Cylinder Coils?

[hykj888]

PI FE discrimination is a joke, placing the same object horizontally and vertically results in two different outcomes, small iron blocks are identified as non-iron

[Carl-NC]

PI FE discrimination is a joke, placing the same object horizontally and vertically results in two different outcomes, small iron blocks are identified as non-iron

Same with VLF.

[Carl-NC]

Carl refers to as "constant current PI," is it the ZVT of the GPZ7000?

Yes, that is one example. The Garrett Axiom uses a CCPI pulse, plus an exponential pulse.

I am using a DD coil at present. It gives FE discrimination close to the coil, but the discrimination does not go the full depth.
Does anybody have more information about the relative depth of the FE discrimination of a DD coil?

I don't, but it's the same thing Minelab has always said about their PI iron discrimination.

[Tinkerer]

Yes, that is one example. The Garrett Axiom uses a CCPI pulse, plus an exponential pulse.



I don't, but it's the same thing Minelab has always said about their PI iron discrimination.

DD coils give a good sweep area, but less depth, a typical tradeoff.
The concentric coils give a reduced sweep area. The concentric, induction balanced coil give less depth and less sweep area.
An option outside the box, would be a concentric coil where the RX coil is about the same diameter than the TX coil. I don't know if anybody has done it yet, but believe it is possible. This could be an interesting project.
For different ergonomics, a tandem coil could be considered.

[Detectorist#1]

What you mean under "tandem coil" - OO coil or some different?

[Tinkerer]

What you mean under "tandem coil" - OO coil or some different?

More like 2 TX coils side by side, like OO, and a RX coil in the center, overlapping the TX coils for induction balance.
GPX 7000 uses something similar, but it is 2RX coils and one TX coil in the center.

The Current in the tandem TX coils can be both clockwise or one clockwise and the other counter clockwise. The magnetic fields f the 2 TX coils can be aiding or opposing.

[Carl-NC]

DD coils give a good sweep area, but less depth, a typical tradeoff.
The concentric coils give a reduced sweep area. The concentric, induction balanced coil give less depth and less sweep area.

DD isn't as great on sweep area as manufacturers would have you believe. Here is an illustration from one manufacturer:


Here are measured results, comparing a White's 950 CC with a Fisher DD:


Only for very shallow targets does that DD (black curves) give the alleged "windshield wiper" pattern. As you go deeper, they converge to about the same coverage.

An option outside the box, would be a concentric coil where the RX coil is about the same diameter than the TX coil. I don't know if anybody has done it yet, but believe it is possible. This could be an interesting project.

Garrett SeaHunter Mk2 and Infinium both had mono-looking coils that were actually split coils (separate TX & RX coils). I assume the GPX6000 does too, otherwise their TX scheme wouldn't work.
If you mean an IB coil where TX & RX are the same size, Alexander Bell used this method in 1881:


There is the "search coil" to the right, and a counter-balance transformer to the left. In this illustration, the coils are not equal in size but they could be.

[Detectorist#1]

I prefer to use configuration with 2 Rx coils inside one big Tx coil. The two Rx coils are connected as planar gradiometer. The planar gradiometer can process weak signals even under a lot of noise. In "Fe" mode, no Tx pulses and with 15cm distance between Rx 300uH coils , Fe bottle caps are surely detected at 15cm. No false signal from non-Fe targets. In "All metals" mode, for stable operation without EFE problems, Tx coil have to be bifilar type or bipolar pulsed. In "ALL metal" mode US 25c coin is surely detected at 30-35cm.

[Detectorist#1]

Next my design will be with oOo configuration with gradiometer connection of the two Rx coils and induction balanced Rx coils. Tx coil will be bifilar with two Tx switches or bipolar pulsed.

[Mdtoday]

MdToday, fair enough, just one more question about 'Forties" road, yeah i do have the yearly pass, but we can't use that now, is Forties road always closed by gate?, i think when i drive past it ,it's always open, but i'm not sure, and since we aren't allowed to drive inside now, how or by what road would you choose to get into that area, thanx mate?

Nova, yes, all closed up for fire ban and yes, Forties always gated. Have to go ridge road then back down into it.
Can park the car on Rocky Creek and walk in. Try using small coil and look for the gravel that has been ripped up when they planted.
There is a large area, just use the Forties as your reference is my advice. See attached.

cheers.

[hykj888]

This is the DOD coil I made two years ago, which uses bipolar emission and can stably sample at 2us. It has very high sensitivity to small gold nuggets,

[Detectorist#1]

Hi hykj888,
Thank you for the picture. I hope to repeat your construction with good sensitivity and only 2us sample delay!

[Detectorist#1]

Hi hykj888,
Give please more information for the coils of your DOD sensor: dimensions, wires, turns, inductivity, SFR frequency of Tx coil and Rx coils, type of used screens.

[GeoMax]

Hi hykj888,
can you also post the front-end and driver circuit for your DOD coil?

[Tinkerer]

More like 2 TX coils side by side, like OO, and a RX coil in the center, overlapping the TX coils for induction balance.
GPX 7000 uses something similar, but it is 2RX coils and one TX coil in the center.

The Current in the tandem TX coils can be both clockwise or one clockwise and the other counter clockwise. The magnetic fields f the 2 TX coils can be aiding or opposing.

Here is a visualization of the magnetic fields of a Tandem coil in opposing and in aiding configurations.