Mono Coil construction help

[Stefan]

SD coils

Hi gef12,

Yes Shield material. There are different methods and materials to use. Tried a few different methods but settled on the following type of shielding. I use an enamelled copper wire shield which will not be picked up by the detector. When the current to magnetic field of the coil is abruptly turned off the coil magnetic field collapses and induces an eddy current in the target material. It also induces an eddy current in the coil screen and the effect will depend on the type of screen employed. The signal received by the detector will then be a composite of the target signal, screen signal and residual ground signal. The idea is to ensure that the screen signal does not mask the target material signal.

I fabricate the shield as follows:

Use thin yellow polyester tape 19mm wide (available from RS Components) and lay approx 30 strands of 0.2mm enamelled copper wire length wise along the tape. ( sticky side). The end result will be about 3 m of tape with 32 lengths of enamelled wire afixed to the tape roughly evenly spaced across the 19 mm width. I then layer another layer of polyester tape on the wire side of the fabricated tape so there is no sticky side. I have a jig to help me do this as it is time consuming.

I then wind the screen spiral wound leaving a gap of 10 mm between the turns onto the coil. I make another tape 15mm wide with the appropriate number of wires and wind this on the coil to fill in the gap.

The other way is to use the 19mm tape and spiral wrap with a slight overlap so the screen fully covers the coil. A final layer of polyester tap is wound over the total shield to hold all in place.

This screen works because the wire in the screen is insulated from adjacent runs, small wire diameter so the eddy currents induced are minimall with no consolidation of the eddy currents in the screen.

Note that one end of the tape is insulated so I usually cut of clean with side cutters and insulate with some polyester tape. The other end is the one you connect to the shield of the coaxial signal cable.

So thats about it?.

Regards,

Stefan

[Reg]

Hi Stefan,

I have to say, your technique of making a shield is a very unique idea. Now, after thinking about it, I would like to add this possibility. One suggestion I might add is to use the shielding off some of the cheaper S Video cable for the wire instead of the individual strands of insulated wire. Many of the S video cables are served shielded. This consists of many parallel strands of very thin wire spiral wrapped around the insulation over the center conductor. In many cases (brands of wire) this shielding wire can't be detected even though it is bare and the strands are touching. By stripping the shielding and using it, the very fine bare wire should work much like what you have done. In fact, one could spread out the strands of the shielding on tape and use it like you do or simply spread them and spiral wrap them around the winding.

Fortunately, some of this S Video cable is extremely cheap so trying it wouldn't be that expensive.

Many years ago, people would spiral wrap a single strand around the windings as a means of shielding. This was done on the early BFO's for shielding. What I don't know is how close the spirals have to be to each other to be completely effective and eliminating the E field. I suspect the wraps would have to be closer for a greater reduction in noise though. However, on a DD coil, just eliminating the E field is all that is required on the tx coil.

Maybe somebody has done some experiments along these lines and are willing to share what they found.

Thanks again for your great idea.

Reg

[Max]

Hi gef12,

Yes Shield material. There are different methods and materials to use. Tried a few different methods but settled on the following type of shielding. I use an enamelled copper wire shield which will not be picked up by the detector. When the current to magnetic field of the coil is abruptly turned off the coil magnetic field collapses and induces an eddy current in the target material. It also induces an eddy current in the coil screen and the effect will depend on the type of screen employed. The signal received by the detector will then be a composite of the target signal, screen signal and residual ground signal. The idea is to ensure that the screen signal does not mask the target material signal.

I fabricate the shield as follows:

Use thin yellow polyester tape 19mm wide (available from RS Components) and lay approx 30 strands of 0.2mm enamelled copper wire length wise along the tape. ( sticky side). The end result will be about 3 m of tape with 32 lengths of enamelled wire afixed to the tape roughly evenly spaced across the 19 mm width. I then layer another layer of polyester tape on the wire side of the fabricated tape so there is no sticky side. I have a jig to help me do this as it is time consuming.

I then wind the screen spiral wound leaving a gap of 10 mm between the turns onto the coil. I make another tape 15mm wide with the appropriate number of wires and wind this on the coil to fill in the gap.

The other way is to use the 19mm tape and spiral wrap with a slight overlap so the screen fully covers the coil. A final layer of polyester tap is wound over the total shield to hold all in place.

This screen works because the wire in the screen is insulated from adjacent runs, small wire diameter so the eddy currents induced are minimall with no consolidation of the eddy currents in the screen.

Note that one end of the tape is insulated so I usually cut of clean with side cutters and insulate with some polyester tape. The other end is the one you connect to the shield of the coaxial signal cable.

So thats about it?.

Regards,

Stefan

Hi Stefan,
yeah it works. I've realized some wire-wrap shields using litz wire...and you technique is similar. My impression of litz wire wrapping was partially bad cause is too trycky and time consuming to get a working shield. Also litz size and number of conductors is an issue. But if properly done it is ok.
A big issue with litz is capacitance added to the coil due to proximity of thin wires near coil windings. Using a spacer , polyethylene type spiral, capacitance could be reduced greatly.

Another good alternative is mesh tape by Scotch that is a kind of small net made of conductors. Very good for PI detector coils...but much depends on frequency too.

Best regards,
Max

[Stefan]

Hi Max,

Basically I am building coils for my Minelab SD2000 and SD2200. One thing to be aware of with the Minelab is that it runs two pulse sequences. One pulse is approx 200 uSec and the other is approx 35 uSec width. The problem was trying to get a screen that would not be detected by either pulse sequence.

The long pulse was not a problem, short pulse yes!

I looked also at copper impregnated tape made by Scotch. I ran a test by placing about 300 mm of the copper impregrated tape on the ground and using the SD2000 to see if it would detect the screen material. Result was that the detector was clearly detecting the tape.

Also did the same test using the Scotch mesh tape. Same problem, the detector was able to detect the mesh also. I was disappointed in the result as the mesh tape is very easy to apply. Expensive also.

I have also used graphite tape and mylar aluminium backed tape used as a shield in instrumentation cables. Short pulse detects the tape.

The tape I fabricate is not detected by the detector and that is why I use it. Hard to make but I think worth it. Why do I use 0.2 mm wire. This is the smallest size I can get readily. I live in a very remote location so the electrical/electronic stuff I have to buy is via mail order.

Regards,

Stefan

[IBGold]

Stefan I agree about the Scotch Tapes but I find by splitting the Scotch 24 it is much better but I will try your method it is the same as used in the coil coax used by Minelab and Coiltek I am trying wes ALC227 and Beldon 1505F they measure lower capacitance than Minelab coax. Regards Ian

[IBGold]

Stefan I have been testing your shielding system I did not have any PE tape But used 25 mm masking tape instead with some .15 enameled copper wire that I had a reel of about 40strands then I wound it on my test coil as follows .1. coil to shield capacitance with split Scotch 24 140.6 pf, with Stefans shield 122.4 pf good improvement, .2. With detector 25 x25 NI/CU tape detected from 69 mm ,75 mm of split Scotch 24 detected from38 mm, but 75 mm of stephans shield with ends terminated one end could not be detected very impresive it raised the resonant frequency of the coil 45 Khz I have put the coil in a 10.5 x 3.5 semi rectangular housing will fully test it when it is set tomorrow and report back. Stefan I have the same problem as you I live in country South Aust and components are a problem all mail order where are you what state. Regards Ian

[Stefan]

Hi Ian,

I live in Karratha, WA on the North West Shelf. The masking tape will work ok. I use the PE tape as it is very robust and thin.I can send you a sample of the screen tape I make if you are interested. I am in the process of building a large DD coil and have some small offcuts left over.

Regards,

Stefan

[IBGold]

Stefan fine on Karratha I am in Yorketown On the Southern Yorke Peninsular in S.Aust. would like to make direct email contact and it would be interesting to see some of your shield for comparison I have the same problem as you with parts and components ect. Regards Ian.

[Woody.au]

You fellows go to a lot of hard work making your coils, just use Teflon or Polyprope 1mm/2 multi strand tin plated wire with a 750V insulation (thick) and use either the stainless steel,Copper or Silver mesh from less emf.com.

The tin plated wire stands have far less capacitance to the shield compared with Litz wire. The small thread size of the mesh will not allow long time constant eddy currents to form. You will not need a spacer due to the thick wire insulation giving ample stand off to the shield.

[Stefan]

Hi Woody,

Thanks for your post. Yes I do take some effort in constructing my coils using home made Litz wire. I don't wind the coils in a straight bunch wind as Minlab do but take special care and a couple of tricks in winding the coils to minimise the interwinding capacitance of the coils for a higher self resonance. As I make my own Litz wire I can ensure that each coil size has the same Q so I have a few reasons why I prefer Litz wire.

As for the screen, a spacer is required to reduce capacitance as you mentioned. When making large coils 21 inches or so the screen to winding capacitance can get quite large so I can adjust the screen accordingly to keep capacitance within limits.

I get excellent results this way. In fact so good that the coils were too sensitive in mineralized ground and drove my crazy on ground balance. I had to de-sensitize a couple of coils.

I use a SD2000 as a preference search detector and SD2200D on small nuggets.

Regards,

Stefan

[Woody.au]

Good to see that you make your own Litz wire, I have also done this but after walnig 6 kilometers up and down the house block I have gone back to tin plated multi strand. I have found that I get a Q of 5 or over using tin plated but a Q of 4 or less using any Litz wire. By the way I have a swag of mods for a SD2000 that will rival their newest offerings, especially on large deep Gold.

[Stefan]

Hi Woody,

I don't have any trouble getting whatever Q I want. If I want a total Q of coil + lead of about 4.4, I make the Litz wire to give me a coil Q of 4.9. For a 14 inch coil this works out to about 33 runs of 0.2 mm diam wire. if I want a total Q of 4.0 then it is about 29 runs.

I got some info from IB Gold about the SD2000 mods but would be interested in what you would be willing to share.

Regards,

Stefan

[Woody.au]

The biggest gain is from doing Ismaels mod on the opamp gain on both channel 1 and 2 and that is simple to do. Put in a lower RDS on Fet as the IRF710 is pitiful. Rip out that LM394 and put in a SSM2210. Remove one 4148 claming diode. Replave the front end fets with lower theshold voltage and lower RDS on devices. I would like to incorporate a decent band pass filter into this design as it would make it a very good detector. I still reckon the original SD2000 with the right tweaks would leave the GP series in the dust on depth performance.

[JC1]

Great Shield Idea

Hi Woody,

I don't have any trouble getting whatever Q I want. If I want a total Q of coil + lead of about 4.4, I make the Litz wire to give me a coil Q of 4.9. For a 14 inch coil this works out to about 33 runs of 0.2 mm diam wire. if I want a total Q of 4.0 then it is about 29 runs.

I got some info from IB Gold about the SD2000 mods but would be interested in what you would be willing to share.

Regards,

Stefan

Hi Stefan,

Really good information you are providing everyone.

Great Shield Idea.

And COPPER also.

Yes any Q desired.

I've used Litz Wire for Eons.

Especially for Large Coils in excess of 1 meter

and several amps xmit. About 100 meters wire.

The "effects" really show up then.

Haven't seen really good coil

posts like this in a loooong time.

JC1

[Unregistered]

Hi All,

New at this, in these posts i read about Q all the time.

Can somebody explain what is Q & it's effects & also how do i measure this, i have a Minelab GP3500 & would like to make some coils but i want to make sure my brain is on top of this before i start.

Is Litz wire the best to use or is there better?

Thanx