Advanced PI coils

[Midas]

Some coil arrangements that have been discussed are described as having the ability to GB. Surely the only thing that can happen is that the amplitude of the ground signal is reduced from that of a mono coil, but that electronic GB will still be necessary. The only situation where a good cancellation takes place is when the coil is perfectly horizontal over a perfectly flat and uniform ferrite mineralised area. As soon as the coil is tilted either way the GB is no more as the amplitudes are no longer equal. Also, most operators swing in an arc to some degree. The GB must be a system that works on the t^-1 decay and be independant of amplitude. Since IMHO you have to do this, you might just as well stick with a mono coil or a coplanar concentric TX plus separate RX. The humble mono can be improved as we have seen by bifilar winding, differential preamp etc, and maybe double shielding. I have been making tests on a double shielded 12in mono and I can sample at 6.5uS with a non differential LME49990 preamp with input diodes. The double shielding, I hope, gives more like a 2nd order filter to rf frequencies. Certainly it is much less noisy and I can pick up more stations if I use it as a radio antenna. A bifilar mono with dual shielding should be even better. Maybe a 2.5 order filter .

One interesting thing I noted when I was doing tests in Australia, was that for mono coils sitting on the ground surface the signal level at the output of the integrator (no GB) was the same for any coil diameter. This makes sense when you think about it. Also, the level was nowhere near saturating the preamplifier as some have thought in the past. This was on one of the worst ironstone areas in Victoria.

Eric.

Hi Eric,

Thanks for all your very informative posts of late. What exactly do you mean by double shielding ? And what sort of pre-amp gain were you running when you say you were 'nowhere near' saturation? How much do you think you could run before it did become a problem? I seem to remember reading on here somewhere that more than 50 could become a problem in some ground.

Midas

[dougAEGPF]

Some coil arrangements that have been discussed are described as having the ability to GB. Surely the only thing that can happen is that the amplitude of the ground signal is reduced from that of a mono coil, but that electronic GB will still be necessary. The only situation where a good cancellation takes place is when the coil is perfectly horizontal over a perfectly flat and uniform ferrite mineralised area. As soon as the coil is tilted either way the GB is no more as the amplitudes are no longer equal. Also, most operators swing in an arc to some degree. The GB must be a system that works on the t^-1 decay and be independant of amplitude. Since IMHO you have to do this, you might just as well stick with a mono coil or a coplanar concentric TX plus separate RX. The humble mono can be improved as we have seen by bifilar winding, differential preamp etc, and maybe double shielding. I have been making tests on a double shielded 12in mono and I can sample at 6.5uS with a non differential LME49990 preamp with input diodes. The double shielding, I hope, gives more like a 2nd order filter to rf frequencies. Certainly it is much less noisy and I can pick up more stations if I use it as a radio antenna. A bifilar mono with dual shielding should be even better. Maybe a 2.5 order filter .

One interesting thing I noted when I was doing tests in Australia, was that for mono coils sitting on the ground surface the signal level at the output of the integrator (no GB) was the same for any coil diameter. This makes sense when you think about it. Also, the level was nowhere near saturating the preamplifier as some have thought in the past. This was on one of the worst ironstone areas in Victoria.

Eric.

Thanks Eric,very interesting observations re: ironstone and mono coils.
Another interesting paper
Improving UXO Detection and Discrimination in Magnetic Environments
http://australianelectronicgoldprosp...cseen#msg26725
dougAEGPF

[Ferric Toes]

Some coil arrangements that have been discussed are described as having the ability to GB. Surely the only thing that can happen is that the amplitude of the ground signal is reduced from that of a mono coil, but that electronic GB will still be necessary. The only situation where a good cancellation takes place is when the coil is perfectly horizontal over a perfectly flat and uniform ferrite mineralised area. As soon as the coil is tilted either way the GB is no more as the amplitudes are no longer equal. Also, most operators swing in an arc to some degree. The GB must be a system that works on the t^-1 decay and be independant of amplitude. Since IMHO you have to do this, you might just as well stick with a mono coil or a coplanar concentric TX plus separate RX. The humble mono can be improved as we have seen by bifilar winding, differential preamp etc, and maybe double shielding. I have been making tests on a double shielded 12in mono and I can sample at 6.5uS with a non differential LME49990 preamp with input diodes. The double shielding, I hope, gives more like a 2nd order filter to rf frequencies. Certainly it is much less noisy and I can pick up more stations if I use it as a radio antenna. A bifilar mono with dual shielding should be even better. Maybe a 2.5 order filter .

One interesting thing I noted when I was doing tests in Australia, was that for mono coils sitting on the ground surface the signal level at the output of the integrator (no GB) was the same for any coil diameter. This makes sense when you think about it. Also, the level was nowhere near saturating the preamplifier as some have thought in the past. This was on one of the worst ironstone areas in Victoria.

Eric.

[Tinkerer]

How about a bicycle coil. It comprises two Tx Rx combinations in two separate wheels parallel to the ground, where Rx coils are at 0.9 Tx radius. Both Tx and Rx coils are connected in series, only the phases are reversed at Rx to achieve IB, AI, and GB. The only trouble is that such contraption requires a real 4 quadrant gear to squeeze the most out of it. It is a true differential coil with distance between Tx coils of ~ 1 radius. It is a wearable configuration with coil sizes exceeding ~ half a meter radius. IMHO it is a perfect substitute for a 2 box.

This is about what I mean with a Tandem coil. A Tandem bicycle has 2 drivers. Tandem coils have 2 TX. It makes a big difference in the field vectors if the coils are same phase or opposite phase. The distance between the coils affects the field differently in either case.

But I think it is possible to achieve good AI, IB and GB, experimenting with the many possible configurations.

[Tinkerer]

Tandem coils

Here are the Tandem coils, same phase and opposing phase

Note the distance of the coils is not the same for the 2 pictures

[Davor]

How about a bicycle coil. It comprises two Tx Rx combinations in two separate wheels parallel to the ground, where Rx coils are at 0.9 Tx radius. Both Tx and Rx coils are connected in series, only the phases are reversed at Rx to achieve IB, AI, and GB. The only trouble is that such contraption requires a real 4 quadrant gear to squeeze the most out of it. It is a true differential coil with distance between Tx coils of ~ 1 radius. It is a wearable configuration with coil sizes exceeding ~ half a meter radius. IMHO it is a perfect substitute for a 2 box.

[Aziz]

Yes, the comparison is difficult and there are many different configurations possible.
For the ground balance it is important for the coil assembly to be parallel to the ground, easier if the assembly is narrow, 2 D's, back to back, or 2 rectangles. This will reduce the depth.

Any smaller than 10"TX is going to reduce the depth.

Maybe more important than comparative depth, would be to look at the ground balance and IA.

The distance between the 2 TX will also affect the detection between the coils.

However, the IA and GB effected at the coil level has some appeal, as Davor insists telling us.

Well, I can make the following:
- 10 inch round mono loop TX (300 µH), inside a back to back DD (RX+, RX- AI) as figure-8 RX, SNR compensated RX

- 10 inch back to back DD (AI) as figure-8 TX=RX (300 µH)

A more or less fair comparison to our coils.
Aziz

[authere]

Hi Ron,

unfortunately, I have no idea, what you really mean. Could you show a pic or a sketch of it?
Aziz

Hi Aziz,

Can't draw but if you imagine a normal coil bundle which is 4rows of 5wires or 5 rows of 5 wires in cross section to make a coil bundle, just imagine a tubular cross section to the coil bundle but laid in the clockwork manner described before

Cheers Ron

[Tinkerer]

Hi Tinkerer,

we have a problem in making the comparison fair.
1. Coil size (should have the same area/dimension of a 10 inch mono coil).
2. The TX magnetic field strength will be distributed on the two TX halves.
3. The total TX inductance 300 µH -> distributed over the two TX halves.

The distance of the two coils? Round coil halves? Size?
Aziz

Yes, the comparison is difficult and there are many different configurations possible.
For the ground balance it is important for the coil assembly to be parallel to the ground, easier if the assembly is narrow, 2 D's, back to back, or 2 rectangles. This will reduce the depth.

Any smaller than 10"TX is going to reduce the depth.

Maybe more important than comparative depth, would be to look at the ground balance and IA.

The distance between the 2 TX will also affect the detection between the coils.

However, the IA and GB effected at the coil level has some appeal, as Davor insists telling us.

[Aziz]

Hi Aziz,

No, no,no, consider this, try thinking of using ribon wire where the wires are laid side by side, use this to wrap around the outside of a tube.

Then you should understand what i explained before

Ron

Hi Ron,

unfortunately, I have no idea, what you really mean. Could you show a pic or a sketch of it?
Aziz

[Aziz]

Tandem coil, what is meant by that:

Place 2 coils side by side.
TX in counter phase, so that the coil fields are aiding, the field vectors are north-south in one coil, south-north in the other coil. This causes the field vectors to go more horizontal at a certain depth, enhancing detection of vertical coins.

If we do this with 2 concentric, coplanar IB coils, the RX+ and RX- signals can be added to cancel the interference and the ground.

Hi Tinkerer,

we have a problem in making the comparison fair.
1. Coil size (should have the same area/dimension of a 10 inch mono coil).
2. The TX magnetic field strength will be distributed on the two TX halves.
3. The total TX inductance 300 µH -> distributed over the two TX halves.

The distance of the two coils? Round coil halves? Size?
Aziz

[authere]

Hi Ron,

you surely mean the low capacitance coil weave technique (see examples below):

http://www3.telus.net/chemelec/Projects//Metal/New-Coil.jpg

http://www.vk2zay.net/article/file/865

http://img172.imageshack.us/img172/2733/silk1ir1.jpg

Yes, I can do such coil simulations as well. But they won't perform better than the mono coil. The inductance law remain the same of course. It's just a means of reducing the coil capacitance.

The benefit I see is, that the magnetic field strength is getting diluted over a wider region range due to a wider coil bundle, which is causing less disturbing ground magnetising effects.

Cheers,
Aziz

Hi Aziz,

No, no,no, consider this, try thinking of using ribon wire where the wires are laid side by side, use this to wrap around the outside of a tube.

Then you should understand what i explained before

Ron

[Aziz]

Hi Eric,

Hi Aziz,

Something I have thought about but never tried is the use of a soft ferrite cored inductor as RX2 in a AI coil system. This inductor would be a small diameter (less than 25mm) and more easily mounted above and on axis than another air cored coil. I have used ferrite cored coils for the main TX/RX mono type arrangement and they work well, provided you get the right grade of ferrite. What do you think?

Also, another unusual IB coil arrangement I saw in a book many years ago, was a ferrite rod with the RX coil mounted horizontally inside the outer TX coil. As the axes are at right angles, there is a point where coupling is zero. The book was about building your own detector which was a simple VLF design.

Eric.

unfortunately, my coil software is limitted to air core coils. Ferrite cored coils would make the task very non-trivial.

Why not. Provided that, it picks up the same amount of EMI noise sure a winner due to its small size. But due to the different coil characteristics, I would suggest to use a matching network (of RX1 and RX2) to maximize the EMI noise rejection ratio. A big pitfall is, that the EMI rejection ratio is frequency dependent (due to different coil characteristics).

The second proposal:
The GB is going to be challenge. Ground noise could be high. Could be a hotrock detector.
I don't know. It should simply be tested.

Cheers,
Aziz

[Tinkerer]

Hi all,

I'm open to other ideas and wishes now as I have covered most interesting coil architectures. Maybe I did forget something. Please, just remind me.

- I have left over the two-box principle (trivial and causing a lot of work, response is dependent on target orientation, would require a coil software upgrade).

- The tandem coil - what was meant by it? I need a description or a sketch so we know, we are talking about the same thing.

- Spiral coils: trivial but causing a lot of processing time (more wire elements required).

Anything else you would like to know and how a new coil performs?

Or should we invent a new coil together? Yes, we can do it.

Cheers,
Aziz

Tandem coil, what is meant by that:

Place 2 coils side by side.
TX in counter phase, so that the coil fields are aiding, the field vectors are north-south in one coil, south-north in the other coil. This causes the field vectors to go more horizontal at a certain depth, enhancing detection of vertical coins.

If we do this with 2 concentric, coplanar IB coils, the RX+ and RX- signals can be added to cancel the interference and the ground.

[Ferric Toes]

Hi Aziz,

Something I have thought about but never tried is the use of a soft ferrite cored inductor as RX2 in a AI coil system. This inductor would be a small diameter (less than 25mm) and more easily mounted above and on axis than another air cored coil. I have used ferrite cored coils for the main TX/RX mono type arrangement and they work well, provided you get the right grade of ferrite. What do you think?

Also, another unusual IB coil arrangement I saw in a book many years ago, was a ferrite rod with the RX coil mounted horizontally inside the outer TX coil. As the axes are at right angles, there is a point where coupling is zero. The book was about building your own detector which was a simple VLF design.

Eric.