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I noticed that I'm having a problem with my homemade coil falsing on the grass, I took a clump of grass and brushed it across the coil and I get a false low foil range false. Any ideas on why this might be? Do I need more shielding maybe?
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It was the shielding
I hosed the coil down in nickel spray-on sheilding and she's quiet as a mouse now. -
Re: It was the shielding
Hi Charles,
I am curious, did you shield the outside of the coil housing or the indside, on this second coating? Do you have a plastic bottom to the coil? Also, since the coil was already assembled, did you electrically connect the shielding to the coax shield? I would think that would have to be done for best operation.
I have found the most important part to shield is the bottom of the coil, but it is best to shield the entire thing. If not, then something touching the unshielded part could create the problem you encountered.
I have not tried the nickel spray yet but will shortly to see how it compares to what I am presently using. It would be easier if the nickel spray works as well.
RegComment
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Your going to laugh but...
Remember I'm an IT manager not an electronics guru, I know enough to trip the household breakers once in a while thats all. ;-)
1st Coil - I sprayed the shielding just on the shell and did not sheild the bottom. I thought that the shielding was just for EMI and figured the top half was probably good enough. This coil of course falsed badly and I thought it was the nickel shielding it was picking up.
2nd Coil - No shielding at all, this is when I discovered it was the grass causing the problem. Someone also then clued me in on this ground capacitance thing. So I hosed the entire outside of the coil down with this nickel spray on shielding and routed a drain wire up to the coil connector and soldered it to the machine ground there. This quieted the machine down a lot, I'd say it got rid of 95% of it but I ran out of spray so the bottom was not completely covered.
I'm hearing from more than once source that the nickel shielding is to be avoided, I get the impression that it will shorten your life if you lack a hazmat suit. So I just ordered a couple cans of EMI-RFI Sheild which is a graphite shielding spray.
I'm going to spray the shell of my next proto-type coil, pour it 75%, let it gell, then shield the bottom and pour the rest. I think I'll be careful to tape off the lower shell so all the shielding is encased in resin.
Lastly I may have to buy some basic electronics books.Comment
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Re: Your going to laugh but...
Hi Charles,
I forgot to ask, what detector are you building the DD coil for?
Many of the present manufacturers of VLF's use or did use the graphite spray type coating for shielding. I have never known anybody to use it consistently for PI's. Today, many manufactures are using conductive plastic for their coil housings, which eliminates the painting process.
Now, as for your knowledge of metal detectors, I don't know anybody who began by knowing it all. Most of us have done similar things as you, so don't feel like you are the only one. Even with a strong technical background, it is impossible to know all the best or even the right techniques on the first try or two.
The shielding can accomplish two things. One is to eliminate the electrstatic field which will minimize the "grass" noise problems. The second item is more relevant to PI's and that is to reduce electrical interference from external sources.
VLF's are generally "tuned" so they can reduce external electrical noise via circuitry design. In other words, the preamp is not a wideband amplifier like it is on a PI. On a VLF, the preamp is generally not specifically tuned to the desired frequency but does have upper and lower frequency cutoffs that will minimize much of the noise problems. They do not eliminate all of them, however.
On a PI, all signals get amplified because a broadband amp is used. So, if they are not reduced by some other means, noise can be a real problem. That is why other forms of shielding are generally used than the graphite spray paint.
The use of lead foil tape, nickel loaded paint, or even the tape I use are used to try to minimize both the electrostatic effects and external noise effects.
Early in the game, I tried a conductive rubber tape made by 3M. It worked fine for minimizing the ground effects, but noise was much more obvious, so I abandoned the idea and stuck with my copper coated polyester tape. Fortunately, the tape is thin enough that the detection of the copper is not a problem.
Now, as for your info about the hazards of the nickel loaded tape, I will keep it in mind when I try it. Respirators are a good idea when using any paint as are wearing a tyvek suite to minimize skin exposure.
RegComment
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Re: Your going to laugh but...
Reg I'm building coils for my Minelab Explorers. I'll generally jump right in and try something I know nothing about like building kitchen cabinets and master it without too much trouble but this coil building has been a challenge each step of the way. But its also been a great learning experience and I'd have to thank quite a few people for getting as far as I have.
If you have cheaper options for shielding I'd go that route, the spray on shielding is expensive at $25 a can and that 12 oz goes fast, three 21 inch coil shells and it was gone.
I have been thinking about the conductive plastic but I'm not sure I want a conductor on the outside, and then theres the bottom left to shield. Maybe for a smaller coil though.Comment
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Re: Your going to laugh but...
Hi Charles
I use refined silver which is as fine as talc powder,it is so fine when mixed with water it turns the water cloudy.
The more you fiddle with coils,the more you come to see how tricky they are.
Regards Frank WallisComment
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Re: Your going to laugh but...
Charles
Detectors are indeed a different sort of beast. The important thing is that you are trying and learning as you go. I personally have learned a huge amount since I became interested in building detectors, and I started with 30 years experience in electronics. I also realize I still have a tremendous amount to left to learn. The guys that frequent this forum who have been doing this sort of thing for some time have been extremely helpful. I don't know what part of NY you are located in, I'm near Syracuse. I certainly am not an expert, but if I can ever be of any assistance with the circuitry side of your detector projects give me a shout.
Russ_NYComment
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Re: Your going to laugh but...
Hello Reg!
Your information is very useful!
For VLF coils: is it necessary also to shield the transmit coil or is it sufficient only to shield the receive coil?
Thanks
NorbertComment
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Re: Shielding the xmit coil
Hi Norbert,
I am not sure since I haven't tried it, but my guess would be you would have to shield the xmit also. I would suspect there would be some phase shift that would occur as a result of the e field of the xmit coil.
However, this is just a guess. I would be interested if someone would try it to see.
Right now, I am still concentrating my efforts on the PI and I always shield both of the coils on my DD's. But, next one I build, I will try it without a shield on the xmit to see what happens.
This is an interesting question.
RegComment
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Re: Your going to laugh but...
Hi Charles,
My guess is the shielding I use wouldn't be much cheaper if any. A lot depends upon the coil diameters, diameter of the wire, etc.
I use a 3M 1190-1 copper coated fabric tape. It seems to to a great job of shielding yet is so thin that it isn't hardly detectable. In fact, I just set up a coil that operates with a delay of 7.5 usec on my PI.
I get the tape from Contact East. It comes in a 1" wide roll 18 yds long. The cost with shipping is about $42 (about $10 for shipping).
I wrap each coil with the tape, minimizing the overlap so it take about 10% or maybe a little more than the circumference of the windings.
RegComment
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