Help wanted to jog my memory

[CaptSlog]

Hi,

Quite a number of years ago I made a Surf PI using a long board from Silverdog. As I made it for the beach and didn't have chance to go there that often, it got put to the back of the shed and forgotten about. I probably only used it a couple of times. Sorry, but these things happen.

This last weekend I was invited to spend a few days with my son and his wife at the seaside, so I thought I would dig it out and take it with me. To me surprise and delight, it still works and we found a few things on the beach. I left it with him there for the rest of his holiday because he like it so much.

When he and it comes home, I'd like to improve on a few things but I've forgotten what I did to make it. The main thing I've forgotten is how I wound the coil, I don't really want to take it apart to find out and this is where you may be able to help please. I realise I could do a load of searching and basically start again, but knowledge a few terms would speed my search somewhat.

Questions...
I know that I made a former from a ring of plastic, and around the inner and outer diameters of this former I cut notches to create segments around both diameters. The winding wire went from outer to inner in sort of Spirograph pattern. What do you call this winding? I have an idea that it was referred to as 'self shielding' does that sound feasible?

I recall covering the coil with tape(?) and then it was painted with Aquadag (colloidal carbon), and a few turns of very fine wire wrapped over the coating which formed a ground. Again, does that sound right?


I hope someone can help.

Thanks

[Carl-NC]

Sounds like a basket weave coil. This type of coil is more resistant to ground capacitance but I would still shield it. Yes, wrapping a layer of tape (of some other spacer), followed by a conductor sounds right.

[CaptSlog]

Basket weave! Thank you. I'd not have found that in a hurry although it seems obvious in hindsight.

A quick google search brought up this

which is just like the one I made.

I was thinking of trying to go bigger, but I'll look into the pros and cons of that.

[kt315]

just reminder you have to do the coil right. much people of the forum got mistakes winding this type of coil with poor result.

[kt315]

... - - - ...

[Eddy71]

The intercoil parasitic capacitance of the coil can be greatly reduced not only by this complicated way of winding, but also very simply by winding it not with enameled wire, but with PVC-insulated wire. The distance between the wires will be larger and the interwinding capacitance will be smaller.

[bbsailor]

The intercoil parasitic capacitance of the coil can be greatly reduced not only by this complicated way of winding, but also very simply by winding it not with enameled wire, but with PVC-insulated wire. The distance between the wires will be larger and the interwinding capacitance will be smaller.

To reduce coil turn to turn capacitance, use a wire insulation with a lower dielectric constant like teflon.

Joseph J. Rogowski

[kt315]

Joseph J. Rogowski, best dielectric is the air - !!!

[bbsailor]

Joseph J. Rogowski, best dielectric is the air - !!!

Yes, dry air in a vacuum has a dielectric constant of 1 at 20 degrees C at sea level. I was making reference to the section in my quote referencing PVC with a dielectric constant of 3.18 while Teflon is 2.1.

Winding coils requires that some design variables remain rather constant. When you make a fast coil to detect small low TC targets like gold nuggets, you need to make sure that no parts of the coil connection to the coax cable become detectable.

Coils wound on a slotted coil form assume that the inside of the coil housing will remain dry. That is why experimenting with different designs is very important to see which design variables work best. All solid wire sizes have a skin effect which is the highest frequency that will reach the full depth of the wire diameter. This is why Litz wire is used for high frequency PI detectors operating a low delays as you do not want to pick up eddy currents still in the coil wire.

Good coil design requires knowledge of the variables to make good tradeoffs for the specific operational environment and desired target optimization.

Joseph J. Rogowski

[kt315]

foamed teflon i had seen in my life much times. every o-scope cable made in USSR uses it. not a matter for me.
yes, you see/observer full degradation of Russia tech now.

[Stevedbk]

To reduce coil turn to turn capacitance, use a wire insulation with a lower dielectric constant like teflon.

Joseph J. Rogowski

Cat 5 cable single wire is HDPE, dialectric constant 2.3, and very cheap and obtainable but has more moisture absorbsion than PTFE.
Steve