Theories about coil-shielding

[baum7154]

I don't think any shielding specifically and directly improves depth. It improves performance in terms of minimizing interference, capacitance effects with the earth, and electrostatic charges. I guess you can say it comparatively improves depth over an unshielded coil in many cases. To me depth is achieved with a larger coil and more transmit power and a more sensitive receiver, i.e. better technology.

I get my spray on graphite from the auto parts store and the powdered graphite is from the hardware store, generally sold for locks. For coil shielding see the thread started by Satdaveuk titled "Synthetic Graphite". It details all you need to know about how to apply and tweak for your desired resistance value.

Dan

[Henrikras]

Hi all. My Test with shielding at the outer on my Velox One Stock coil DID work Very good. I was out today 4-5 hours at my hotspot and gave the Velox a chance.. The Velox One sens. pot is from 1-10 and I had Allways got the limit 7 / 7.5 and on good days 8.0, then is was chattering. (In wet grass often only 6-6.5 or 7.0) The depth is like many know VLF turn on and go detectors arround sens.6.5- 7..
Today I was running sens 8.5 nice and calm and could even go up to 8.8-9.0 before the chattering was annoying.
I did a depth test this morning with 8.5 gram GOLD coin, (0.900 Gold), just to Be sure that I hadn't lost something. It went out perfect.
I did the same test - max dept with this Gold coin at my 3 other used detectors (Deus, Vista Gold and Blisstool) and the Velox did win with the Blisstool close to the same.. That was good to know and should allso Be like this, If it possible to run it with so high sens.

This was my way to make a shielding outside at this Stock coil...
A; 2 places - front and back Remove a small part of the plastic outside of the shell with a file, to find the inside shielding lay, to get connecting to this new graphite lay outside.
B; Control measure that you got connect from the inside shielding to the plug pin 5 (the RX cold side). If not! you had filed to much or to less. The inside shielding lay is Grey and the Shell / plastic i Black, so it is easy to see the different.
C; Outside drainwires must Be made with a connecting to those two places you had filed... Glue the drainwire at some points to hold it.
Remember drainwire at the up and the down side of the searchcoil. The drainwires I gave some graphite 33 spray at those two places I needed to get connection. ( Again use pin 5 and measure to the drainwires, you should get same resistance like from the inside graphite lay to the pin 5)
D; paint a grahite lay outside down to 4-500 ohm, measured from pin 5 at the plug to no matter where at the new outside graphite paint. Remember some 1-2 hours your graphite paint must Dry up, before you do the measuring.
Sand or paint more until the lay is perfect.
E: When the shielding is perfect, a protection paint is needed... I was using the Plastic 70 spray paint. Ist easy to work with.
F: The Stock Shell cover is now ready to put on.. Be carefull at the corners. I had to file and sand the cover at some places, Otherwise it had taken off new paint. A Heather can allso help to soften up the Stock cover.
This outside paint will not last for ever.... This is made like a test, but allso like a copy of my XP Deus coils, which was the way I got my Deus coil to work. I got no idea about capacity which could change LC or even the resonans) or what ever this could make a different. It was only a test to lower down the chattering, Otherwise I got no advantage to use this Velox one. I got the Velox One only because I needed a VLF Detector with good dept and a good disc. system, but I was disapointed with the chattering at high sens.
The next test will Be a Rain full day out... If that allso will end succesfull, my 9" Stock coil will get same work.
Henrik.

[Henrikras]

Test 2 in wet grass Nokta One with outside graphite lay - PERFECT outcome. Wet grass field after rain this morning and it was working nice. Full sens. (8.8-9.0) and no problem. I wonder more and more why they hadn't done something like that at the factory ! Maybee small nuggets will Be a problem ! Don't know because I never do nugget hunting.
Henrik.

[Skippy]

I find this puzzling. I would expect an unprotected conductive coating to be useful, particularly for eliminating static discharge problems, like dew-covered grass. But if you then cover it with a layer of protective resin/varnish/paint etc, would you not expect the benefits to disappear?

Maybe it is possible to obtain carbon-filled plastic sheet off-the-shelf in low resistance form. Obviously stuff they use for making vacuum-formed anti-static trays is conductive, but probably nowhere near good enough. If you could get such sheet, a coil-cover coild be made from it. I tried this several years ago, using regular back anti-static bag material, joined to the coil cable's screen. I even made a heel-grounder (copper wire around my shoe, with a wire going to anti-static foam, tucked in my sock) and a conductive foam handgrip joined to electronics ground. Sadly, it wasn't much use, despite the idea being good. The plastic sheet just wasn't low enough resistance.

[Henrikras]

I dont know Skippy... Maybee its only a lucky punch I made. The way I see it.... When we got one of those VFL detectors with Very high sens., just a microscopisk signal will make they way through.. It could be anti static, EMI, or just a coil not made solid, and so one.
I dont know what the reason is in this case, but I know so much... If I make a air test like Nokta does to sell those, this high sens. didn't work when I took it out to Hunt. There were complains, not only from me but allso from some in UK. Nokta answered that we must lower the sens., and it still one Very deep detector, but I did test and could see that with sens. 6.5 -7.0 it was similar to the Deus, and that we all know isn't one of the deepest in the market. I needed to make it run stable with sens. + 8.0 and I will get what I was looking for, buying this Detector.
I our work with the Deus 7" coil, we learned that XP had made a conductive Shell, which wasn't a mix with graphite in the Shell plastic but a outside lay... A Very good idea to help short block those small signals we dont like, and by doing the same (not the excatly the same) at the Velox coil, some how it did help and the sens. could be turn a fraction up..
Here comes the reason why at think this Helped: The inside Sheilding is arround 1K ohm and connected to the RX cold side. When I make a outside lay with Very low resistance and connect this to the inside lay, (not direct to the Stock ground wire and cable) it will work like a sandwich shielding with two lays like the Deus, but each lay got different thickness and resistance (like the Deus) then it would work like the inside shielding got a capacity to the ground. Like this - low capacity/ low resistance (outside)+ high capacity/higher resistance(inside) in serial. (The Deus Stock coil got the Otherway around - low inside and high outside, but my 7" coil got low inside and low outside)
Another way to say it; with a closed shield case outside and connect this to the inside lay, I got now a higher capacity down to the ground. If I had connected the outside drainwires directly to the cable, Properly the inside shielding had been out of work, because the outside is made thicker that the inside. now I got the Stock Sheilding working and a Very closed shield outside allso.. Sandwich construction like XP did at the Deus coils, not excatly the same, but the idea is the same.
I need to record a video, so everybody and you can listen to the chattering at high. sens, with a Stock coil and with my newly made coil, Maybee then you will come up with some usefull ideas there could be made at some other brands..
Henrik.

[golfnut]

We dont really know what the problem is here do we.

If you have a less than adaquate e field screen, what goes wrong? What is the mechanism for the problems observed?

[Henrikras]

Good quenstretion Gulfnut.... I can only geuss. But If you watch the video I just made quickly this morning, it sounds like sparks..!
And those sparks sounds like high Freq.... I know its impossible to remove all the chattering, but If the Detector can Be run with sens.8-9 stable with a 11" coil, you got something usefull. My shielding is one little step in the right direction, sure more can Be done, but I think that would mean a new coil design.

Link : http://www.youtube.com/watch?v=br5uYMy51ZE&sns=em
Ps. Sorry this thread is been interrupt with this Nokta shielding.
Henrik.

[golfnut]

The real noisey one may have a rx coil tuned TOO close to the Tx - try an air test on small target.

Use the same target on the 11 inch.

If the air sensitivities are similar it may prove that the Rx on the 9" is tuned too close to Tx.

If the Rx on the 9" is tuned a little below the Tx you can move it away a little with a small cap. say 1, 2, 3 nF this will reduce the sensitivity and depth a little.

It should air test less than the 11 in this new case.

S

[Henrikras]

Thanks Gulfnut. Just made an air test. The 9" got less airtest dept like 3/4"-1" all type of conductivity and small objects.
The preamp is inside the searchcoil, so it not easy to finetune with capacity.
Notice that the 11" coil got less sparks after the extra shielding was made outside.
I will play a little with a piece of ferrite, Maybee a better Null can Be found. If not try to see what happens, If the drainwire at the outside goes directly to the cable...(now its connected to the inside graphite lay).
Thank you for your ideas.

Henrik.

[golfnut]

"The preamp is inside the searchcoil, so it not easy to finetune with capacity."

If the Tx frequebcy is an osillator ( rather than generated in SW) you can move the Tx up or down with capacitor in control box.

Moving the Tx up works the same as moviing the Rx down

[Henrikras]

okay Gulfnut - I'll look into that when I got more time.
Got Some other project going and the Velox is performing pretty good with the 11" coil, so it can wait.
Donald (DB Shell) is close to finishing the new design small eliptical coil, until that is ready, I'm working on a 15" Deus coil.. (Had a 15" Shell left from last years projects) To do that I'm allso working at Graphite mixture using some ideas from this thread.
If the graphite mixture succeed I will post the results in here. (With this square sample with electrodes, your idea).
Henrik.

[golfnut]

I recently bought some printer Toner for Ricoh commercial copier/printer - I was hoping it was Carbon Black.


I had intended to mix it with the graphite powder. It was unfortunately NON conductive.


Im interested to mix graphite with carbon black powder for more repeatability

[Skippy]

I think the key to obtaining the correct amount of shielding is to understand the effects of too much shielding. We know that way-too-much will affect sensitivity - the Faraday cage is TOO good, TX signals can't get out so easily, RX can't receive so well. But how much loss can we tolerate? We know that there is an "Inverse 6th" power law affecting raw air detection depth ( I use inverse 5.5 power in all my calculations, as I think the typical detection depth is not far enough away for the 6th power approximation). So, as an example, if our shielding loses 10% of the signal, that's 10% of TX, 10% of RX, total loss = 20% approximately. This equates to a depth loss of about 3.8%, example: 10mm at 250mm distance. This is an acceptable loss, and it could be recovered possibly, if RX gain were increased by a modest 20%, this could be completely recovered, with little side-effects.
If we had heavier shielding, say 20% loss, it becomes more serious, 20mm loss on a 250mm target, and +55% gain (TX or RX) would be needed to recover it. This is probably the limit, any heavier shielding and we have problems. I think from Henrik's figures, he possibly has this level of shielding on his recently modified coil.
But if we have too light shielding, and we only lose 2%, say, the depth loss is just 0.8%. We are probably not getting the best shielding here.
George Payne, in one of his online documents about coil-building, liked a value of 10 K Ohms / square, but not lower than 1 K Ohms. I suspect the lower figure of 1 K / sq is the value we should be looking at, maybe 500 Ohms is viable, maybe 2K is where we are not trying hard enough.
One problem is measuring an irregular surface, to find out the surface resistivity. A seperate "test-piece" might be one solution.
The other thing to consider is the shape of the Faraday cage. Would a complete circular case be worse than a spider-type case, where the cage is closer to the coil wire?
Maybe an experiment worth doing is to make a single TX coil (circular would be easiest) and accurately measure field strength with a sniffer coil. Then enclose it in a graphite Faraday cage, and measure the difference. Repeat for different resistivities, different distances from the coil etc.

One final thought I had: I saw a link, in an earlier post, to graphite paint with VERY low surface resistivity, 50 Ohms / sq. This would be great to use as a "drain wire". One of the recognised problems with an actual wire is the painting of it with graphite. I imagine painting high-resistance (1K) graphite onto low-res (50 Ohms) would work well. You would have to avoid complete loops (like you would with a wire) and you still have to make a wire connection somehow, unfortunately.

[Skippy]

@Golfnut: Toner is regular plastic, and an insulator. That is why static electricity attracts it to the paper, and then a heated roller melts it onto the surface permanently.
Graphite powder is easily obtained, a bit of Google searching soon turns up suppliers. It's quite cheap from some distributors, too.

[Henrikras]

That was some interesting and valuable information Skippy