Theories about coil-shielding

[green]

[I think another way to test the effect of your shield is to do an air/detection distance test with a much smaller and thinner target (perhaps 1/2" X1/2" X .001" thick) with the coil unshielded and shielded. Don't know if it is possible for you to remove the shield or not at this time. This test would give a real world idea of shield degradation of detection distance.]

I will try the 1/2 x 1/2 foil shielded first to see how fast I can sample. The coil was over damped on the first recordings. Wanted to keep the amplifier the same and didn't know how much adding the shield would effect things. The amplifier comes out of saturation at 4 usec after gate off signal with shielded and unshielded coil. Not sure why target size would effect when it comes out of saturation unless target signal is too strong. I bought a large spool of awg 28 magnet wire a long time ago and am trying to learn with what I have. I cut the foil before putting it on the coil. Thought I saw it somewhere on the forum but can't find it now. Maybe not. I put 2 inch masking tape on the foil long enough to go around the coil and cut it for desired width. Then mark the tape every 3/8 inch. Cut on marks from one side. Turn tape around and cut between cuts from the other side. 3/8 inch seems to work, a different dimension might work better. The thread was about signal loss and I didn't see any. I tried only one time, maybe If I tried again it would be different. I would be interested if graphite effects the signal. I think it would be a lot easier and faster to apply. Picture included

[baum7154]

Don't have any graphite so I've been using aluminum foil for the shield. Was wondering how much signal loss I am getting. I have a single layer coil wound on a wood hoop. Recorded target with coil not shielded and shielded. The plotted data is the difference of a no target recording and a target recording. Didn't see much loss.

I think another way to test the effect of your shield is to do an air/detection distance test with a much smaller and thinner target (perhaps 1/2" X1/2" X .001" thick) with the coil unshielded and shielded. Don't know if it is possible for you to remove the shield or not at this time. This test would give a real world idea of shield degradation of detection distance.

A few years ago I shielded one of the first spider coils with 0.001" thick copper foil and it was 'really pretty' but destroyed the coil sensitivity and speed. While aluminum foil decays much faster than copper foil we know from our 'Target Time Constant' thread tests that even very small amounts of .001" thick foil will be detected at short delays. I would think that the effect of your aluminum coil shield would make it hard to sample at short delays because there is a relatively large amount of it whose decay masks desired fast target responses.

Dan

[Waikiki_Sweep]

Aluminum foil acts like target. May be signal of it is not too strong so you can not see it on chart but it is here.

I have a rule: not to use in coil any material you can detect.

You can detect:
1. cooper magnet wire
2. aluminum foil
3. single wire core coaxial cable or coaxial with aluminum foil shield.
4. single wire you using to connect shield

You can not detect:
1. stranded wires (I like Teflon insulated silver plated 19 strands AWG 24)
2. graphite shield
3. stranded wire core coaxial cable and coaxial with Mylar or wire mesh shield.
4. stranded wire you using to connect shield.

So you have a chance to not add any targets into you coil design. Otherwise you are at risk to have longer decay from coil plus decay from those targets.

To make good connection to the shield I am using silver plated stranded wire and winding it with step 1.5 - 2.5 cm. as spiral over coil winding (remember to make a gap.)
Than paint coil with graphite + marine epoxy 1:1 (remember to make a gap.) You can buy a cheap graphite powder on eBay (I got 1 pound for $12) or in hardware store (graphite lubricant for locks).
So you will have reliable connection to the shield over all surface of your coil. And because silver plated stranded wire in undetectable it makes no gesture to your detector.

For several coils I have made I see good results with PI and with VLF detectors.

Also if you build VLF coil like DD please remember that shields on RX and TX have to be separated (NOT CONNECTED anywhere)
and your coil cable have to be build with two separated NOT CONNECTED coaxial cables.

I have purchased many S-video cables for VLF coil cables and found them wrong build like two wires in one shield or separated shields contacting inside cable or no shield at all.
Finally I have found Belkin Synapse S-video cable with two insulated coaxials insuide just like a cable for Minelab Excalibur. It gives best results for VLF coils.

[green]

Don't have any graphite so I've been using aluminum foil for the shield. Was wondering how much signal loss I am getting. I have a single layer coil wound on a wood hoop. Recorded target with coil not shielded and shielded. The plotted data is the difference of a no target recording and a target recording. Didn't see much loss.

[greylourie]

We need to have the graphite powdered to a universally acceptable state. Like concrete formulas use different sized particles of aggregate in different combinations of mix volumes. Where viscosity of the carrier medium is low - the risk of the graphite settling out is high ! Need to have the graphite powder reduced to as fine a state as possible, so that the graphite is less likely to settle out during drying/curing stage.

Density of natural solid graphite has a density of around 2. The density of graphite in pencils and carbon motor brushes varies dependent on binders used, and this has an impact on the final electrical properties. Communition of graphite to easily reproducible state could be accomplished by using a simple ball mill (like used by home pyrotechnists), and milling for fixed amount of time. Best done in a shed, the stuff will probably settle all over surrounding surfaces.

[Gwil]

Re post #3 -

In principle concentric coils can be screened with foil but it has to be applied in the same way as to DD coils - wrapped around each individual coil and with a small gap. Lining the shell with foil means there is a large flat shorted turn present. Not likely to be helpful.

[golfnut]

Barry in oz got some good results using thin cellulose car paint and graphite.

This is something I struggle with, tried a few paints/epoxies etc but not low @8k/sq and very variable result

[Skippy]

I'm not sure which graphited paper you are referring to, some commercial type, or home-made. But I have home-made graphited paper with a surface resisitivity of 1.5 K Ohms per square. Compare to black ESD bag of 300 K Ohms per square. I used a 6B artists drawing pencil, on normal writing paper., applied heavy.

[bklein]

I think you'll find they are static dissipative and not conductive...

There is a thread here somewhere (mine??) that suggests using a high input impedance fet amp to test for efficiency- should this be a good test for the esd bag effectiveness? How does sq. inch resistance of the black conductive rubber bag material compare to graphite paper?

[sinclairuser]

yes i used dave's thread as the basis of my experiments, the only difference was i used clear acryllic plastic lacquer(i'm actually allergic to white spirit/tups subs), so used a different solvent it works no problems, its flexible and dries quicker the standard varnish.
if you grind your graphite dont forget to make it as fine as you can.

[baum7154]

The thread I am referring to is called 'Synthetic Graphite' started by SATDAVEUK in the coils forum in 2012.

[baum7154]

On coils for PI use I have found that no actual metal should be used as a shield if you are trying to detect small gold targets, i.e. 10 grains or 1/2 gram or less. In fact I don't even have solder joints in my latest coils.

If you look up the threads on Graphite Coil Shielding you will find posts by Guru SATDAVEUK and he is the man on this topic. He worked closely with me on getting graphite/varnish shielding to work on my first coil. There tricks in that thread you will see on how to connect to the graphite, how much resistance per inch, lightly sanding the graphite coats to get the proper resistance, under layer materials to get best results on the graphite coatings, resistance changes as the coatings cure and appropriate wait times to get stable results. It is worth the search.

Regards,

Dan

[Skippy]

As I commented recently on Henrik's Deus coil thread, I measured a black conductive anti-static bag as having a surface resistivity of 300 K Ohms per square. About 30+ times too high to be useful. Unfortunately.

[Silver Dollar]

I think you'll find they are static dissipative and not conductive...

[bklein]

ESD bag as shield?

Before I go and spend time experimenting with it... has anyone tried using an ESD bag for shielding. They can come in large sizes - see photo.
They can be heat-sealed. Any advice? - should I shrink to the coil then cut a clearance ring? Attaching to cable ground may be challenging.