Pi figure 8 coil to cancel salt water response ... possible ... or not

[Polymer]

Thanks for posting this Carl! It supports what I am looking into. Made an extra thread as not to muddy green?s thread.

https://www.geotech1.com/forums/show...coil-at-Tx-off

Generally speaking, for a fixed power supply voltage fewer turns on the TX produces more ampere-turns. Ideally you would want a 1-turn coil but then your power supply current would be stratospherically high. So you compromise to something you can live with.

On the RX side the opposite is true: more turns produces a higher induced voltage. Ideally you would want a gazillion turns but then you couldn't lift the coil, and the parasitic C would kill you. And, yes, more turns will also induce more EMI noise and generate more thermal noise. So you compromise to something you can live with.

If using a mono coil, then the two coils must compromise to a single winding, and that seems to have settled out to be around 300uH. If using 2 coils (whether IB or not) you can individually optimize them. I would look at 100-300uH on the TX and 500-1000uH on the RX. IB is preferably if you want seriously early sampling.

Given a PI salt water coil ... hooked up to the MPP E. Mono Tx, figure 8 Rx coil.

Here are my assumptions:

- The figure 8 coil would cancel out the salt water response to a great extent (no 100% perfect geometry possible)
- The earth field signal would also cancel out
- I could effectively only detect with one half of the figure 8 coil ... at least in the MPP E setup

Are these assumptions bonkers or ... any input appreciated.

I am in coil building mood and have time now ...

[Carl-NC]

Yes, a Figure-8 coil should do a pretty good job of canceling salt. Also Earth field, and far-field EMI. I would turn the 8 sideways so as you sweep the coil both lobes sweep over the target. Think of the DOD coil on the Minelab GPZ... that's exactly what it is. The raw output form the demod then looks like a first derivative signal, and the output from the SAT circuit looks like a 2nd derivative signal. The glitch is this: when you sweep in one direction over the target, the SAT out is positive, and in the other direction it is negative. You can fix this with an absolute-value circuit.

[Melbeta]

Yes, a Figure-8 coil should do a pretty good job of canceling salt. Also Earth field, and far-field EMI. I would turn the 8 sideways so as you sweep the coil both lobes sweep over the target. Think of the DOD coil on the Minelab GPZ... that's exactly what it is. The raw output form the demod then looks like a first derivative signal, and the output from the SAT circuit looks like a 2nd derivative signal. The glitch is this: when you sweep in one direction over the target, the SAT out is positive, and in the other direction it is negative. You can fix this with an absolute-value circuit.

this gives me a question. So was Jack Roach's SEMTECH detector coil, a figure 8? I think it was myself... He used it to identify positive signals from negative signals.
Melbeta

[Carl-NC]

this gives me a question. So was Jack Roach's SEMTECH detector coil, a figure 8? I think it was myself... He used it to identify positive signals from negative signals.
Melbeta

No idea, don't think I've ever seen one.

[6666]

Yes, a Figure-8 coil should do a pretty good job of canceling salt. Also Earth field, and far-field EMI. I would turn the 8 sideways so as you sweep the coil both lobes sweep over the target. Think of the DOD coil on the Minelab GPZ... that's exactly what it is. The raw output form the demod then looks like a first derivative signal, and the output from the SAT circuit looks like a 2nd derivative signal. The glitch is this: when you sweep in one direction over the target, the SAT out is positive, and in the other direction it is negative. You can fix this with an absolute-value circuit.

Like this ?, does a folded FIG 8 need damping on both sides or only if you use two separate coils, my guess is if you use two coils for the fig 8
also if useing a fig 8 where is the correct feed point for the feed line, my guess at the cross over.

[Carl-NC]

Like this ?, does a folded FIG 8 need damping on both sides or only if you use two separate coils, my guess is if you use two coils for the fig 8
also if useing a fig 8 where is the correct feed point for the feed line, my guess at the crossover.

Yes, that's what I meant. I've never made a Figure-8 by folding a coil, always use 2 separate coils. But on a PI I suspect the coils need individual damping so the folded coil won't work. You can place the feed point(s) anywhere.

[6666]

Thanks, just for fun might try and 3D print a simple Fig 8 coil former

[Polymer]

Yes, that's what I meant. I've never made a Figure-8 by folding a coil, always use 2 separate coils. But on a PI I suspect the coils need individual damping so the folded coil won't work. You can place the feed point(s) anywhere.

Thank you for all the input so far.

I have made a rectangular figure-8 PI coil where it is both Tx and Rx in one. Good with emi and EF as far as I remember, but salt and ground effects, no difference.

Am looking up simple absolute value circuits now, have not found many with a lot of bandwidth eg. 10MHz. Would that actually be an important criteria?

I am only aware of the absolute function from the XLT in conjunction with the BIGFOOT coil, a figure-8 coil, which I still love using.

[Carl-NC]

A Figure-8 mono coil (both TX & RX) won't cancel ground or salt because, for targets, it ends up with additive polarities for both lobes. It only subtracts for non-energized signals, like EFE and EMI.

The absolute value circuit comes after the SAT circuit so it's low BW. Look at ideal rectifier circuits, they can be made to do absolute value.

[green]

Don't need much band width for the absolute value circuit. You absolute value integrator out which should be low frequency.

[6666]

Made up a 12 inch coil, 311uH, twisted it to a fig 8, it dropped down to about 265uH, but needs a couple more turns to get back up to 300uH, interesting though my folded over coils dont drop much in uH, but the fig 8 does.

Yep needed about 365uH, when folded dropped to about 305uH. 2.6 ohms, 24AWG.

The folded fig 8 shape equates to about a 6 inch coil each side so I guess for raw depth , its performance will equal a 6 inch mono.

Which is the ideal way to wind a folded fig 8, should the windings at the cross over point all cross as two separate bunch's, (as they are now) or could they be scrambled at cross over ?, would make it easier to wind if scramble cross over worked, and you would not have to twist it.

[Eddy71]

Yep needed about 365uH, when folded dropped to about 305uH. 2.6 ohms, 24AWG.

The 24AWG wire for the pulse detector coil is too thin.

[green]

Made up a 12 inch coil, 311uH, twisted it to a fig 8, it dropped down to about 265uH, but needs a couple more turns to get back up to 300uH, interesting though my folded over coils dont drop much in uH, but the fig 8 does.

Yep needed about 365uH, when folded dropped to about 305uH. 2.6 ohms, 24AWG.

The folded fig 8 shape equates to about a 6 inch coil each side so I guess for raw depth , its performance will equal a 6 inch mono.

Which is the ideal way to wind a folded fig 8, should the windings at the cross over point all cross as two separate bunch's, (as they are now) or could they be scrambled at cross over ?, would make it easier to wind if scramble cross over worked, and you would not have to twist it.

Hi 6666
Wondering why you are making it folded instead of two round coils(reply 6)inductance for the two coils would add.

[Polymer]

A Figure-8 mono coil (both TX & RX) won't cancel ground or salt because, for targets, it ends up with additive polarities for both lobes. It only subtracts for non-energized signals, like EFE and EMI.

The absolute value circuit comes after the SAT circuit so it's low BW. Look at ideal rectifier circuits, they can be made to do absolute value.

Thank you. Found a few relatively simple ones. Here a pdf of some.
PrecisionRectifiers.pdf

[Polymer]

Made up a 12 inch coil, 311uH, twisted it to a fig 8, it dropped down to about 265uH, but needs a couple more turns to get back up to 300uH, interesting though my folded over coils dont drop much in uH, but the fig 8 does.

Yep needed about 365uH, when folded dropped to about 305uH. 2.6 ohms, 24AWG.

The folded fig 8 shape equates to about a 6 inch coil each side so I guess for raw depth , its performance will equal a 6 inch mono.

Which is the ideal way to wind a folded fig 8, should the windings at the cross over point all cross as two separate bunch's, (as they are now) or could they be scrambled at cross over ?, would make it easier to wind if scramble cross over worked, and you would not have to twist it.

I am wondering about that myself. Have only made one longish rectangular figure-8 coil using the "scramble crossover" method you mention. It works. Have no comparison though.
You have nice precise way of describing that method actually. I would have struggled to find the right words.