VLF Coil Sheilding - Tape

[Mike]

Hello Charles,

In another thread (https://www.geotech1.com/thuntings/s...7648#post37648) you mentioned:

"...we are experimenting with conductive fabric tape at the moment...I already built a prototype, the tape as applied was too conductive and the Explorer would detect it if the coil was bumped, but overall it was quite useable...We are thinking it may be possible to shield the coil with smaller sections of tape, each insulated from the other, but tied together with a fine drain wire creating what may be akin to a tiny gold chain that the detector can pick up."

I wondered if you had done any more testing with conductive tape, and if so what your results were like. Was the tape you were using like 3M's 9712 ? (http://products3.3m.com/catalog/us/e...er/output_html)

Tape would be a whole lot easier than Electrodag.

Thanks,

Mike

[Charles (Upstate NY)]

Tape would be a whole lot easier than Electrodag.
Mike

I'm not so sure about that. The tape 3M was making that we were testing has been discontinued. It seemed to work well on Sovereign coils but not on the Explorer coils. It was too conductive in my opinion.

That asside, assuming you could find some tape with similar conductivity to eletrodag here's the deal. First you have to wind some spiral wrap around the windings to properly space the shielding tape from the windings. That not only requires a fair amount of time but we found it was then difficult to overlap the windings e.g. TX, RX, two spiral wraps were taller at the overlap than the shell was deep. Then there is the time consuming part of wrapping the tape. The tape is also fairl stiff, its not like applyin say some sports tape to a bandage.

I dont know if you saw my later post but I solved the electrodag application issue. I use hot dip to space the shielding from the winding, dip it and toss it in the freezer and its ready for eletrodag in 2 minutes. If one were building a lot of coils you could dip the whole winding in eletrodag and hang it to dry. I apply it with a small brush. I find this approach to be the easiest by far of all that I have tried.

Charles

[Mike]

Charles,

I can see how if the conductive tape is somewhat stiff, then winding it could becomes a pain. Yes, I've seen your liquid spiral wrap post. Thanks for the update.

Regards,

Mike

[woody]

Fabric tapes

The best by far is the "Phantom fabric " sold by less emf.
It is copper plated nylon at 90 threads per inch ascis absolutely invisible to any type of detector. The trick is to cut a length and wind a spiral winding over you coil.

[Reg]

Hi Woody,

If you get the delay down to well below 10 usec then the "phantom fabric" is detectable. The "see through conductive fabric" works well also and is a little harder to detect. Also, since it is about 3 times as long, the cost is not that much different.

BTW, the cu/ni fabric tape they sell also works.

Cheers,

Reg

[Mike]

Woody & Reg,

Thanks for the information. LessEMF looks like a useful site to know about.

(Direct link: http://www.lessemf.com/fabric.html)

Thanks,

Mike

[Charles (Upstate NY)]

Lessemf if located about 10 minutes from my house. The Ni/Cu tape is too conductive for VLF shielding. I can easily pick up the tape with my Explorer. The less conductive 3M tape is better, I can't pick up a 1 inch piece but I can pick up a 4 inch piece.

Woody & Reg,

Thanks for the information. LessEMF looks like a useful site to know about.

(Direct link: http://www.lessemf.com/fabric.html)

Thanks,

Mike

[Reg]

Hi Charles,

Thanks for the clarification. I should have mentioned the fabric tape seems to work ok for PI mono coils.

Now, one tape that might work for the Explorer is a rubber semiconducting tape made by 3M. I think it is called Scotch 13. I found it worked for PI's and is not detectable.

Reg

[J_Player]

I have a question: has anybody measured the electrical resistance across an inch of shield material that is the right conductivity?

A second thought about the foils and tapes: The foils are made of the same metals as the fabric tapes. But the fabric tapes do not have a continuous flat conductive area as the foils do. The fabric weave prevents currents from flowing in an unobstructed path, thus we may have a mechanism similar to what happens when a lot of very small grains in a target are not detectable compared to the same weight of a solid lump of the target.

[Reg]

Hi J,

You are right, the foil tapes and the fabric tapes are not the same in structure. The foil tapes consist of a thin sheet of copper material with an adhesive on one side. One can solder to it very easily.

The fabric tapes are clearly a special polyester type tape with an extremely thin layer of copper material plated on the tape and one side has conductive adhesive also. When you hold the tape up to a light, you can see through it and see the very fine weave of the material. Any copper plating is mainly on the individual threads. So, there is a dramatic difference in thickness of the two as far as the amount of copper involved. This is probably why the tape is not detectable as easily.

It is almost impossible to apply solder to the tape. Bbsailor has done it, but I prefer to simply tape a shield wire to it. This seems to work as well.

Reg

[Charles (Upstate NY)]

I have a question: has anybody measured the electrical resistance across an inch of shield material that is the right conductivity?

Yes, the carbon black shielding on coils I have tested measured 80-100ohms with the probes 1 inch a part. To put things into perspective this shielding paint will not pass a continuity test even with the probes 1/10th of an inch apart. Both the 3M copper and ni-cu fabric tapes will pass a continuity test as will nickel paint.

A second thought about the foils and tapes: The foils are made of the same metals as the fabric tapes. But the fabric tapes do not have a continuous flat conductive area as the foils do. The fabric weave prevents currents from flowing in an unobstructed path, thus we may have a mechanism similar to what happens when a lot of very small grains in a target are not detectable compared to the same weight of a solid lump of the target.

I had the same thought but the fabric tapes are detectable.

I think its important to stop and think about what one is trying to shield against. To shield from EMI/RFI requires a pretty heavy shield and a good ground. Explorer coil carbon paint is ineffective against EMI/RFI, it shields against static and ground effect.

A shield heavy enough to shield against EMI/RFI would be too conductive for the Explorer type VLF machine. Therefore if one were considering alternative shielding materials you may as well focus on something for static and ground effect.

I have built coils with the nickel paint which measures about 1 ohm per inch and if you bump the coil the machine detects the nickel paint.

[Reg]

Hi Charles,

Have you found different sources for the electrodag shielding paint you use?

Reg

[Charles (Upstate NY)]

Hi Charles,

Have you found different sources for the electrodag shielding paint you use?

Reg

I buy Electrodag 502 by the quart bottle from this place BUT I would recommend looking for a supplier of Aquadag instead. Aquadag is water based versus Electrodag which is MEK based which is pretty nasty stuff and ships hazmat. I personally have not tried Aquadag so I don't know how long it takes to dry (Electrodag dries very fast) or how well it sticks but Beachcomber has and says it works well. I'm still on my first quart of Electrodag with maybe 75% of the bottle left. A little goes a long way. Keep clear of the spray can stuff, you will get about 10% on your winding and 90% blown elsewhere, its quite wasteful.

http://www.laddresearch.com/General_...ng_paints.html

[MojaveRed]

Hi Charles,

Thanks for the clarification. I should have mentioned the fabric tape seems to work ok for PI mono coils.

Now, one tape that might work for the Explorer is a rubber semiconducting tape made by 3M. I think it is called Scotch 13. I found it worked for PI's and is not detectable.

Reg

Hi Reg,

With respect to PIs, it seems from what you've said here and elsewhere that the suitability of a given shield material (lead foil, Cu/Ni fabric, etc.) depends also on how long your sample delay is.

Is this generally a true statement? If so which of the various materials mentioned are best in your opinion for a PI operating below 10 us, say at about 8.5 us?

My own experience is that to use lead foil, for example, I'd have to increase the delay to 12 or 15 us.

[bbsailor]

It is almost impossible to apply solder to the tape.Here are a few tips to solder to this 3M copper fabric tape. I use a variable heat 25 watt soldering iron set at about 660 degrees. Then I use a flat braided desoldering wick (about 4" long) that is pre-tined as the ground lead. I try to solder about a .375 to .5 inch length of wick to the shield. I tried several methods and failed until I came upon this method. Place a small piece of flat metal (I use a small horseshoe magnet keeper) to act as a heat sink under the point where the joint will occur. Use very fine solder as it melts faster than thicker solder. This prevents burning a hole in the copper fabric. Practice on a few pieces of short 1" pieces first to get a feel for how long heat can be applied before you burn a hole through the copper fabric tape. Solder the wire in line with the circumference then apply a few tight turns of tape to secure the solder joint and take the strain off the solder connection.

Soldering or wrapping a bare lead wire tightly taped should work equally well.


bbsailor