Basket coil question

[bmwbiker]

Hi all, in an attempt to make my Barracuda behave, I've just wound a basket coil as per the instructions here:

http://www.treasuresweeper.com/wp-co...asket-coil.pdf

I used 0.71mm enamelled wire to wind a 280mm x 200mm coil and have wound 24 turns as the 180mm x 110mm coil of 32turns seems a little on the small side for effectively covering a beach. If my L-C meter is to be believed the inductance is 205 uH, with a DC resistance of 1.2 ohms.

My question is: does this type of coil need to be shielded, and if so what is the most effective means of doing it? Also, since it has distinct inner and outer turn connections, does it need to be connected to the PCB in any particular way (i.e. inside connection to battery -VE or such?)

Any advice please?

Clive

[Chet]

Hi bmwbiker
The outside wire winding acts as a shield and should be connected to the printed circuit ground. The inside wire, the one that you started the winding with, should be connected to the output transistor. If they are connected properly there should be no need to shield the coil. if in doubt grip the coil with your hand with the wires connected one way. Then reverse the wires and grip it again. With the correct connection your hand should have little effect on the detector.

Do a comparison test between your 32 turn coil on a small gold ring or similar small object. Then repeat the test with the 24 turn coil. You may need to add more turns to improve the new coil. If so a small soldered splice will not hurt the performance. Insulate the splice with tape or goop/glue and it will be fine.

Chet

[bmwbiker]

Hi Chet,

Thanks for your reply to my question.

I didn't try winding a 32 turn coil as I thought it would be on the small side, as I explained, so can't compare them side by side. I do have a 10" unshielded mono coil which gives great results in air but is unstable in use, so I could compare the basket coil with this.

Sadly, it's not going to be possible to add any more turns to the basket coil since it's already been covered in expanding foam and I think I would probably damage the wire trying to dig it out. The thought did occur to me after I measured the inductance that I could probably double the amount of turns and still end up below the 453 uH required for the Barracuda, but that I was already too late for that.

It's cold and dark here right now so I'll try the coil tomorrow (when it will still be cold but at least it will be light )

[Chet]

Hi bmwbiker

Coils with more turns (higher uH) work well with silver coins, iron and other metal objects that keep the eddy currents flowing longer.

The 205 uH coil will not have as much stray capacitance because of the fewer turns. This will allow a faster collapse of the flyback pulse which is good.
This will allow you to adjust the sample delay to a earlier timing than a coil of 300-453 uH.
Shorter delay time is good for small gold objects. The eddy currents die faster in small gold.

The downside is the amperes X turns factor is less which means less power will be transmitted. The loss in overall performance may not be too bad.
So try comparing a small gold object with your 10" coil.

Good luck and good hunting,

Chet

[baum7154]

Hi bmwbiker
The outside wire winding acts as a shield and should be connected to the printed circuit ground. The inside wire, the one that you started the winding with, should be connected to the output transistor. If they are connected properly there should be no need to shield the coil. if in doubt grip the coil with your hand with the wires connected one way. Then reverse the wires and grip it again. With the correct connection your hand should have little effect on the detector.

Do a comparison test between your 32 turn coil on a small gold ring or similar small object. Then repeat the test with the 24 turn coil. You may need to add more turns to improve the new coil. If so a small soldered splice will not hurt the performance. Insulate the splice with tape or goop/glue and it will be fine.

Chet

_______________________________

After carefully looking at this coil construction in the attachment I doubt that this coil is self shielding as it is composed in 8 distinct layers and is not over wrapped with it's successive layers as in the unique geometry of the CHANCE PI COIL I have detailed in the COILS forum. The last winding layer of this basket coil design is a flat plane on top of the other winding layers. In my opinion this coil will need a separate shield, the best type being a graphite shield. There is much written about graphite shielding on these forums.

Regards,

Dan

[Chet]

Hi bmwbiker

I'm sorry I made a mistake and thought that you had built the coil that baum7154 built in the CHANCE PI forum. I agree with Dan that the coil you have is not self shielding.

Dan thank you for correcting my error,
Chet

[bmwbiker]

My thanks to both of you for taking the time and trouble to re-visit the thread to analyse it for me.

Luckily (?) I've been so busy that I haven't had time to even test the coil so I haven't got as far as making a housing for it, which would have been pretty much the end of any further work on it.

I must have missed the articles on the CHANCE PI coil so I will look it up when I get the chance . . Got a very busy week coming up which means I won't be near a computer. Is there a PDF I could download so I can read it on my e-reader?

I've seen some reference to Gorilla Glue and graphite powder for shielding material - as it happens I have both in my workshop (the graphite powder was for a Victorian recipe for a rust-inhibiting compound for my machine tools) and also some carbon fibre cloth. It strikes me that if I could make a reliable electrical contact with the carbon fibre it would be an ideal shielding material - at one stage I was entertaining the idea of making a coil housing from it but now see how that might be a bad idea . . . I did buy some self-adhesive copper foil to use for shielding but gather that the copper might be detected, instead of shielding the coil?

Thanks again - will read up on it when I can.

Clive

[bernte_one]

hab an idea for a basket coil body for everybody who is to lazy to cut plastic rings (like me)
search the net for jonglage rings

[Qiaozhi]

hab an idea for a basket coil body for everybody who is to lazy to cut plastic rings (like me)
search the net for jonglage rings

I guess you mean "juggling rings"?
http://www.bilboquet.com/juggling-ri...m226,p568.html