Induction Balance Stuff - Single/Multi Frequency Response, GB, Disc, Measurements, Ideas, Fun, etc.

[Aziz]

Hi all,

splitting the big mono coil into two different concentric co-planar halves makes also sense. It is a combination of a small and large coil, so sensitive to small and sensitive to big targets. A compromise for different reasons.

I have put 1/3 total turns count for the inner coil half and 2/3 total turns count for the outer coil half. The bigger coil dominates.
The total mono coil acts like a mono coil of diameter D using the geometric mean formula:
D = sqrt( (1*D1/3) * (2*D2/3) ), where
D1 is the mean diameter of the inner coil,
D2 is the mean diameter of the outer coil,
The null-zone (B=0) is easier to access for soldering the coil connectors and capacitors. This is the only reason for making such a mono coil. A compromise of all.



Aziz

[Aziz]

But I will build the version in post #360 with a ribbon cable. Leaving a local gap is easier to wind the coil.
I will leave a gap of 10 - 15 mm however. The capacitors will have enough distance to the windings (capacitive coupling problem) and I can make the soldering easier at the end.

[Aziz]

Hi all,

it seems, that I have to mix different NP0/C0G capacitor types. Not all values or package types are available to me.
For the mono coil VLF, a voltage rating of 50 V is enough. When I can get 100 V ratings, it is preferable to me.
The coupling capacitors are all easily available (< 1nF).
For the resonant capacitors (>1.5 nF), I might be forced to change to the wire connector types (well, they are also small enough but more expensive).

Oh well, another 100 bucks must be spent.

[Aziz]

I have ordered over 2,000 NP0/C0G capacitors now. Only the ones I can really see & solder.
Approx. for 120 bucks this time. Now can't wait to get and measure them.

[Aziz]

Hi all,

I have 35 m long ribbon cable (AWG28, 10 strands). I am planning a mono coil with 30 turns count. The coil bundle height is 12.7 mm, width approx. 30 mm. The AI will tell me the starting radius, the expected coil inductance, resistance and coil capacitance. After 20 turns, I'm going to leave the local 10-15 mm gap for the capacitors. And will wind the rest of the 10 turns around the local gap.

As the coil bundle width reduces our effective coil diameter by approx. 30 mm, we can cheat a bit.

This is the magic cheat (optional):
A secondary coil (RX): 2-3 turns count outside the mono coil. A thin magnet wire is enough and won't make the total coil diameter bigger. We are voltage dividing by number of turns count relation.
30/3 -> 10x voltage division. And we can leave the LC-tank output coupling capacitors completely. The differential signals on the LC-tank won't be measured. The secondary coil will be operated as a balanced RX coil (L-ch,R-ch direct input).
So we would have total 2 input coupling capacitors (balanced/differential feed-in) + LC-tank capacitors.
The secondary coil is bigger and effectively increasing the total diameter of the coil system. And the 10 kOhm input impedance of the sound card will be translated upto 1 MOhm or more and does not degrade the Q of the LC-tank.

A few additional turns outside the mono coil won't make the coil difficult. So a "quasi" mono coil. But it solves a few issues. Win-win.

Cheers

[Aziz]

Hi Kenny,

Auction on ebay.... Ends in 4 days for
https://ebay.io/m/mSF3P3

this is really sad what to White's happened.
But I don't need any equipment right now.

But my first ever contact with a metal detector was a White's metal detector in 1974/1975 when I was a young boy.
In an US army base in Felden/Chiemsee, Bavaria, Germany. The soldiers and their families spent their holidays there.
My parents worked there and I have lived 1 year there too.
One nice soldier had such a White's metal detector and took us childs to metal detecting.
We have spent several hours metal detecting.

Since then, I was very fascinated about the metal detecting technology.
Aziz

[Aziz]

Ooops, I have obviously ordered some of the tiny 0603 SMD NP0/C0G capacitors (1.6 x 0.8 mm) as well. They cost only 2 cent per piece.
I will see how to handle them.
I have a pure thin copper foil roll. I could make a flexible small SMD "PCB on paper" with it and solder the SMD capacitors there. It could work.

[Aziz]

Hi all,

this is my first test attempt on winding the ribbon cable mono coil with gap. I have used 34.2 m (so it wasn't 35 m).
Inner diameter 30 cm. Total N=33 turns count. And there is even enough gap space left for bigger capacitors (the fallback option).
It is very easy to wind such a coil.

Aziz

[Aziz]

Oh well,

I can't use the ribbon cable mono coil. The proximity effect will degrade the Q of the LC tank heavily. Although the skin effect is reduced, the proximity effect dominates here. Very very much.
fuck!
I can even measure it's effects on inductance change and ESR of the coil.

Weave it (making a weaved ribbon cable) or using a true HF litz wire?
Weaving would take a lot of time as I would separate each strands and weave it by hand, that every strand changes its position across the geometric position evenly. Every 2-3 cm.
This is the art of weaving carpet science. Really.

Aziz

[Aziz]

Hi all,

the chat with AI is really enlighting. It clearly says, when which effect dominates if you make a low frequency (100 Hz - 1 kHz) and a high frequency (100 kHz) LCR measurement of the coil.

BTW my ribbon cable must be weaved this way to minimize proximity effect:

The grey outside lines are nonconductive threads to hold the weave structure. So every strand must change its position evenly over the pitch length.
No, I'm not going to weave a ribbon cable.
It is much easier to build a basket-weave coil with simple litz-wire. Coil capacitance and proximity effect will be radically reduced.

[Aziz]

Hi all,

the LCR measurement of a coil is really interesting.
One can even calculate the coils capacitance C (low, high frequency measurement).
And the high frequency losses.

AI is teaching me how to interpret die LCR measurement parameters.

[Aziz]

Hi all,

I have been looking for optimal place for the capacitors in a two row basket weave mono coil.
The mono coil with lowest capacitance and low proximity effect.



If I take the number of turns for the inner coil (N1) 0.866 times lower than for the outer coil, the zero-zone (B=0) is in the middle of the large gap area.
There is even enough space for bigger capacitors.
The outer coil will have more turns (N2) and this is good: more depth.
The cross-over points are interleaved to reduce coil capacitance further.

N1 = 0.866*N2

BTW, sqrt(3) / 2 = 0.866 (rounded). It is a good harmonic formula. Google AI found this for me for my initial 0.8 and 0.85 value.

Cheers

[Aziz]

Oops,

my fault. Looked at wrong figures.

N2 = 0.866*N1.
The outer coil will be slightly lower turns count. This is bad.

[Aziz]

Now for a three row basket weave mono coil with N1, N2 (N1=N2) and N3 turns count:
N1 = 0.866*N3
N2 = 0.866*N3
This time N3 > N1, N2. This is good.



The B=0 zone is approximately (averaged over the large gap area) in the middle.

[Aziz]

Hi all,

regarding my sad ribbon mono coil:

Here is a good explanation from the AI, why this coil with ribbon cable will degrade the Q of the LC tank.
Ribbon cable consists of PVC isolation. There is much more PVC than copper wire in it. It's permittivity is quite large (increasing the coil capacitance further more).
PVC is polar, so it will (the PVC molecules) polarize in an electric field. This is frequency dependent. At high frequencies, the polarisation character
will cause heavy losses (reducing radically Q of the coil).
In conjuction with the increased coil capacitance, a relative big portion of the capacitance will contribute to the LC-tanks capacitor.
So a very "bad capacitor" with high losses is connected inherently parallel to the LC-tank.

These losses can exceed the proximitiy effect losses. Potentially several order magnitude (10^x).

A plain magnet wire woudn't have much losses compared to the PVC wire cable.

So we must avoid PVC cables. (Except for very low frequencies).
Better isolation would be Teflon (PTFE). Very expensive. Difficult to get the wire.
Next better PP (Polypropylene). Difficult to get the wire.
Next better PE (Polyethylene). Cheap, still difficult to get the wire.
Next better PU (Polyurethane, thin magnet wire isolation).

Unfortunately, almost all standard cables are made of PVC. It is cheap and good temperature resistive.

I have a nice tinned PVC litz cable. Pealing off the fckn PVC isolation and isolating the wire thin PE stripes?
Aziz