Optimum coil shielding method/material

[Rov]

I Agree with your comment WM6,on the copper shielding an solderability,May be too much for shielding an detrimental.
I look towards the simplistic,an maybe commercial manufacture of coils an their shielding.
Do you see nickel spay,No.do you see Scotch open weave tape No.do you see copper foils No. No you see graphite papers,foils,an tapes Yes.Maybe Aluminium foils Yes.Nothing else.
Why. cost effectiveness!
Something that has been extensively researched an developed to do the job at minimal costs.
I stick with this an good results can be obtained consistantly.Simple!
The Military an Aerospace Industries use Graphite paints,foils an laminations for all their screening purposes Still! for good reason.
Regards Rov

[maikl]

For those who Faraday shield aluminum foil I would advise to do it as shown. This method has many advantages. Author, it is Thomas Scarborough (CCO metal detector).

[guiseppi]

I have used a roll of 17 micron aluminium foil I bought years ago.
Also use this for general screening.
It is much thinner than the thinnest kitchen foil. Cut thin strips and wrap in a spiral over taped up coils using some DS tape to hold in place, pressing and rubbing after smooths it all down, then tightly roll with electrical tape.

Don't know where to get this foil now, but it was intended for making capacitors/condensers.

I also had a problem making coil heads and couldn't stretch to getting it together to form plastic ones, so I looked for another way with the tools and skills I have. I have a lathe Used some light, cheap pine and turned a hollow disk/dish exactly the right size for the TGSL standard coils. Good enough and pretty light weight once finished.

[maikl]

We should not exaggerate the thinness of aluminum foil work will become "micro-surgery". I neither have worked TGSL but maybe better for him to reel with aluminum foil space of 3mm-4mm(film reel with no overlap).

[cyclops]

Some years ago, I opened an early DD IB coil from BH and was astonished that there was no shielding exception made some red paint on the plastic shells. And the coils are performing exceptionnally good.
At second sight, I noticed that the greyish epoxy enclosing the windings was stuffed with aluminium powder and that one connection from ground cable was hooked into that epoxy.

[guiseppi]

If I had exaggerated, my roll of foil would be at least 20 microns thick
In practice, it's probably not miles better than ordinary foil, but it does form to a very thin, smooth layer easily, regardless of how many creases are in it.

A good source of the metalised mylar (or mirror mylar, whatever) is from model aircraft suppliers. This can be sourced down to low micron thick ranges. Pretty cheap.

Another idea is to use ordinary 5-15 micron mylar, cut it into strips and rub graphite powder into the surface quite hard with some tissue or cotton. By measuring with a multimeter, you can control the resistivity to a certain degree by rubbing more in, or off. The graphite does adhere to the mylar without treatments. This was commonly used for making electrostatic speakers, but I don't see why it shouldn't be fine for this. I have used this method and it works.

Graphite powder can be sourced from locksmiths as a dry lubricant. It goes a long way and could be experimented with in paints and epoxies/glues to create conductive applications. Knowing how much conductive graphite sprays cost, it's worth the effort.

[simonbaker]

Pictures of my coil winder...

Cool stuff Monk; sorry I missed this thread earlier, just found it. Lot's of good ideas here.

I'm interested in any data on using EZ Slide graphite paint. Particularly, conductivity measurements of painted areas.

-SB

[dfbowers]

Cool stuff Monk; sorry I missed this thread earlier, just found it. Lot's of good ideas here.

I'm interested in any data on using EZ Slide graphite paint. Particularly, conductivity measurements of painted areas.

-SB

I have also tried metallized mylar from a survival blanket. For a few dollars you can get a life time supply. It's only conductive on one side. you cannot solder to it, but you can run a bare wire around the outside of the coil. Then cut the material into long strips and wrap around the coil, conductive side inward.

Don

[simonbaker]

I have also tried metallized mylar from a survival blanket. For a few dollars you can get a life time supply. It's only conductive on one side. you cannot solder to it, but you can run a bare wire around the outside of the coil. Then cut the material into long strips and wrap around the coil, conductive side inward.

Don

Mylar sounds better than al foil which rips easily. Maybe perfect for my "stockade peg" winder.

Shielding not well studied. I think we need to compare some unshielded coils to different shielding ideas to compare performance; how much needed; how much coverage; etc.

-SB

[taliesin]

Pictures of my coil winder.








This is my "epoxy coater".






The amount of epoxy deposited is determined by the size of the exit orifice. For the .012 diameter magnet wire that I've been using I've settled on a .018 inch orifice which gives me the amount of epoxy I need to produce a good coil. It's quite easy to use either five minutes or 30 minute epoxy in this form of a two-part syringe. Depending upon wire size I can get 3-4 coils from one syringe. A syringe cost approximately 3 dollars USA. If I'm winding a single coil I will use five minute epoxy. If I'm winding to coils I'll use 30 minute epoxy.

Setup is fairly easy takes about 10 minutes. And once you are done winding there's nothing else to do other than let the epoxy cure. You end up with very consistent, very solid and very repeatable coils.

The forms are made from three-quarter inch plywood. They are faced with .100 Plexiglas. I have tried different release agents and find that Teflon spray works fine. The real secret is the Plexiglas facing that has been glued to the plywood forms.

Using this setup I can make at least six different coil configurations. 2 DD, 2 concentric and 2 Mono.

As stated previously and from my "too many years" working with low-level instrumentation and data acquisition, the transducer (search coil ) and its "care and feeding" (construction and interconnect) is probably the weakest link that the home constructor faces in producing a quality detector,

From a testing/performance standpoint you must be able to reproduce consistent and repeatable coils. My plan is to build several different configurations of the same coil form to test shielding and construction variations.

From a grounding/shielding perspective there is a wealth of information in these forums, especially early on. I've attached some of the input/comments that I think are most pertinent at the end of this post. For the DD, IB coil I'm going with a build up to different configurations .

Configuration number one: starting with the coil then adding spiral wrap then adding Millipore tape than spraying with graphite, adding a spiral drain wire, and covering with .002 mill mylar tape.

Configuration number two: starting with the coil then adding spiral wrap then adding 3M AL-25BT Aluminum Foil with Conductive Acrylic Adhesive, adding a spiral drain wire and covering with .002 mill mylar tape.

Not sure what I'll do for concentric PI mono coils. For PI mono coils I will experiment with graphite, nickel spray, copper spray and 3M aluminum foil.

Connecting the coil to the electronics is the next hurdle. My instrumentation experience says that I want to shield both transmit and receive lines. The transmit line to keep noise in, and the received to keep noise out. Also want a separate conductor for the shield.

For the DD, and concentric IB coils I'm going to use this cable:




I haven't settled yet as to what I'm going to use for the PI mono coils.

Any comments/suggestions are appreciated. I especially appreciate the work of dedicated hobbyist/enthusiast such as dfbowers and simonbaker. From my readings of these forums (and specific to coil construction/shielding) there were some great early forum pioneers such as Eric Foster, Reg, Charles (Upstate NY USA), bbsailor to name a few. And from a general metal detector construction and tinkering perspective I miss very much previous contributors such as ivconic and max, posters who were not only more than willing to offer their help, support and opinions but also who put in tremendous amount of hours building and testing metal detectors.

Some posts that I found particularly informative regarding coil construction:

nice idea with the epoxy applicator

[bklein]

Stupid me - I don't get how the epoxy is applied. You have to mix that epoxy 50-50. Where is that done? How is it applied to the windings - as you go on each separate turn? So it looks like you have two pieces of plywood and you say there is acrylic sprayed with teflon facing each one. Then the coil is wound in the center, over the dowels... But the epoxy doesn't stick to the dowels?
Since you can make so many of these have you experimented with the number of turns? If you have too many what do you see?

[Monk]

I don't get how the epoxy is applied. You have to mix that epoxy 50-50. Where is that done?



The epoxy injected into the "extruder reservoir" ( yellow arrow pointing to extruder reservoir) and mixed there.

How is it applied to the windings - as you go on each separate turn?

The magnet wire runs through the bottom of the "extruder reservoir". As the wire is pulled through the reservoir the epoxy coats the wire. the amount of epoxy applied to the wire is determined by the size of the opening in the output of the extruder reservoir. This puts epoxy on the wire continuously i.e. every turn automatically has a epoxy applied to it.

But the epoxy doesn't stick to the dowels?

I forgot to explain that the wooden dowels are first inserted into one side of the plywood/plexiglas form and then the form is sprayed (with the dowels) with Teflon or other release agent. The other half of the forms is also sprayed with Teflon and then both forms are bolted together with the appropriate spacer in between.

Since you can make so many of these have you experimented with the number of turns? If you have too many what do you see?

Unfortunately after going through all the effort in building this and winding some coils I haven't had a chance (time) to do anything else with the coils other than making one complete search DD coil which I have yet to test. The only thing I can say is at the close of very strong, very tightly wound, and very repeatable in terms of physical size and electrical characteristics. This setup also can be used to wind single layer coils for PI type detectors.

[bklein]

Thanks. It was right there but I didn't see it. drrr....

[simonbaker]

Very nice gadget!

What did you use for the reservoir? And how do you keep it from caking up with epoxy after several uses?

-SB

[Monk]

What did you use for the reservoir?

The reservoir is made out of 1" thick PVC, approximately 3" long by 2" wide (height). The actual reservoir is "carved" out of the plastic. A 1/8" thick aluminum "wear plate" is attached to the end in the direction of the plywood forms (coil form). The wear plate is necessary to prevent the PVC from elongating (enlarging) due to tension/friction on the front end of the reservoir facing the coil form. Holes are drilled front and back ends ( in-line) approximately 1/10 inch from the bottom of the reservoir for the magnet wire to pass through. Diameter the hole in the aluminum wear plate determines the thickness of the epoxy being applied to the magnet wire.

And how do you keep it from caking up with epoxy after several uses?

Before the epoxy has a chance to set up a simply clean up the reservoir with a paper towel making sure that I aggressively remove the epoxy. I use a small stiff wire to clean out the wire passage hole on each end. And sometimes if necessary I use a small drill to clean up the wire entrance and exit holes after the epoxy has set up.

I've actually built two sets of coil forms so that I can make two coils at a time which makes the process (cleaning) less tedious i.e. I can make a set of coils before I have to clean the reservoir.

When all said and done the actual construction of the coils goes very fast (typically 5-10 minutes to set up the coil forms, 10 min. for epoxy/reservoir set up and maintenance, 15-20 minutes to wind two coils (depending on number of turns per coil) and 45 minutes to allow coils to harden. In summary 30-45 minutes to actually build the (2) coils and another 30-45 min to let them harden up. The only real pain being having to maintain the reservoir i.e. cleaning the reservoir.