100mm (4") coil with ferrite inside

[belax2018]

Dear people

I have successfully built MD-20 pinpointer. I am inspired by how it performs on small flakes of gold.
Briefly about MD-20:
It is a 300 kHz Colpitts oscillator. The change in rectified signal from oscillator defines a presence of a target nearby. Also this rectified voltage is used for auto-ground balance - you can dig with pinpointer inside soil or even underwater. The pinpointer head is a D10mm ferrite rod with 60 turns of 0.2 mm wire.

Now I am considering using this concept with bigger coil for finding bigger and deeper targets.
The coil I am considering is in the picture attached. It is like 10 times scaled MD-20 head.
Running frequency planning to make lower – down to 90 kHz.
Questions:
Will it work for finding small gold nuggets (around 3mm in diameter) 10” deep in soil?
What pros and cons of this monocoil concept?

[Dutchie]

Hi.nice plan.
I wonder if the inner 7 ferrite rods will add to the effect or can be replaced by any inert (lower weight) material.
From memory the following (by please checkout the forms on the topic coils!):
If you plan to increase the number of windings too - the resistance will increase proportional with the number of windings and the diameter.
The inductance will increase too.
When you double the diameter you could lower the number of windings by approx. sqrt(2) and still have the same inductance. Your diameter is 5 times so with approx half the number of windings you still with have the same inductance...Or have an higher inductance.
Larger diameter will also mean more interwinding capacitance.
In all the optimum frequency will be lower?
Lots of success wit the mods.

[belax2018]

I remember seeing something similar from Nupy (I don't know how to spell it right).

[Skippy]

I agree with the earlier reply: You only need the outer ring of ferrite rods, the inner rods may actually reduce performance. I think your coil is too large, for practical reasons, something like 2.5 inches would be more manageable. As you are just experimenting, it's worth trying several different arrangements.
This is an interesting read:
http://www.zen22142.zen.co.uk/Media/fsi.htm

I've done a lot of experimenting with a Sherlock pinpoint probe, based on the Cherry CS209 chip. I was intending to try larger rods, starting with a bundle of 6 around a nylon rod, which would be about 27mm diameter overall.

[belax2018]

Skippy.
No. It is a thought experiment.

I agree about central ferrits. No need of them

I actually had in mind to give up this idea, before you showed me this link. I am afraid it will not give me enough depth for small nuggets.
Oh, I think I shall try it now.



Right now I am looking to a constant current PI, analog of GPZ 7000.

[Dutchie]

Skippy.

I actually had in mind to give up this idea, before you showed me this link. I am afraid it will not give me enough depth for small nuggets.
Oh, I think I shall try it now.
[ATTACH]41118[/ATTACH]

It would be nice if you made a coil and would tell about the properties:
number of turns, wire type and thickness and resulting inductance, resistance and capacitance

[Davor]

I believe Graham Maynard is SK now. I have no idea whether this construction works as advertised, but generally it stands that a ferrite antenna is more effective if the ferrite rod is longer.

[Skippy]

I too think that if you're planning on making large diameter bundles/tubes, longer rods would also make more sense. The longest you are likely to find an 8 inch / 20cm , though there are some specialist distributors that may stock unusual / 'surplus' ferrite in odd sizes.
I bookmarked this supplier a few years ago:
http://www.stormwise.com/page26.htm
But they only ship to US, no export.

If the ends of smaller rods are ground nicely flat, you should be able to join them end-to-end to make 20 cm rods. I think that rods are often slightly bent, they must shrink during manufacture, and they twist a little. So bundling many together won't produce a perfect job.

[belax2018]

I have plenty of 10x200 mm rods with me.
In addition, I will check if I can get 19x200 mm rods.

I will post results when I try.
Thank you all for your answers.

[Dutchie]

Please keep us updated with your results. Very nice experiment.
Longer ferrite rods not better - often times worse. http://www.zen22142.zen.co.uk/Media/fsi.htm
Hollow bodies better then (very thick massive) same reference.
Even one ferrite core with with a large air space and isolation is better then a coil tightly wound around the ferrite. same reference.
if large diameter coil : use silver coated wire suitable for the amps used. keep them as widely spaced as possible to have low capacitance.(litze probably not necessary)
Because the coil will be high Q you may need to add external resistor.