Surf PI - successful build

[Chet]

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/

[chrisw]

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.

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.

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

[godigit1]

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.

[chrisw]

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!!

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.

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?

[godigit1]

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!!

[multieagle]

I use this: http://www.ebay.ca/itm/2PCS-Roll-3M-...item259a51fabd (check out other sellers).

[chrisw]

HI Guys,
The coil to shield spacing is two wraps of spiral wrap; I dont really know how to test the capacitance. But I have always grounded the shield at the coil not higher up, I dont know if this makes a difference. When I have used foil it is just normal sheet cooking foil which I have cut into strips. I cant seem to find the article where Carl preps his foil. Anyway, I have found and ordered some cheap graphite powder off ebay. I read somewhere about mixing this with varnish and painting it on, so I will give that a try.

[kenny444]

Hello regarding the Surf PI and its hardware, I mounted mine a Project box with all the controls at one edge and the coil input connect close to the end but on the side, so I can use a right angle plug to keep it all compact, I also think I shall use it in a bag or on a belt mount to reduce swinging fatigue. As for the search coil I bought a cheap cake icing spinner and used that to house the coil. I do hope this image is here and useful.

[godigit1]

Hi Chris
Also what is the coil to shield spacing? Capacitance between coil and shield? Are you leaving a gap in the shield? when I use the copper shielding it is from a 99 ofc free microphone cable and the weave is very fine. It has two wires and a shield wire for independant grounding. In the kit instructions it is stated that only solid core cable will work. This is not true. I had problems with coax. It is better to join the shield to the ground in the box. I drilled a new pin in for this. Also when I attach the shieldind I fold the wire over itself and use shrink tubing to lock it together. If I solder the shield at the coil I loose sensativity( coil goes dead ). The solder joint becomes a target and nulls my coil. My coil to shield capacitance is .31 nf I am using both teflon mega tape and silicon rescue tape as the insulator but the rescue tape has to be wraped tight so it might not work on a spider coil.
Hope you get it figured out. Oh Yea Just a thought on the foil are you cross hatching it before you wrap or using solid foil? See the hammerhead article how Carl preps his foil. That might help.

[bernte_one]

use graphit paint for shield, and how it is been told everywhere connnect the shield too

[multieagle]

Maybe off base, but are you grounding the shield?

[chrisw]

Hi Guys
I cant seem to get any shielding working. I tried first of all wrapping with some copper braid stuck to tape, but it was very thick and didnt work. Then I tried wrapped with foil. The waveform is below. But the detector made no noise at all. The only way I could get at working again was slowly changing the load resistor. It seemed to work ok but had a crazy ringing waveform. But this was the only way I could get it working. How can I shield this coil and get the proper waveform and depth?

[chrisw]

Its strange, I just now went outside to the slabs outside and now its working ok, but maybe a little less depth, strange. I like your shielding idea I will give that a go, thanks.

[godigit1]

Hi Chris,
It sounds like your coils may be fine but you are getting overload when metel is too close. The concrete outside may have rebar reinforcement. My coils do the same thing, overload and just go silent whereas my VLFs have a ring tone for this I prefer the silence!! If you check a clean area outside with your VLF you can sit the coil down on the ground and it should be ok, otherwise?? Ive been playing with copper shieldind a little and it works great but my method is pretty ruff and I am restricted to a minimal coil to shield spacing. I am lacing my coils into the shielding from my coil cable.
For the spiral coils Maybe if you cannot find carbon paint some conductive mylar (space blanket) might work. I have not tried the scotch 24 it looks a little thick on the guage.
Here is another way to experiment with a similar method. http://www.geotech1.com/forums/showt...-shield-method
This is a picture of what Ive been playing with for shielding. Fast coils wound inside of the shielding, .
Hope you get it figured out.

[chrisw]

That second video that you posted is great depth and only a small coil, I would like to know how he did that but cant get any information.
How do I pump up the performance to get 40cm, I would be very happy with that.

I am still having trouble shielding my bigger coil. If I put foil round it , it stops working, so I think if I bought scotch 24 copper tape I would get the same effect so I dont want to waste my money. But if I place the coil on the paving slabs anywhere outside it just doesnt work at all, so I think it is detecting the floor. Also something that confuses is if I place a small copper coin close to the searchcoil it doesnt work with any other targets till the coin is removed, why is this? If the coil and coin are stationary I thought it should be ignored. The reason I tried this was I was going to use a steel bolt to secure the coil housing to the shaft, thinking that if both were stationary that the bolt would not be detected. So I will have to look for strong nylon.

It seems my knowledge about metal detector design is very lacking.