you make a few tests for me? Compare it's depth with a 9 1/2" mono coil?
Do you sweep the coil in it's longitudinal direction or the narrow direction?

My coils,
The 1mH, 1Mhz coil is wound on the cut-off bottom of a plastic bucket. I have not tried it out yet. If it works good I will remove it from the mold and use it together with a TX and a Bucking coil, shielding all 3 coils.
I lost count on how many turns this coil has, kind of hard to count the turns after it is made.
It is wound with #30 magnet wire.
I am planning to make the TX coil of larger diameter and the bucking coil the same diameter as the RX coil.
The bucking coil in close contact with the RX coil, above the RX coil.
The TX coil wound in a square bundle, adding Polypropylene/air spacer between the windings.
The bucking coil wit the same kind of spacer, but spiral wound to have the same maximum and minimum diameter as the RX coil and then lay it on top of the RX coil.

The drawing of the coil with the funnel-like RX sensitivity field refers to a coil that is spiral wound with only a small hole in the center. I have built several coils with this configuration and observed the RX sensitivity field in experiments.
Tinkerer

[Tinkerer]

Try the wire mesh shield called Scotch24. The specifications are at this link:http://multimedia.mmm.com/mws/mediaw...666WTUCOrrrrQ-

This mesh is made from AWG36 sizes wire. It has some air space so that the area is about 1/2 to 1/3 that of a solid shield. This stuff is not detected at very short delays.

For those doing experiments, try this exercise. It will be very educational.

1. Measure the coil self resonance without the shield but tightly bound together to obtain the final inductance. Deduce the capacitance to cause this self resonance.

2. Add a shield and measure the coil-to-shield capacitance with a capacitance meter.

3. Measure the new coil self resonance with the shield added and connected to one coil lead. Deduce the capacitance to cause this self resonance.

4. Compare the deduced capacitance is step 1 to the capacitance in step 3.

Questions.
1. How much capacitance did the shield add to the coil to lower it's self resonance?
2. What percentage is the added capacitance in question 1 to the total measured coil-to-shield capacitance in step 2?

Doing these measurement with thin coil bundles (about 4mm cross section) as well thicker coil bundles (8mm + cross section) will provide some interesting results and conclusions about what makes faster coils (lower capacitance).

bbsailor

Hi bbsailor,

Nice to have you helping with this thread. As yet, nobody makes faster coils than bbsailor.

Here is the way I see the problem. My view might not be correct, so please correct me.
For every increase of diameter, the circumference increases pi or about 3,14. This means that the coil to shield area increased that much and adds that much more capacitance.
When we add a spacer around the coil winding, the area increases much more, however, the distance between the coil and the shield also increases, diminishing the capacitance.
The matter that fills the space between coil and shield is therefore very important. Air is best.
At some given distance, the relation between the distance and "capacitive area" is ideal.
What is this distance? How do I calculate this relationship?
Tinkerer

[Tinkerer]

Coil to shield capacitance

Try the wire mesh shield called Scotch24. The specifications are at this link:http://multimedia.mmm.com/mws/mediaw...666WTUCOrrrrQ-

This mesh is made from AWG36 sizes wire. It has some air space so that the area is about 1/2 to 1/3 that of a solid shield. This stuff is not detected at very short delays.

For those doing experiments, try this exercise. It will be very educational.

1. Measure the coil self resonance without the shield but tightly bound together to obtain the final inductance. Deduce the capacitance to cause this self resonance.

2. Add a shield and measure the coil-to-shield capacitance with a capacitance meter.

3. Measure the new coil self resonance with the shield added and connected to one coil lead. Deduce the capacitance to cause this self resonance.

4. Compare the deduced capacitance is step 1 to the capacitance in step 3.

Questions.
1. How much capacitance did the shield add to the coil to lower it's self resonance?
2. What percentage is the added capacitance in question 1 to the total measured coil-to-shield capacitance in step 2?

Doing these measurement with thin coil bundles (about 4mm cross section) as well thicker coil bundles (8mm + cross section) will provide some interesting results and conclusions about what makes faster coils (lower capacitance).

bbsailor

Hi bbsailor,

Nice to have you helping with this thread. As yet, nobody makes faster coils than bbsailor.

Here is the way I see the problem. My view might not be correct, so please correct me.
For every increase of diameter, the circumference increases pi or about 3,14. This means that the coil to shield area increased that much and adds that much more capacitance.
When we add a spacer around the coil winding, the area increases much more, however, the distance between the coil and the shield also increases, diminishing the capacitance.
The matter that fills the space between coil and shield is therefore very important. Air is best.
At some given distance, the relation between the distance and "capacitive area" is ideal.
What is this distance? How do I calculate this relationship?
Tinkerer

[Tinkerer]

not inductively coupled

The method c) is not inductively coupled to TX coil. It is only inductively coupled to any possible external magnetic field.
Reason: flux area is perpendicular to magnetic field generated from TX coil.

A spacer like the shielding should be used before winding the super shield. An insulated thin wire core with thick insulation would be fine to keep the inter-wire distance constant.

Anybody interested on the super shielding?
Aziz

Aziz,

you are right, the flux area is perpendicular to the coil field so the coupling would be minimal.
How about the coil current?
Tinkerer

[KingJL]

Hi KingJL,
your elliptical coil looks very interesting. Could you make a few tests for me? Compare it's depth with a 9 1/2" mono coil?
Do you sweep the coil in it's longitudinal direction or the narrow direction?
Tinkerer

Tinkerer,
Sorry but I have no mono coil to test with. Only a 11" DD and this figure 8. In the future, I am going to make a bigger figure 8 and a 11" coplanar concentric.

The eliptical figure 8 is mounted with the long axis pointint to the front and back. The front half of the figure 8 is the 'in-phase' signal (giving a positive response) and the back half gives a 180 degree 'out-of-phase' signal (negative response). The housing is from Hays Electronic. It is the 'Goldbug' style.

[Aziz]

Aziz,

you are right, the flux area is perpendicular to the coil field so the coupling would be minimal.
How about the coil current?
Tinkerer

Which coil current? I have no idea, what you mean by that.

The second fact of this super shield is, that only middle part is connected to coax shield. Other endpoints of the shielding has no potential to any electronics nor to each other (because not connected).

The shield acts as a very low eddy current causing EMF shield.
Aziz

[Tinkerer]

Which coil current? I have no idea, what you mean by that.

The second fact of this super shield is, that only middle part is connected to coax shield. Other endpoints of the shielding has no potential to any electronics nor to each other (because not connected).

The shield acts as a very low eddy current causing EMF shield.
Aziz

Aziz,

The RX coil is inductively coupled with the TX coil. At any time a current flows in the TX coil,there is also a current in the RX coil.
With the standard PI it is the endeavor to have these currents decay as fast as possible in order to measure the secondary currents generated by the fields from the target in the RX coil.

I have observed that the amplitude of the currents in the RX coil is much influenced by the size of the damping resistor that is across the RX coil.

A conductive spiral that is wound around the RX coil will be inductively coupled to the currents running in the coil.

Tinkerer

[Stefan]

Hi Aziz,

I have been using method "C" for about 12 years. I make the shield material using enamelled 0.2 millimeter enamelled copper. This shield material is optimum for a PI and there is no coupling of the magnetic field through the shield. I wind the shield onto the coil so that the wires are at 90 deg to the coil winding. I also keep a spacer between shield and coil winding to minimise capacitance effects..

Regards,

Stefan


The sepearationI

Hi Tinkerer,

there is still improvement to squeeze out:

1. Coil capacitance and effectivity:
Coils making not too much wider. Instead, one could wind in the height direction too. So some distance ring could be used for this. The result will be a square cross-section coil. The wire intercapacitance can be kept low to make PI coils faster.

2. Shielding:
a) Shielding should not be directly made on the coil wires. Some distance 1-2 mm should be used, to lower the shield capacitance.

b) Using low eddy-current shielding:
High impedance (high resistance) coil shielding can be used. Such as with graphite shielding.

or
c) Using very thin enamelled wire wound across the wires. This wire goes from the coax shielding (soldered to shielding) to half of the coil. Another wire is wound on the other half. They are not connected together - they are only connected on one point of the coax shielding. The windings could be spaced on every 1-2 mm. It depends on the wave length, which EM fields you want to screen out. The closer the windings, the higher EM's could be screened out. But the closer the windings, the more capacitance it will be.
c) is probably not used yet. But this gives much improvements on PI coils and one should make some tests on it.

Regards,
Aziz