Coil encapsulation

[dbanner]

The lead/lag phase shift inherent in the search coil(20/180) for tesoro, is a function of the phase shifting circuitry, more specifically the ground balance . So designer make coil with phase shift circuit in mind. And also the tx resonant frequency Rx resonant frequency spread.
I am sure engineers who design such devices have firm grasp of the science involved.
Not easy to wrap head around.
White's classic series use different parameters to arrive at same destination. However, there are strengths and weaknesses inherent in the subtle differences.
So tesoro devices perform better than White's in certain conditions and vice versa, all having to do with the coil and shifting topologies. Do you want better discrimination separation or better ground balance capabilities? See.
At least for the older devices. Haven't got a clue about the new ones

[dbanner]

Deep null is not preferred because weaker targets have further to climb before breaking threshold, lower the threshold and detector becomes unstable.
It is a trade-off.
Either side of deepest null is better for max sensitivity without unstable. It is the amplitude of the null that becomes the baseline for the target signal.
Just my guess.

[Carl-NC]

EDIT: question marks are degrees, it still does not show proper characters...

Testing @ 27?C

Yep, it looks fine in preview but not after it's posted. Sorry, it gets fixed when vBulletin fixes it.

[Koala]

I built some Bandido coil and if I went for best null on the scope it only had a 50/50 chance of working well

After wasting much time I gave up with best null. Instead I use the scope to get to the null point the get some various targets and adjust for best results. These always work.

my shield is made from cardboard discs covered in graphite by adding to one side the null alters but comes back to normal when the second is add.

Bandido doesn't go down to zero discrimination. Small nails are always notched out. The phase doesn't have to be far out to move this up towards large nails and pull tabs. On my concentric 5.5" coil there is a s all phase shift either side the null point. Whether this is the reason I doubt it.

Really? This is another phenomena I want to examine more. I used to null DD on 20? (plus minus a LOT does not really matter) and CC on 200? without obvious reason - its possible to null it where ever you want. Even when I played with oposit null, it worked well. I do not remember exactly, but for DD on 200? side, its little bit more sensitive to ferrous and on 20? side its little bit more sensitive to nonferous. But more than result of measurement, it was only feelings. Difference was hardly noticeable. Deep null was somehow unstable, bad ranges of GEB and Disc. Never understand why, even when I read all Tesoro threads several times, my brain is too weak for such amount of different and often contradictory informations

For example... several times is mentioned, that correct GEB range is caused by phase shift. Well, try it. On both sides including deep null I set potentiometer almost into one spot to reject ferrite. Guess that it is more affected by Tx to Rx inductance ratio and Rx parallel capacitor.

Please, get it with reserve, I spend 15 hours at work in row and these are only my faded memories.

I have never found satisfactory answer. And yes, I have ITMD

EDIT: question marks are degrees, it still does not show proper characters...

[Qiaozhi]

20 phase-shift is not a magic figure that works for any detector. This is specific to Tesoro detectors (e.g. TGSL). This offset is required for the ground balance knob to be somewhere around the middle position, and for discrimination to work correctly. You can confirm this yourself by measuring the phase-shift on a commercial Tesoro coil.

If you try to "deep null" the coil, it just won't function correctly, and will be unstable in use. Alternatively, if you null the coil with a phase-shift much greater than 20, the ground balance setting may be outside the available range of adjustment.

On some coils you have to use 200 (which is 180 + 20) because the RX signal is inverted with respect the TX.

P.S.
Use [latex]^{\circ}[/latex] to create the degree symbol.
You can also use: [latex]\textdegree[/latex].

[Hyena]

Good question.

I know from experience if it just the wrong side of null it preforms badly. But don't know why.

Really? This is another phenomena I want to examine more. I used to null DD on 20? (plus minus a LOT does not really matter) and CC on 200? without obvious reason - its possible to null it where ever you want. Even when I played with oposit null, it worked well. I do not remember exactly, but for DD on 200? side, its little bit more sensitive to ferrous and on 20? side its little bit more sensitive to nonferous. But more than result of measurement, it was only feelings. Difference was hardly noticeable. Deep null was somehow unstable, bad ranges of GEB and Disc. Never understand why, even when I read all Tesoro threads several times, my brain is too weak for such amount of different and often contradictory informations

For example... several times is mentioned, that correct GEB range is caused by phase shift. Well, try it. On both sides including deep null I set potentiometer almost into one spot to reject ferrite. Guess that it is more affected by Tx to Rx inductance ratio and Rx parallel capacitor.

Please, get it with reserve, I spend 15 hours at work in row and these are only my faded memories.

I have never found satisfactory answer. And yes, I have ITMD

EDIT: question marks are degrees, it still does not show proper characters...

[Koala]

Can anyone explain why the nulling point must not be at the minimum but a little bit overcoupled?

Good question.

I know from experience if it just the wrong side of null it preforms badly. But don't know why.

[Teleno]

Thanks chaps, .....this looks like a worthwhile mod to the TGSL coil..... https://youtu.be/XnrStRklcUQ?t=539

Can anyone explain why the nulling point must not be at the minimum but a little bit overcoupled?

[waltr]

I suppose that spray 'builders foam' would be OK for the Minipulse coil though.....?

No issue at all for a Mono-coil.

It is only with IB coils that there can be an issue.

[Koala]

I tried this difference on one coil. Coil unshielded, with thin aluminium kitchen foil and with adhesive copper tape. No difference in possible deepest null, detection depth or shift. Only difference was that unshieded coil detected my hand. Did not tried graphite spray, I use only for shielding plastic enclosures. And I hate it

thanks

[Hyena]

correct the screw just flexes the wires slightly. Bit of over kill as you get just as good results by sliding and temporary gluing in place

at the end of the video he depth tests two coils with different shield method's aluminium poor graphite good. I suspect its more to do with the coils than the shield unless they used kitchen foil.

I tried this difference on one coil. Coil unshielded, with thin aluminium kitchen foil and with adhesive copper tape. No difference in possible deepest null, detection depth or shift. Only difference was that unshieded coil detected my hand. Did not tried graphite spray, I use only for shielding plastic enclosures. And I hate it

[samsearcher]

I suppose that spray 'builders foam' would be OK for the Minipulse coil though.....?

[Koala]

correct the screw just flexes the wires slightly. Bit of over kill as you get just as good results by sliding and temporary gluing in place

at the end of the video he depth tests two coils with different shield method's aluminium poor graphite good. I suspect its more to do with the coils than the shield unless they used kitchen foil.

[samsearcher]

It looks like he is flexing the Rx coil to achieve the final null point.

.........not sure the difference in coils at the end will be down to shielding.

[waltr]

White's used to use expanding foam for encapsulation. I would be concerned about things moving around as it cures.

I unsuccessfully tried expanding foam a while back. That was exactly the problem.

Foam would reduce coil weight but can have the issue Carl & KingJL stated.

This should be an issue if you first use a small amount of epoxy to fix the coil into the housing.

I did this when building a DD coil to ensure the balance didn't move before encapsulation the entire coil. I use just enough epoxy to anchor the coils into position. Then recheck balance.
If balance needed tweaking I could have cut away a little epoxy to move a coil.