DD coil balancing again

[ivconic]

I've been making DD coils for long time, maybe made 200 so far, maybe more, never really payed much attention on numbers.
Each time i made my coil the same way. As i explained and showed in PDF file here on forum.
It is obscure and primitive method... but efficient enough.
Usually inductances are known, so only thing left in such cases is to put both "D" coils in enclosure and balance them.
Balancing... usually most delicate part!
I noticed that sometimes it goes easy and sometimes not.
Modern coil designs seems tending to reduce coil weight more and more, reduce coil mass too and overlapping area is narrowed and straightened more at modern coils.
So now we have quite a number of modern DD coils which are light weight, interesting design and with ultra narrow overlapping area.
It is much harder to balance coil with narrow and straight overlapping area (figure B on posted sketch below), i realized that recently.
It is much easier to balance coil with wider and "belly" overlapping area (figure A on posted sketch below).
So far i had success only with case "A" (figure A).
But recently i got salvaged enclosure from XP coil, 36x30cm and i tried to make coil to fit that enclosure, for another type of detector.
Once i put coils in enclosure and tried to balance them; i realized it would be tough!
After ten minutes somehow i got some balance, but it was damn critical!
My coils are close with dimensions and i don't have enough space inside enclosure to move them more to balance them better.
So i was wandering; how it was done by manufacturer?
How they calculate coils so later to have sure comfort that coils will be easy balanced in such narrow and straight space where overlapping area is?
How to take overlapping area, shape and dimensions into account to be later sure that you will balance coils easily?
If you look some of the commercial coils and their design; overlapping area is so narrow and precise... that from this perspective i can't think on method how to calculate the coils to fit perfectly in such enclosure!?
What is the secret?

[Koala]

I have only built type B

Shape of Tx and Rx has to be identical for them to null to a very small voltage.


A small bit of ferrite or copper moved around the overlap can be used to fine tune.


Been making concentric lately. Don't go for best null but best depth. Get it close with a scope then test with a various objects. Just making another and looks like best depth on 6x6 mm square cut from can is going to give best results for most things.


Guess if you are manufacturing to tight tolerances a jig will be used. There is a bit of differences in performance between Deus's. I have seen two people running the same setup/program on two different machines same size coil everything identical. One gets a week signal. The other nothing not even a hint. Just manufacturing tolerances on s probably hotter than average and the other below.

[ivconic]

I have only built type B

Shape of Tx and Rx has to be identical for them to null to a very small voltage.


A small bit of ferrite or copper moved around the overlap can be used to fine tune.


Been making concentric lately. Don't go for best null but best depth. Get it close with a scope then test with a various objects. Just making another and looks like best depth on 6x6 mm square cut from can is going to give best results for most things.


Guess if you are manufacturing to tight tolerances a jig will be used. There is a bit of differences in performance between Deus's. I have seen two people running the same setup/program on two different machines same size coil everything identical. One gets a week signal. The other nothing not even a hint. Just manufacturing tolerances on s probably hotter than average and the other below.

Thanks Koala!
Yes i noticed that coils must be identical, i am using exactly the same former when winding up those.
But it is possible later those shapes to distort a bit... like i said i am using primitive and obscure method yet.
I guess there must be a proper method to calculate close to perfect those D shaped coils, so later to be easy to balance them in such tight enclosure?
Only way i can think on is to make few windings more on each and since those are made using coil winder and wire with adhesive layer; later when put and positioned in enclosure: it would be easy to unwind it to the point where easy balance is achievable... (though looks like a stupid method...)

[bernte_one]

Hello Ivica,

i you have reached the area where it becomes critical to balance coils by move them more left or right
just stop, fix both parts, then take a small Piece of alu or copper foil and move this over the middle area and
glue it on, on of the coils in the Position where you can get best null voltage (fine tune)
simple trick but works

[Koala]

Tesoro use enamelled wire with a glue on it. Then run it through a solvent bath before winding. After it dries it retains its shape.


Seen another video of a DD being made. This time the windings were taped to a plastic former. The former was not removed but became part of the coil


In another video I saw enamelled wire impregnated with glue used then the completed coils being loaded in to a oven and then baked.


I cover my former with a cut up plastic bag then run the wire through a epoxy bath and let it set solid.




Not seen any videos of the nulling process.

[bernte_one]

hello Koala, i think you mean Garrett with the coil former

[ivconic]

Hello Ivica,

i you have reached the area where it becomes critical to balance coils by move them more left or right
just stop, fix both parts, then take a small Piece of alu or copper foil and move this over the middle area and
glue it on, on of the coils in the Position where you can get best null voltage (fine tune)
simple trick but works

That's an option too.
I tried that before in the past, with concentric coplanar coil, it works.
But i am not sure at what degree "foreign body" in coil shifts the phase from default position?

[ivconic]

...
Not seen any videos of the nulling process.

Exactly!
That's why i started this topic and asks questions!

[Davor]

That's an option too.
I tried that before in the past, with concentric coplanar coil, it works.
But i am not sure at what degree "foreign body" in coil shifts the phase from default position?

Ferrite balances in-phase component, exactly as pinching or stretching coils would. A small piece of tetrapack with aluminium foil in it balances quadrature component. Both are possible even after the coil is potted.
Of course, you must find a spot on a coil body where you may fix the piece.

[6666]

There are a few pictures on the net that show a small portion of one turn left free, and twisting and moving it to get final balance.

[Koala]

https://www.youtube.com/watch?v=6adQvxOugcU

There's a bit on coil tuning in here but she's too fast too see.

[ivconic]

https://www.youtube.com/watch?v=6adQvxOugcU

There's a bit on coil tuning in here but she's too fast too see.

Yes, but those are concentric coils.
Those can be precisely calculated to balance very well.
Later finest balance may be achieved with electronic tuning.
I seen that before and i like to watch it again.
Nice "birds" there at Garrett!
Robert Podhrasky is enjoying there like pig in a mud!

[Fabian]

Here there are another example of a DD coil with a loop for balancing.

[Maurizio2]

Hi!

I have a question.

It is better to have TX or RX under?

[Carl-NC]

RX under, usually.