Advanced PI coils

[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

[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

[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.

[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

[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

[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.

[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

[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.

[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.

[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

[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

[Aziz]

DD PI AI Figure-8 Coil Added

Hi all,

the DD PI AI Figure-8 coil (TX=RX, formed as back to back DD figure-8, 300 µH) doesn't go deep even with additional gain of 10x. Forget this coil once and forever if you want to chase deep targets.



Cheers,
Aziz

[Aziz]

...
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.

Hi Eric,

yes, my coil software is making the same observation too. The total magnetic flux doesn't change (much) when the all sized mono coils are designed to a specific inductivity (300 µH).

Aziz

[Ferric Toes]

Hi Eric,

yes, my coil software is making the same observation too. The total magnetic flux doesn't change (much) when the all sized mono coils are designed to a specific inductivity (300 µH).

Aziz

Yes, I try to make the inductance as close to 300uH as I can for any coil size, also I use a constant current source for the TX supply so that any changes in coil/cable resistance is compensated for as well.

Eric.

[Aziz]

Yes, I try to make the inductance as close to 300uH as I can for any coil size, also I use a constant current source for the TX supply so that any changes in coil/cable resistance is compensated for as well.

Eric.

When looking at SNR, the SNR (the N(RX)*A(RX) product) gets worse with larger mono coils due to picking up more EMI noise. That's the drawback with large mono coils.

But we can split the large mono coil of course:
Large TX, seperate small concentric co-planar RX.
The magnetic flux linkage gets better too (reduced) to reduce the overall ground noise.

Cheers,
Aziz