Single layer Spiral coil

[mario]

I like IBGold's suggestion for polyolefin wire. That has got to be cheaper and easier to glue than Teflon. It also has a low DC, although not quite as low as Teflon's. But polyolefin insulation can be "puffed" to make DC lower than Teflon.

I had some time to to ponder the discrepancy between the inductance values you and I calculated with the wire you specified.

With the calculator you used, I believe you must take insulation thickness, and double that number to obtain minimum wire spacing. If you make a calculation based on having only a single insulation thickness, your measured result will be much lower than what you calculated.

The MiscEl spiral calculator has fields for entering wire od and copper od and takes into account insulation walls on both sides of the conductor.

You can possibly use PVC wire if you intend to make a large loop for relic hunting where a fast response is not necessary. You might want to make some tests with the wires tied together but not potted to check this out.



This one is still open. Somebody?

The discrepancy is largely due to the spacing as you originally suggested. I entered a generous number, taking under consideration an imperfect scenario. Anyway, that point is now mute. I did some calculations and realised using litz 660/46 wire (18 AWG equivalent - please correct me if im wrong about it being 18 AWG) will get me very close to the .300 uH, .500 OHM target using coils up to 18 inches (mainly coil bundles, not spirals).

Here is an interesting question i have yet to see posted: For discrepancies in ohms between your coil and target coil, is it ok to use a resistor to bring your coil up to specification? will it degrade coil performance? Idont think any body has posted anything about the use of resistors in regards to spec compliance and their effect on performance.

[porkluvr]

I like IBGold's suggestion for polyolefin wire. That has got to be cheaper and easier to glue than Teflon. It also has a low DC, although not quite as low as Teflon's. But polyolefin insulation can be "puffed" to make DC lower than Teflon.

I had some time to to ponder the discrepancy between the inductance values you and I calculated with the wire you specified.

With the calculator you used, I believe you must take insulation thickness, and double that number to obtain minimum wire spacing. If you make a calculation based on having only a single insulation thickness, your measured result will be much lower than what you calculated.

The MiscEl spiral calculator has fields for entering wire od and copper od and takes into account insulation walls on both sides of the conductor.

You can possibly use PVC wire if you intend to make a large loop for relic hunting where a fast response is not necessary. You might want to make some tests with the wires tied together but not potted to check this out.

Last question, what about using the flat spiral design on large coils 16inch+? Has any one experimented and obtained results regarding advantages/disadvantages in depth/sensitivity?

This one is still open. Somebody?

[mario]

Thank you for the suggestions. I appreciate the attention given to shielding. I have to agree with the issue of tinned plated vs. silver plated wire being minor to the thicker pvc shielding. I purchased the wire assuming that because it had a .41mm thickness
(compared to the listed .254mm teflon), the thin pvc shielding would make the wire have a similar dielectric constant. However, it doesnt, so I shouldnt even consider using pvc. I'm going to see if i get the litz 660/46 or the silver plated 20 awg at surplus of Nebraska. Having said that, what are your thoughts on using pvc shielded wire on large coils? I would prefer not having to waste this fine wire. Should I avoid pvc altogether?
Last question, what about using the flat spiral design on large coils 16inch+? Has any one experimented and obtained results regarding advantages/disadvantages in depth/sensitivity?

[porkluvr]

Just make your connection to the coil as direct as possible. There is no use tearing your hair out over that.

I used the calculator from the URL, and also the flat spiral coil calculator in MiscEl. Using numbers from your datasheet, both calculators agreed that you could only get about 250uH with 45 turns and an outer diameter of 8". One of us probably used incorrect wire overall diameter od.

But I am not here to argue the accuracy or inaccuracy of your coil calculator or mine or the methodology.
Here is my issue:

The choice of stranded vs solid and the choice of tin vs silvered wire, are relatively minor points compared to the choice of PVC insulation vs Teflon. This is because of PVC having about 1.5x (or more) greater Dielectric Constant. Your wire datasheet does not list DC, but it will likely be 3 or more. The DC of Teflon comes in at 2.0.

If you go to the trouble of hauling a heavy, large gauge coil, you should not waste the effort by using PVC insulation material. It might not matter for other types of targets, but looking for BB sized nuggets you must avoid adding unnecessary interwinding capacitance. Using PVC is wire is shooting yourself in the foot. Some people say that winding a spiral coil might be inferior to winding a bundle, but I'm not going there.

I wish they had a better selection but Surplus Sales of Nebraska lists solid 20AWG Teflon insulated wire on their website. Solid, silvered, but good enough.

I would prioritize the wire's insulation material over whether or it is silvered or stranded but that is just my opinion.

[WesP]

See if this is what your looking for. Gary can wind you a custom coil just about any size.

http://www3.telus.net/chemelec/Projects/Metal/Coil-Jig/Coil-Jig.htm

[IBGold]

Hi I would try tinned copper with polyolefin insulation 32/.2mm strands (Marine Wire) that is what I used to wind mine start from the inside and wind out using super glue to hold turns together you could also use 660/46 Litz wire which is available on EBay in your country probably your best option I had no problem with the outer strand crossing the coil although I did space it 5 mm from the coil as long as the strands are .2mm or less the coil will not see eddy currents in the wire shielding is the tricky bit.
Regards, Ian.

[mario]

Hi, is there a right approach to winding spiral coils. In my winding, there's an area where the wire entering the winding overlaps with the spiral body. And I can't figure out a way to make the wire not overlap the spiral body. 1) is this area a problem? won't the detector constantly be set off since it's metal wire. and this over lapping area is in the coils field of view?

2) whats the proper way of winding spiral coils?

3) are spiral coils that take up the whole coil diameter deeper and sensitive, than traditional "ring" coils? My Minelab sucks when it comes to the really small gold, so I'm trying to know if spiral coils are the way to go. I'm hoping to get at least 9" depth for a BB size object.

By the way, I'm working on a 8 inch coil for my Mine lab SD 2100v2. using spiral coil calculator here, I was able to target a 310uH Inductance using 45 turns. I know I need to target .5 Ohms. Also, I'm using single threaded Tin plated 20 AWG copper wire. I couldn't find any teflon insulated wire so the insulation of this wire is PVC. The full specs of the wire are found here. I tried finding multi stranded tin plated wire but was not successfull at 20 AWG. I read that when making small coils you need tin plated stranded wire because the eddy webs are much stronger and therefore the coil much sensitive.

Can any one please answer me the questions above?

Thanks in advance.