Ferrite Rod Coils for Pi Detectors

[bernte_one]

Hello,

i have made several Ferrite Coils for Pi Units in past.
But i often ask myself if they are right constructed or not.
I think the Performance is not only a result of the wire ond ferrite material which is used i suppose the Geometrie of the windings will have a big effect too.

So i hope we can discuss here how to make a good Ferrite rod Coil for a Pulse Induction Detector (for example surf pi, baracuda or itmd pi Projects).

[chemelec]

I assume you want this for Pin Pointing?

Obtaining a Low Capacitance is Good.
Not Very Easy to do, but the Best way is to "Pi" Wind it.
(Pi winding is like a Cross Hatch Layering of the wire.)

Or for normal layer winding, using "Nomex" Insulation Between the Layer of wire, Might Help a bit.

Also, Not all Ferrites are the same.
They come with various characteristics and AL Values.
(Power Types, High Frequency, Low Frequency, RF, etc)

I Prefer a High Power type such as Material 77.

[bernte_one]

yes i want to use it for pinpointing
made a single layer of 0,4mm magnet wire on a lenght of 50mm direct on the ferrite core fixed with carpet tape

result are nearly the same wit 10mm ferrit or 16mm ferrit
yesterday i bundled three pieces of 10mm (like from nupi pinpointer) - result is a Little bit better- but the coil/ferrite pack is too heavy and too big

i think i will try yv wire 0,5mmx1,2mm to reduce interwire capacitance

[chemelec]

Bundling 3 Rods like Nupi does or using a larger Diameter Ferrite rod, Increase Distance for Larger Objects.
But Less Accuracy on actual Pin Point.

Not sure What your Expecting, But Pin Pointing Coils on Ferrite Rods will Never give a Great Distance of detection.
Usually just a few inches at most.

You can also use a 15mm or 20mm diameter POT Core for greater Distance, But Definately Less Accuracy and less sensitivity to Tiny Objects.

[kt315]

my DP was tested with all kind of erasing heads that have C form ferrite corn (a ring with gap).
try to check U form ferrite.

[bbsailor]

my DP was tested with all kind of erasing heads that have C form ferrite corn (a ring with gap).
try to check U form ferrite.

Only use "soft ferrites" for a pulse induction probe. See this: http://www.magneticsgroup.com/pdf/mmpa%20SFG-98.pdf . Eric Foster said this many years ago.

Joseph J. Rogowski

[chemelec]

Only use "soft ferrites" for a pulse induction probe. See this: http://www.magneticsgroup.com/pdf/mmpa%20SFG-98.pdf . Eric Foster said this many years ago.

Joseph J. Rogowski

I Agree, I Only buy the Soft Ferrites. (MnZn)
Mostly I get Material 77, good for up to 100 Khz from Fair-Rite Corporation.
Or Material 78 is good up to 200 Khz.

[golfnut]

one thing to watch for is the peak power that is being pumped into the ferrite.

If the core saturates then the inductance you wanted wont be the same and maybe it limits the flux density as the core kind of max's out - range impact?

[Nupi]

Maybe also good to take into account the residual magnetic field after each pulse, and the closer to the saturation point comes as Golfnut rightly predicted, you get more affected from the residual magnetic field.
I do not know how far this negatively impacted on the detection.

[Nupi]

A possible good solution in order to prevent residual magnetic field and saturation.




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[6666]

Only use "soft ferrites" for a pulse induction probe. See this: http://www.magneticsgroup.com/pdf/mmpa%20SFG-98.pdf . Eric Foster said this many years ago.

Joseph J. Rogowski

Thanks, They are a whole science in themselves, maybe you could stack two hole balun cores?

or 26 Way Flat Ribbon Cable Suppression Sleeve which is hollow.

[bernte_one]

maybe i will post some Pictures the next days for the differrent types

so here the Facts which will give better performance
short area from ferrite is covered by coil
spacer between ferrite and coil
litz or yv wire better than magnet wire (because of lower capacitance)?
Diameter not too big (loss of pinpoint Feature)
possibilty to add a shield
i have seen ferrite core coils with ribbon cable as shield

[Nupi]

I thought that the benefits of litz wire are less skin effect and not capacitance.
I am never too old to learn.
Thanks

[bbsailor]

I thought that the benefits of litz wire are less skin effect and not capacitance.
I am never too old to learn.
Thanks

At low delays, below about 10 microseconds, the wire diameter can begin to look like a target, like a small piece of gold. Eddy currents are decaying in the thicker metal core of the wire like decaying currents in a small target. Generally, the thickest strands you want to use is about AWG 31 to prevent this from happening. In Litz wire the strands are individually insulated but in tin plated stranded wire they are not and only the tin plating creates a slight resistance between strands to minimize eddy currents. The thickness of the wire insulation keeps the wire farther apart to reduce coil capacitance but reduces the inductance so a few more turns of wire are needed to get the desired coil inductance. The nature of the insulation also affects the turn to turn coil capacitance. Lower insulation dielectric constants are more effective in reducing coil capacitance. Teflon and polyethylene insulations have typically lower dielectric constants than PVC insulations.

These techniques for making coils are mostly important when making fast coils for seeking small, fast decaying targets. Coil size and construction techniques should be optimized for the primary targets sought, TX pulse width and lowest delay time needed for the primary targets sought and the ground characteristics in your hunting area.


Joseph J. Rogowski

[Nupi]

Thank you for your clear explanation Joseph.

I have no experience with PI detectors.

What strikes me is that the Q-factor is not mentioned in your explanation.

These are some thoughts that come to mind:
-The dielectric constant reduces the electromagnetic field and the Q factor
Why not air coils.The dielectric constant of Air(1 atm) 1.00059 - Teflon 2.1 - polyethylene 2.25
-Spectral noise with high Q (narrower bandwidth) is lower than with low Q (wide bandwidth).
- A high Q-coil is faster than low Q-coil ?!
-Also Inductor DC Resistance (RL), Capacitor Loss Resistance (RC)

Perhaps these thoughts are crap and not applicable in a PI detector.

Thank you for your time.