What did I do wrong?

[Al1g]

Does anybody know what I did wrong. The coil is made with the core of 75 ohm coax. It has two layers of 16 turns each. Shielding is graphite glue. But I can only get it down to ~30 us. What did I do wrong?
Its all enclosed in fiberglass.

[homefire]

Multi Layer is one thing.

You would have been better to make the coil bunched up.

Do you know what the Value of the coil ?

[Al1g]

The inductance is ~250uH. What is bad about Multi Layer?

[Tepco]

From pictures, it looks like "shielding" wire is not only too thick, (too many metal in search head) but covered whit resin, is it in electrical contact whit outside graphite shielding layer at all? Also, what type of detector, or TX power stage you use, connecting cable etc? You don't need to make "notch" in graphite shield.

[Al1g]

The wire is a single strand from some braided wire. And yes it is in contactt with the graphite. I am using the Hammerhead. Its connected to the cable via standard 75 coax. What about the dreaded "eddy currents" ? Also the damping resistor is about 680 ohms though that isn't perfect.

[porkluvr]

It looks like you painted your shield very close to the wiring. You will have quicker response if you have some appreciable distance separating coil and shield.

I think that when you wind a spiral, capacitance tends to rise at a faster rate compared to inductance, compared to a scramble wind. Unless you really need the particular field shape that your spiral shape affords, you may be better off increasing the inner diameter and then scramble winding.

[Al1g]

What is "Scramble Winding"?
Actually there is some space. Windings were covered with fiberglass then the shielding applied.

[Al1g]

Sorry my questions but I am new to metal detecting.

[porkluvr]

What is "Scramble Winding"?
Actually there is some space. Windings were covered with fiberglass then the shielding applied.

Don't you mean that you coated it with a resin that came with a fiberglass repair kit? You should try to find out what the dielectric constant of your particular brand of resin is.

"Scramble wound" lessens inter-winding capacitance because the wires from one turn to the next are not lying exactly parallel to each other. It should not be that hard to envision, or Google, but in all honesty judging by the apparent thickness of your wire, it probably makes more sense to wrap your windings neatly.

When I said "appreciable distance" I should have probably added that the insulating material should have a low dielectric constant. And, 1/8" thickness is not really my idea of appreciable but then I can't really give you a hard fast number either. I can't tell how thick your resin coating is, but if the resin has a high dc you are probably not going to get much improvement no matter how far the shield is from the windings, because your inter-winding capacitance will rise.

That really is a nice looking coil and I hope you can get it to work better.

[maikl]

I think it is too many metal in search head.

[Al1g]

The reason for the copper wire is (I read somewhere) to make good contact with the graphite.

[Al1g]

Is that necessary??
Again I'm new to metal detecting.

[Comp]

I'm new, too, {Hi, BTW...!} but, it looks like (to me) if you have wound the copper wire all the way round, spirally, from inside to outside,... and then covered with graphite/conductive paste/paint, you now have a 'shorted turn',.... the screen must not cover the whole of the coil, you need a small gap, ???? But,as I say, I also am a 'newbie', its easy for me to be wrong,...... {I'm having enough trouble, winding a simple P.I. coil myself??!! LOL}

Cheers, good luck with it, hope you can sort it, it looks very nice!!! Mine where 'potted' in resin, so are scrap!!

Fred

[Al1g]

Since I live in Brazil I don't know the D-C of my brand of resin.

[Al1g]

So is the copper wire necessary? Or can I just paint the coil with graphite. Is the break in the graphite also necessary? Also I was wondering if laying the coax across the coil (as shown in the pic of the completed coil) could mess it up.