Minelab 8000

[Aziz]

Regarding the low capacitance coils and the benefit of spiral coils.

This is a good option as it allows tight and stable coil designs. The coil is split into two halves. Upper and lower layer with some distance gap.
A spider-web weaved tight coil design with two z-layers (see below).
I see, I forgot to rotate the upper layer TX coil wires a bit. This is important, to have large distances of the coil windings between the z-layers.


This is a good compromize option for a coil design having still some kind of spiral coil.
Aziz

[Aziz]

Or instead of radially weaved, a vertical weaved coil with two parts. A double basket weaved coil.

[Aziz]

Now rotated the upper layer of the spider web spiral coil. You can see it now, what I mean.

Such a coil is quite easy to make. And it will have low capacitance too.

[Aziz]

these types of concentric coils can suffer from target holes close to the coil surface ... eg within 5 cm. Basically the are in the nulling zone.

Nevertheless ... we must progress the uber bucking coil.

140 db optimally nulled bucking profile from 3d print files :

I am interested in the detection hole zone. If I have some more time, I'll do the sensitivity map cross section below the coil. The detection hole will be visible then.

Let's make the uber uber super duper coil.
Ask your very smart AI, what we can do. I don't have an AI account for such complex analysis.

[Aziz]

Or instead of radially weaved, a vertical weaved coil with two parts. A double basket weaved coil.

Of course one can combine a spirally (horizontal) weaved spider web coil with the vertical weaved basket coil. This makes even the coil building more easier and can be done in one step. You begin with the horizontal weaving until the space is filled and then it goes into the vertical direction. You use L formed round plastic angles mounted on a round coil former (wood).
After gluing the coil windings, you can cut the final coil from the wooden coil former. There you have it.

[Carl-NC]

In fact spiral wound coils have more self capacitance than say your solenoid wound coil.

Here is some data from built coils:


This shows that, yes, a solenoidal coil has the lowest capacitance (26.6pF) but not by much: the spiral is 28.1pF. And it comes with a price: for the same number of turns, the solenoidal coil produces a weaker field, both at the surface (B₀) and at a 1 diameter depth (B₁₀). Up until I built these, I had always assumed that the solenoidal coil would have the tightest and therefore the strongest field at depth, but measurements say no. To increase the solenoidal field strength would require more turns, which then will give it higher capacitance, resistance, etc. There are always caveats.

[moodz]

Here is some data from built coils:


This shows that, yes, a solenoidal coil has the lowest capacitance (26.6pF) but not by much: the spiral is 28.1pF. And it comes with a price: for the same number of turns, the solenoidal coil produces a weaker field, both at the surface (B₀) and at a 1 diameter depth (B₁₀). Up until I built these, I had always assumed that the solenoidal coil would have the tightest and therefore the strongest field at depth, but measurements say no. To increase the solenoidal field strength would require more turns, which then will give it higher capacitance, resistance, etc. There are always caveats.

Yes I should have qualified that post with the prefix " The AI reckons that .... " I put your post back to the AI and it admitted it was saying that 1.8 pF was a major improvment in coil capacitance

However having said that from my own experience PI detectors using commercial and hand built coils ... the flat spiral seems to be more sensitive to smaller gold nearer the surface but for deep targets the flat spiral struggles to match a plain old bundle wound mono or DD coil.

[Aziz]

I have good news to the Z18 coil users (the figure 7 coil):

The detection hole isn't in the bottom side of the coil.
If the ground is z=0 (coil laying on the ground), it is approx. 4-5 cm (2 inch) above the ground level at the coil center axis. So the detection hole is above the ground level.
I have checked this for the coil center axis only.

[Carl-NC]

However having said that from my own experience PI detectors using commercial and hand built coils ... the flat spiral seems to be more sensitive to smaller gold nearer the surface but for deep targets the flat spiral struggles to match a plain old bundle wound mono or DD coil.

Good to know, that's what I would have expected.

[Carl-NC]

So the detection hole is above the ground level.

The Minelab engineer at Wedderburn mentioned that the coil "focused more energy in the ground than above the coil." However, even if the RX arrangement results in a hole above the coil that doesn't mean the TX energy is asymmetric.

[Aziz]

The Minelab engineer at Wedderburn mentioned that the coil "focused more energy in the ground than above the coil." However, even if the RX arrangement results in a hole above the coil that doesn't mean the TX energy is asymmetric.

Yep. I will show the high resolution magnetic field strength cross section images soon. One can see, that the transmitted TX energy is fully symmetric.
I will show the target sweep resonponse voltages along the center axis of the coil. Each for the RX and nulling coil. One can see, how much target response is left due to the nulling coil.

[Aziz]

Hi all,

now the interesting stuff. The target response of the Z18 (figure-7) coil.
I have increased the target size to 2 inch diameter wire loop (1 mm wire thickness). Excitation current 1 A at 1 Mhz. Target sweep at z-axis (center of the coil), going from 10 inch above the ground level to almost 30 inch below the ground level. The detection hole is dependent of target size. But it is above the ground level and won't degrade the coil below the ground level.
The interesting stuff is, how much the target response signal is beeing subtracted due to the nulling coil (bucking RX coil).

All results are in a zipped Excel-file. I have put some pics to the attachment.
Next time, we are going to look at the magnetic field strength.
Cheers,
Aziz

[Aziz]

These are the missing parameters:

The above TX coil has 12 turns.
The below TX coil has 10 turns.
The main RX(+) coil has total 2.44 m² flux area. It consists of 2 x 8 turns ( = 16 ).
The nulling RX(-) coil has total 0.92 m² flux area. It has 11 turns.
So the total RX coil will induce approx. 38 % less EMI noise due to the nulling RX coil.

[Aziz]

Now the nice high resolution magnetic fields of the Z18 coil configuration (magnetic field strenght, H-Field). 1 A coil current.
See in the attachement please.

[Aziz]

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