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I use this: http://www.ebay.ca/itm/2PCS-Roll-3M-...item259a51fabd (check out other sellers).
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Hi Chris,
Page 9 is where It shows how Carl notches the foil. It sounds like you may be using 2 wire Coax and have your coil ground and shield ground tied together. To measure the capacitance between the shield and coil you have to measure between the coil wires and shield, Some have recommended that both coil wires should be checked and Take the higher #. Two layers of spiral wrap should be good though. Do you still have the wire wrapped in the coil that is not attached to anything? Some where there is a formula for the proper mixture of carbon powder varnish. I think I would use a exterior product though as they are engineered to remain pliable, less chance of cracking down the road where you bury your wire into it as the plastic the copper and the varnish all have different expansion and contraction coefficients. I hope this works Ive been trying to figure how I want to shield my DD coil Im building.
Hammerhead Article (PDF-1114K, 300dpi) - Rev 1.5 -- 21 July 2006
Kenny 444 Nice job I like the industrial looking coil cover!!Comment
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Thanks for Carls page info, I will check that out in a minute. My coax is just a single core with the braiding wrapped around, is that the right stuff? I dont have a meter that measures capacitance tho. The coil with the extra winding is connected to my braiding, and the other connection goes to nothing. This is still the coil that performs better than all the others I have just made. So I will try my carbon powder on this first. I will experiment with the powder and varnish ratio, painted on some masking tape and try to get the correct resistance per inch, which I think I have read somewhere needed to be 1500-1800 ohms per inch.Originally posted by godigit1 View PostHi Chris,
Page 9 is where It shows how Carl notches the foil. It sounds like you may be using 2 wire Coax and have your coil ground and shield ground tied together. To measure the capacitance between the shield and coil you have to measure between the coil wires and shield, Some have recommended that both coil wires should be checked and Take the higher #. Two layers of spiral wrap should be good though. Do you still have the wire wrapped in the coil that is not attached to anything? Some where there is a formula for the proper mixture of carbon powder varnish. I think I would use a exterior product though as they are engineered to remain pliable, less chance of cracking down the road where you bury your wire into it as the plastic the copper and the varnish all have different expansion and contraction coefficients. I hope this works Ive been trying to figure how I want to shield my DD coil Im building.
Hammerhead Article (PDF-1114K, 300dpi) - Rev 1.5 -- 21 July 2006
Kenny 444 Nice job I like the industrial looking coil cover!!
Kenny 444, I agree, that coil shell looks good.
Multieagle, whatever I seem to wrap around my coils seems to stop them working for some reason. Thats why I'm giving the powder a go. Does yours still work with the copper tape on it? Is the depth reduced?Comment
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Hi Chris,
It seems allot of kits are using regular coax and there is allot of debate on type. Im no expert and I hope someone chimes in but to me it seems to be defeative to use one wire coax on a shielded coil. I know it all ties to ground but if you have two wires in the cable and the shield tied in the box now the shield actually shields the signal back to the box then it mixes with ground. At least you have a half clean signal coming back to the box not polluted by what you are trying to shield out. Thats my thought on it anyway. I further intend to add a choke between the shield and the ground to see what happens.
I started with shielded two wire coax (Mic cable) with my kit and thought it was great but the instructions said only solid core wire would work. So I tried it and my performance suffered. I think the cable I found is a good one http://hosatech.com/product/hmic-000/.
The silver plated connectors are super nice as well thats what Im using on my Hammerhead.
Wow thats allot of resistance to the shielding I may be doing something wrong. I have a max of 99 ohms from my coil to the highest point of resistance in my enclosure. All my shielding is copper, I scrapped out a compac and they have really nice copper shielded plastic on the inside so I made a shielded box out of it. Seems to work
Im not sure about that extra wire I might have to try that. Maybe it is acting as a kind of drain removing capacitance or static from the coil? Its almost like you added a FM antenna to the ground maybe somehow it is working to help the RX side of the coil maybe by draining flyback or overshoot. Here is a thought if it is a drain some how pulling voltage out of the coil through resonation could it then be fed back to the battery for consumption. (Makes my brain smoke!)If I get a loop of wire close to my coils as they are now they go dead. Maybe having it hooked to ground and under shield insulation will change that. More stuff to play with I cant understand yet but thats what its all about anyway.
There should be a emoticon with smoke coming out its ears for me.
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You know, you may have something there with the twin coax. I thought about this when I first started but when I saw the screen is still connected to one side of the coil I thought it wouldnt matter, so I used old tv coax that I had lying about. Its all good fun experimenting.Originally posted by godigit1 View PostHi Chris,
It seems allot of kits are using regular coax and there is allot of debate on type. Im no expert and I hope someone chimes in but to me it seems to be defeative to use one wire coax on a shielded coil. I know it all ties to ground but if you have two wires in the cable and the shield tied in the box now the shield actually shields the signal back to the box then it mixes with ground. At least you have a half clean signal coming back to the box not polluted by what you are trying to shield out. Thats my thought on it anyway. I further intend to add a choke between the shield and the ground to see what happens.
I started with shielded two wire coax (Mic cable) with my kit and thought it was great but the instructions said only solid core wire would work. So I tried it and my performance suffered. I think the cable I found is a good one http://hosatech.com/product/hmic-000/.
The silver plated connectors are super nice as well thats what Im using on my Hammerhead.
Wow thats allot of resistance to the shielding I may be doing something wrong. I have a max of 99 ohms from my coil to the highest point of resistance in my enclosure. All my shielding is copper, I scrapped out a compac and they have really nice copper shielded plastic on the inside so I made a shielded box out of it. Seems to work
Im not sure about that extra wire I might have to try that. Maybe it is acting as a kind of drain removing capacitance or static from the coil? Its almost like you added a FM antenna to the ground maybe somehow it is working to help the RX side of the coil maybe by draining flyback or overshoot. Here is a thought if it is a drain some how pulling voltage out of the coil through resonation could it then be fed back to the battery for consumption. (Makes my brain smoke!)If I get a loop of wire close to my coils as they are now they go dead. Maybe having it hooked to ground and under shield insulation will change that. More stuff to play with I cant understand yet but thats what its all about anyway.
There should be a emoticon with smoke coming out its ears for me.
I would like a go at the hammerhead next, is it better than a surf PI ?, There seems to be lots of versions so I dont know which is best to build. But I sent two emails to Silverdogs site asking questions but he doesnt answer. I would like a board where the holes go straight through, not this surface mounted stuff, and then a price for all the parts to build it. I contacted another chap on the internet who sells kits for a Mirage PI, which is supposedly an upgrade to the hammerhead, have you heard anything about this? But I can only get this kit from Canada if I remember right, so its a little expensive.
Keep up the good work
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Hi godigit1; chrisw ;
Shielded coax or twisted pairs are used to reduce external interference which the atmosphere is full of; radio, power lines, ignition noise, lightning from around the world, etc.
If the two coil wires are not twisted or shielded they act as two antennas. The interference hits them unequally. This produces a noise voltage between the wires that at times can be stronger than the weak Eddy Current signals from the target.
If you twist the two wires together the external noise hits both wires at many opposing turns which creates opposing voltage polarities, thus self-canceling the external noise with little effect on the weak target signals.
Coax cable prevents one wire from being exposed to the external interference. This prevents external noise voltages from being generated between the shield and the center conductor.
An important factor is capacitance per length. Twisted pairs of small (22-24) gauge wire with Teflon or thick insulation has less capacitance than large gauge wires.
Coax with a small center conductor with a low capacitance insulation is best. Stay away from any coax or microphone cable that does not provide a capacitance per ft. specification. For our purposes two conductors shielded has no advantage and would only increase the capacitance across the coil.
With either choice consider keeping the length as short as possible.
I use Blue Jeans LC-1 Audio Cable at 12.2 pF/ft, it gets close to matching small gauge twisted pairs. It is very durable and handles better than twisted wires. And it looks better. If you plan on making a number of coils it is cheaper to buy one long cable and cut to length as you use it. It does come with high quality audio phono connectors on each end.
Regards,
Chet
http://www.bluejeanscable.com/store/audio/Comment
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Hi Chet, thanks for that info that clears much of my confusion. I think I will try some twin screened cable and remove my coax because my circuit is a little noisy and see if that helps. Thanks for the link.Originally posted by Chet View Post
ChrisComment
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Hey Chet
"If the two coil wires are not twisted or shielded they act as two antennas. The interference hits them unequally. This produces a noise voltage between the wires that at times can be stronger than the weak Eddy Current signals from the target."
"If you twist the two wires together the external noise hits both wires at many opposing turns which creates opposing voltage polarities, thus self-canceling the external noise with little effect on the weak target signals."
Ok, Thanks This kinda gives me a little insight to a question I have been pondering and that is why the big brand coils do not have a shielded coil cable? Just a shielded coil and they use twisted wire that is twisted inside the cable.
When I tried Coax I was looking at it wrong and got the biggest center conductor I could find. Im also thinking that coil type and construction may benefit more from one cable type more than another. Say a fast coil with a high resistance may benefit more by a lower resistance cable than a lower capacitance attribute.
Also dielectric material used for shield spacing how much capacitance your coil to shield allready has.
I wonder about spiral wrap beacuse of the air gaps in wraping It. Do those gaps effect anything ?
My Dialectric is a pain but I think it is good.
One wrap white teflon to tighten the coil bundle. One wrap of rescue tape but it ends up two or more thick and then a wrap of teflon mega tape then shield then another wrap of megatape.
What should a maximum acceptable coil to shield capacitance be?
Hi Chris,
As far as the Hammerhead Kit I got the rev d board from his site. It cost I think around $70. I built the option one and with no mods just calibrated per instructions it performes really close to the SMPI. I would give the edge to the HH as it seems a little quiter than my SmPro. Pluse with the mods you can do to it I think will be better. The board is tight and I dont want to riun it so I am real hesitante to do any mods to it. I have started making my Franken Hammer.
I have allready converted the Rev d to 4 seperate PWB's so I can mix and match or mod any part of the board. It also will be a hammerhead 2 as I am making seperate PIC board and dual front end board. I am configuring them for both 1 and 2 options. and from there I also intend a Uno shield.
End disire would be touch screen programming and three to five preset timing programs with Usb support and ground balance.
Currently I have biult it to the clocking and waiting on parts. So far so good. When it is done and tested I will put the boards on the forum for anyone that might be interested .
I have not looked into the mirage I will check it out when I get a chance.
Thanks!
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Hi Chris
I cant seem to find the mirage as a kit or a schematic. Can you provide a link it sounds interesting.
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Hi godigit1;
It is common for Commercial VLF coils to have multiple twisted wires in the coil cable. Cable capacitance at low VLF frequencies (2-50 kHz) normally is not a problem.
Most commercial Pulse Induction coils use low capacitance coax cable. The flyback pulse represents a frequency somewhere between 500 kHz to 1500 kHz. At these frequencies stray capacitance from the cable, the coil wire and shield slow the coil down.
Think of a slow coil effect as tuning an AM radio, you are trying to tune in a station at 1500 kHz but the maker of the radio had poor construction habits and the stray capacitance he caused will only allow you to tune in stations around 550-650 kHz.
I have repaired a few commercial PI coils for friends. In each of three manufactures; they used unmarked coax cables. They evidently don’t want reveal information or it is probably cheaper to not have the coax labeled.
Two of the companies used paper attached to the coils with masking tape. The shield/ground connection is made with a long very small bare held in place by masking tape then graphite painted the entire surfaces. Before painting the small wire was routed through the center of the coil through small holes in the paper and stretched across and held in place on that side by tape and paint.
The third company sprayed graphite paint on the inner surfaces of the top and bottom coil covers with the very small ground/shield wires held in place by the paint.
All three companies wound their coils on Styrofoam molded forms. The forms created approximately 1/8” to 1/4” space top and bottom and around the circumference between the coil and the shield. The space is to reduce the capacitance across the coil.
Air is the best dielectric so air gaps are good. Styrofoam and Teflon are good dielectrics. Dry materials should be used, glues and sticky tapes are not as good.
Your wrapping ideas will help reduce capacitance. The small center coax conductor is also good. It reduces capacitance; the small added resistance is not a problem. If necessary any added resistance in a coil circuit can be compensated for by a small lengthening of the charge time (transmit pulse width).
You should review the Basket weave coil in the; Coils Forum _ under Chance PI around Posts #110 -----126.
Also review;
http://www.geotech1.com/forums/showt...-the-MPP/page1 KRinAZ has some improved Basket weave templates in the last pages.
This paper by bbsailor / Joseph J. Rogowski is a must read for making fast coils.
http://www.geotech1.com/cgi-bin/page...oils/index.dat
ChetComment
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Thanks for all the info Chet
I have a joey coil that been acting up Im thinking its the small ground wire you mention that is faulty. When I get return home form the road I just might tear into it.
That was a interesting bit of reading. I will give a spider coil a try when I can get some Litz wire and may try some with the 30 guage poly insulated I have.
The patterns that KRinAz made are great Thanks go out to him as well!!
BBsailors article has been my staple so far to my coils other than size, shielding and insulation methods. I have some that perform pretty good. With better wire thay might perform a little better. So far Im not able to make anything bigger than 6 inches that will pick up my 1/2 gram 14 k gold stud earing with any effictive depth. I would be happy with 2 inches deep on a 8 inch coil. I get it at almost three with my 4 inch coil and the 4 inch coil will hit a dime right at 7 inches Quarter is less about six in the air and my stainless drafting ruler at 14" Thats pretty good for a 4 inch coil.
Thanks again
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Thanks for the link Multieagle.Originally posted by multieagle View Post
I will have to put out a email to him and see if he is doing any kits still. Looks like a fun detector project.
Be well!Comment
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Hi Guys,
Ive finished. My objective was I wanted around 12" on a coin and good sensitivity on small rings. Well, with an english two pence I am getting 13-14" in the air. My small gold ring, about 10-11". So I am happy with that. These are in air tests indoors.
Outside is another matter. I tried outside and the detector found signals everywhere, I could find a piece of quiet ground. If I reduced the delay it helped a little. So I am not sure if its doing this because my shielding isnt effective or just because its mega sensitive and theres junk everywhere. In the end I think I upset it because I just got a continuous tone, (sensory overload). Its ok on the living room floor; it found some funny signals but gave me a good depth with my coins. So I was wondering if there was some test I could do indoors to test if my shielding is working. I used carbon powder in varnish and wrapped a few turns of wire for a few inches then taped it up. It doesnt detect my hands so I dont know if this is a good enough test.
Anyway Take a look at the pictures. My next project is a Mirage PI from Sven in Canada. So just waiting for the board to arrive.Comment
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