Coil Shielding

[authere]

Hi Eric and all,
A discussion we are having regards shielding materials, it has been said that shielding on a coil produces a static electrical shield around the coil i think so that the coil recognises the shielding instead of the changing ground that we walk over, static electrical charges will run thru plastic and if you rub fur with plastic it will produce a static elect .charge. These materials fall into a category of TRIBOELECTRICAL materials and they have a nobility chart, some materials have +ve charge and some a -ve with a few nuetrals like steel,copper falls into the middle of the -ve side and that means that electrons are absorbed, whereas a +ve material would give up electrons.I was thinking of using a more Noble if that is the word, material like gold leaf to use as a shielding material or for that matter maybe the spiral wrap could be hooked up to be the shield.

I know you have mentioned that you used to use lead sheet which is a +ve Tribo. Elect Material, have you ever looked into this side of things regarding shielding materials or am i barking up the wrong tree(again)

Regards Ron

 

[Eric Foster]

Hi Ron,

Coil shielding and static electricity are very interesting subjects. I have a problem at the moment with static and seem to be barking up some wrong trees., so don

 

[authere]

Hi Eric,
You sound a bit like the NUTTY professor Sum Demillar or whatever his name was,You do as i do with things, they have to work right or i gotta fix it, can't say i've wired up anything in the kitchen yet though!!!, it sounds like it the angle of the cutters will have to be modified unless you can measure an amount of static electricity in the nuts....see youv'e got me sounding like him now.......back to the topic.

There is a lot to be said for a variable shield and automatic dampening, i can control the amount of shielding by trimming out wires in the shield but once i have this worked out i set this in concrete and don't adjust it any further but it could be made to react with the ground if needed with electronics, as for the dampening resister that is a bug bear as i have tried different coils and they all seem to want there own sized dampening resister to suit the coil.

I make my coils to an exact Q according to the accuracies of my meters and this still does not help to control what dampening resister is reqd for each coil that i make, they can be a fickle thing to keep under control, but as you say coming contact with the ground changes the capacitance in the coil which affects the dampening resister anyway, so the idea of a self adjusting D/R would go a long way to solving some of our problems, i imagine that this would work very well with a salt or I/B coil.

this gets back to the point of what should we use for shielding, i take it the conductor or or's would need to be conductive in that i couldn't use the spiral wrap as a shield.Should the shield have the ability to prevent eddy currents flowing around the shield as does the wire used for the coil wires ie; tinned cu. wire is used for the conductor in the coil could or should this be usedfor the shield as well.


i find this a facinating subject andi see why this has kept you going all these years,good on you and keep up your proven good work,

Regards Ron

 

[Eric Foster]

Hi Ron,

Here are a couple of pictures of my shielded coils that use ribbon cable helically wound on top of spiral wrap. The wire is stranded and each conductor appears to be 6 or 7 strands of 0.08mm tinned copper. Hard to see as it is so fine. The picture shows two coils, one 8in diameter and the other 11in. You can see the different number of conductors in each wrap for the two coil sizes. This is necessary if you want the overall wrap to be flat and butt up neatly to each other. Stranded wire ribbon is best as you not want to cause eddy currents in the wire cross section. I cannot see any advantage in leaving out a connection to any conductors as a floating conductor may cause other problems. These coils date back to the 1980's and I gave up this method when I discovered that lead tape was easier to apply as it had self adhesive backing. To apply the ribbon shield, you need first to wrap a double sided adhesive tape on top of the spiral wrap. Too much labour intensive wrapping. This is why few manufacturers use shielding wrapped on the coil.

Eric.

 

[authere]

Hi Eric,
i would dearly like to show you the method which i am using except it is not mine to show, it is the brainwave of Stefan to whom i keep mentioning, but suffice to say it is not unlike your method but the use of singular strands put side by side between two layers of special tape. The thing is like you have said that we could use tinned cu. wire as the shield, one thing though as a compatriot on another forum has asked does it matter at what spacings we lay the wires and how far apart could these be before they are not effective, point in topic is my 28" dia. coil i used a 32 strand shield and had to keep stripping off strands until it started performing properly, i ended up with 5 strands of .15 enamelled cu. wire at about 5-6mm spacings and it was purring, yet when i make a smaller coil of around 11" i would use 32 strands, have very little capacitance and the coil sings.What i havn't done though is try to remove some strands off the shield and see how it performs, they do perform well mainly because the cap. is only at about 180pf, but on the 28" coils experience i could trim some strands away, by the way, once i have worked out how many strands are req. i remake a shield withthe right amount of strands in it and reaply it to the coil.

The question remains how low can the capacitance be for shielding, i imagine the small coil(11") being trimmed out would tell me that, i still feel that an adjustable shield depending on the capacitive hookup from the ground that we walk over needs to adjust automatically as we detect as we seem to overdo the shielding to cater for all types of ground and coupled with an auto dampening resistor, which is already patented in America would solve a problem of the coil getting maximum depth in all types of ground, another reason why so many nuggets get left behind.

Regards Ron

 

[IBGold]

Hi Eric,
I posed a question to you on another forum but no reply so here goes again I have tried most of the coil shield options but because I build coils only at this stage for my Minelab machines and have found most are seen by the coil with the exception of the commercial Electro-Dag type coatings and the spiral wrapped wire that Ron mentions I hand make this shield tape using PE tape and .2mm enameled copper wire I have been using .75 wire spacing and can do 1mm or 1,2 mm spacings.
I have found on the larger coils this may have a dampening effect on coils performance but it is hard to quantify.
I have found that on one particular type of Mono coil that I build is not affected by ground mineralization in other words I can use this coil where historically you could use only a DD coil on a SD or GP machine my test machines are self Modified SD2000 and a self modified GP3500 I don't know if this is because of the coils construction method or the coil type my question is there a formulae for the shield wire spacing to the size of coil for optimum results my current one under construction is a 3 to 1 rectangular of 12" x 4" what spacing would you suggest for this coil for optimum results the coil is wound with 32 strand tinned copper wire with polypropylene insulation with two wraps of PE spiral wrap and one layer of 2mm Double sided tape.
Regards, Ian. [VK5KIB]

 

[Eric Foster]

Hi Ian,

Making coil shields with PE tape and enamelled copper wire seems like hard work. As I mentioned earlier, the purpose of the shield is primarily to eliminate the effect of variable coil/ground capacitance but with a secondary benefit of reducing r.f. pickup. There is no way that I can see of such a shield having any effect on reducing the signals due to magnetic iron minerals. If it did, then it would also affect the detection of the nuggets and other items that detectorists want to find, as they fall within the same passband.

Minelab PI's are notoriously critical on search coil characteristics and small changes in electrical characteristics can have big effects. This includes overall coil capacitance, which includes the shield. I made some large coils (30in x 10in rectangular) for the late Chris Hake some years ago and used lead tape for the shield. Unless I got rid of the overshoot in the response due to the coil/shield capacitance, they were noisy and unstable. Having done this they were fine.

I would use one of the woven metal tapes such as Scotch 24 or 3M CN3190 which also has an adhesive backing.

I am in Ireland at the moment on another computer and don't remember the long password for the other forum. Anyway, feel free to raise any further points here.

Eric

 

[IBGold]

Hi Eric,
The reason I use the tape is with the Minelab machines I have found I can put a 3" piece of my tape down and the coil will not see it but if I use a 3/4" square of Scotch 24 0r Cu/Ni tape or lead tape the detector will see this from quite a distance.
I will continue this with you in a week or so as I am going to Adelaide for a trip into hospital and expect to return in about a week.
Regards, Ian.

 

[Eric Foster]

Hi Ian,

Hope all goes well at the hospital. In the meantime I will measure the eddy current decay of the materials you mention.

Eric.

 

[wm]

Hi Eric,

as I am also using scotch 24, would be very interested, if it may limit the detector performance? Which sample delay becomes critical?

Another question is - where to look for the lead tape? - could not find any in Germany so far... What is it used for (usually)?

And finally, there is a zinc paint for car sheet protection available - did somebody made any experience with that?

thanks & best regards
Walter

 

[IBGold]

Hi Eric,
I am back home now I have now modified my jig for making shield wire to allow a variety of wire pitches and wonder what your recommendations would be for the pitch for the shielding for the 12 x 4 coil or what coil to shield capacitance would be optimal to aim for in coils I have Electro-Dag that the commercial coil builders use but after many tests find the spiral wrapped shield produces more sensitive coils.
Walter you can get nickle spray to use for shielding but it is very expensive or you can make your own by recycling old heavy duty batteries.
Regards, Ian.

 

[IBGold]

Hi Eric,
Have you had a chance to do those measurements yet.
Regards, Ian.