Very Large PI coil construction?

[authere]

large coils

Hi The Digger,The Digger, The Digger,
Have you got a capacitance reading for shielding to coil, my large coil has a cap. of about 263pf coil and 146pf coax for a total of 409pf and the 6" has 92pf for the coil and 79pf for the coax for a total of 171pf, the outside dia. of my finished coils with shield is about 16-18mm so yes i agree with the 3mm gap between shield and coil, i use 1 layer of spiral wrap and a double layer of woven shadecloth. It sure sounds like the shielding is the problem i am having, i can't waite to try out your method and i was wondering if you have tried 1/2 hitches all the way round with 12mm gaps as i keep thinking that the connection between your spiral wrapped wire and the single run wire which is just an overlap would have problems down the track. Definetely food for thought, so thanks "The Digger" i have been rejeuvinated.
Regards Ron

[Stefan]

Hi The Digger,
Well done with the shielding of your 36' coil,with your .1mm wire was it magnet wire or S.S. or what?, as i am in the throws of shielding several coils and they are being a pain in the a#s. I'm using a method developed by Stefan whereby several strands of .14- to .2mm magnet wire are laid side by side and put on tape which is then wrapped, spirally to make a shield. I'm making(trying to) coils for minelab machines and the tolerences are tight, i get the tolerences spot on ,as the ones i make have a Q of 3.8 to 3.9 to which i maintain so the coils should work, but putting the shield on and i have all sorts of problems. The main problem i am having is the coil when stationary gives good depths in air (testing) but as soon as i ground balance and tune the detector and start swinging they sound off, stationary they are quiet.
My thoughts are that the shielding is wrong, but have not as yet found a solution to the problem,but looking at your design i would say there is too much shielding. My 6" coil i have 40 strands x .14mm magnet wire and the 28" coil has 20 strandsx .14mm m/w, can anyone direct me to the right solution, i have put a lot of work into these mother su## ers and i have run out of ideas.
Regards Ron

Hi Ron,

I'm surprsed that you are having problems I've made many coils for the SD2000 and the Extreme with no problems. At one time many years ago I tried to make my own coax and had the symptoms you described. The other problem I noticed with the extreme was that if I made the Q too high on the coil the ground balance was real noisey. If you are using my shield you don't need a gap as the shield is insulated from from the other strands.

If you are using a conductive screen then you must leave a gap as you can have a shorted turn through the shield . Ron if you require any further help drop me an email.

Regards,

Stefan

[authere]

large coils

Hi Stefan,
I actually wrote about 3 sentences in a pm to you tonight, got distracted and saw this heading on Geotech and thought i might learn something, which i have and posted a quick couple of posts. Its strange how i get some coils to work and for some reason others play up, it could be the coax's it could be shielding, as for the Q i work that pretty well using the formula, don't get me wrong your shielding works fine the coil doesn't recognize the shield if you wave it over top, it's just my application of it to certain coils or as you say it could be some of my coax's, i need to experiment a little more and try to isolate the problem, and i am very happy with what you have taught me so far. So hopefully i will have something sorted by tomorrow night.
Regards Ron

[authere]

large coils

Hi Roughwater,
I answered your post last night twice in a PS at the end of my post with an edit and pressed something and it went into cyber space, so i will try again, scotch 24 is good and expensive hence i like to grow my own(or Stefans to be more precise),today i tried using a 20 strand shield instead of 40 strand on my 6" coil and guess what, it is all quiet, i can wave it around all i like and it's quiet. It looks like i'm on the right track, less shielding is the go, i just reduced the strand count on my 28" coil and it's still noisy so i am going to take it out bush away from power lines and give it a try, while i am out there i will keep snipping off strands until she quietens down(i hope).
Regards Ron

[Alexismex]

Hello Forum ,
I see a good idea for making big coil 1 o 2 meter X 2meter :
IT IS THE BLANKET COIL i see video in you tube .
The coil is insert into a "blanket made of big plastic so you can roll it and in the video you can see how the blanket coil pass above the rocks in the desert very easy ....I think it is a very good idea for desert ...now i must think how i incorporate best the coil in the blanket . Another think it is that you need only ONE person to move that blanket coil !!!!
Alexis.

[Alexismex]

Here the video:

http://www.accuratelocators.com/pi_blanket.html

[Reg]

Hi guys,

I haven't spent that much time trying to build a coil for the ML PI's but the times I did try, I did run into problems. The coils almost worked or worked poorly but not like they should.

Now, building coils for the TDI are a piece of cake. There is no real worry about getting the shielding just right or getting the exact Q on anything. In simple terms, the TDI is so forgiving that most coils will work providing the inductance isn't totally off or the shielding isn't so thick or of a type of material that it is readily detected.

What I am trying to say is the difference in making a coil work on the two different types of detectors is like night and day.

I have never really tried to figure out just why the Minelab is so touchy and requires such tight building specifications. What is worse is I am not sure just how to determine what needs to be done since I don't take my ML SD 2200 apart and looking at the signals. Instead, I simply built a couple of coils and they didn't work well.

I have been sidetracked for some time on other projects enough that I never got back to trying to find a solution or what I would call an easier to follow method of building a coil for the ML PI's. I even went so far to buy a SD 2200 to use as a basic detector I could use to determine whether my methods worked or not.

Well, now I am so disgusted with the ML mindset and arrogance here in the US that I have decided I really do not want to pursue the issue any farther. Instead, I will sell my SD when time permits and concentrate on other interests.

I do wish luck and success to those on this forum who try to build a coil for the ML's, but I won't put any more time or effort into it. Instead, I will be building coils for the TDI and similar PI's and will try to provide my ideas or suggestions for building those coils. Hopefully, some of those ideas will help readers. I may point out at times what I did find in terms differences relating to ease of building coils for different detectors, but will try to do this such that it doesn't appear I am biased. Unfortunately, some people will take it that way because of the fact I do point out how simple it is to work with the TDI or the GS 5 when it comes to building coils.

Actually, this ease of coil building also works well for those building coils for Carl's Hammerhead. Usually, it is the shielding or maybe the coax that can have enough of an impact to make bulding a coil that works at a very short delay more difficult. Fortunately, the Hammerhead also has the ability to adjust the delay, thus allowing coils to work that normally would not if the delay were fixed.

The ability to adjust the delay like what can be done on the TDI, GS 5, Hammerhead, or other PI where that feature is available, does provide a technique or method of sort of troubleshooting any coil since a person can note the delay setting where a coil now works, and then try different shielding techniques and check to see if the delay setting changes.

One thing about all this coil building is the fact it shows that even if one has the inductance correct, there are still issues that may cause a coil to not work as planned. What is fun is trying to find out why. On the down side, this "fun" can get frustrating.

Reg

[Slick]

Hey Slick,

I copied the Meteorite Men show to my Dish Network DVR. I was just reviewing the show in order to better see how the meteorite men built their large coils. Early into the show they open up a corner of the PVC on their 1 Meter coil to expose the actual coil winding. It looks like a fairly thick round insulated cable but it appears to be laid loose inside the PVC and not potted. They may not have even glued all the pvc together in order to allow it to pack smaller?

They also (very quickly) show the meteorite men putting their PVC sleigh together and the coil is not inside any of the PVC tubing. They actually show them taping and straping the coil to the outside of the PVC sleigh.
I'm sure this would also allow compact packing of their coil. They seem to do it smart and simple.

Terry

Terry,

Apologies for the delay in responding to this; been distracted with some other issues!

Anyway, THANKS for taking a good look at that setup. I viewed it when it was aired and did not have a chance to record it and look more closely at the construction details.

If the actual coil is strapped to the outside, then the pvc pipe is simply for support as a 'frame' rather than a 'container'. It does make for a relatively cheap, easy to build/modify platform.

Lorenz makes a commercial version based on this theme as well:

http://www.lorenzmetaldetectors.co.u...pmax/index.php

The "Blanket" idea in the youtube video is a good idea, but I can see "wear & tear" issues with extensive use, and a major 'tear' doesn't appear to be an easy fix with that design....cool idea tho. But I think I want to keep some separation between my coil and the ground!

I need to better understand the differences, pros/cons of the flat ribbon style winding vs. the more simply 'bundle' method. I understand capacitance is a prominent issue, hence the various methods used to add 'air space' between the successive layers of wire with various types of insulators. In a ribbon cable design, that could be accomplished by simply spacing the wire further apart.

Reading the posts here and in other places, it seems the larger you go the less you need to be concerned with shielding. I hope that is true, but in any case where I plan on using mine, external electrical interference (other than the noisy alternator/generator circuit in the supporting vehicle) is non existent

Also, the control unit will be some distance from the coil itself in the final version, so that wire distance may need shielding of some sort as well.

Anyone have a cheap source of magnet wire? So far have found it priced at $25-$30/lb. in small qtys; seems pretty steep!

Thanks again!!

Slick

[authere]

Large coils

Hi Reg,
Minelab coils can be finnicky, i've spent 3yrs making coils some would work well others not, but at long last i have discovered what was giving me grief, i was using the same shielding for all the coils not realizing that large coils need less. Since the "discovery" all my coils are starting to settle down as if by magic, a difference of only 30pf's made the coil work or not work, my next problem to solve is how much is good for the coil,do i leave them just of the raggidy edge or do i take them lower. I think i read where the shielding stops EMI to an extent but also helps to ground balance, i think they said the coils shielding is recognized before the differences in the ground ,something like that. The problem is that the place where i test my coils has only sand, which makes it hard to tell if there is enough shielding on the coil, looks like i need a test bed or like The Digger says to take them out where you prospect to set the shielding right.This is turning out to be a long, long winding road and i will just get there and Moodz will have his shieldless coils up and running and it will all be obselete, "SUCH IS LIFE" .
Regards Ron

[thedigger]

Hi Ron
I think you will find that bricks should give enough hot ground to work out your coil balance.
The coil lead is another area to reduce your capacitance,I wind my own using plumbers teflon tape and head shrink tube.I achieve this using a hand drill with a fishing swivel on one end,allowing it to turn,also require the help of my son.
Hope this helps.

[Reg]

Hi Slick,

You might try surplus sales of Nebraska for magnet wire. Here is the link;

http://www.surplussales.com/Wire-Cable/Wire6.html

Also, there are others that can be found by using google and search for magnet wire and you can end up with a website like this;

http://www.magnet4less.com/index.php...FRYiagodQylfLA

Then there is ebay which seems to have a wide selection of magnet wire at this time.

Now, I am not sure what you are calling small quantities either, though and you didn't mention the desired size you are wanting.

Hopefully, one of the directions will help you find the wire you want.

Reg

[authere]

large coils

Hi The Digger,
A couple of years ago a fellow called Digger(Frank)told me how to make my own coax exactly how you describe so if your the silversmith, i thank you. It takes a little effort but helps to control the Q and i have had no problems coil wise and you can make the coax to order and as i have plenty of time to spare it becomes very cost effective.
The house bricks at my place are made of mud brick, 18 to 1 mix gravel and cement, but i am sure i can get hold of a few from somewhere easily enough.
Slick, i use magnet wire and whatever the cost it ain't going to cost as much as buying a ready made coil, best of luck with your endeavours.
And Frank, Thanks again for your great ideas.
Regards Ron

[Roughwater]

Near done with my large PI coil

Hi all,

I'm nearly done building a large coil (1meter).

Rather than rewrite my expierence so far here is the link if you care to look. Scroll to bottom of pg2.

http://www.geotech1.com/forums/showt...t=15459&page=2

Terry

[Slick]

Reg,

Thanks for those links! Still a bit disappointed in most of the prices, and with the added issue of shipping to Mexico, I kept on looking for a source here.

Finally found one (ANYTHING can be found here, if you just keep looking, LoL) in an unlikely store in Guaymas, near by. They have the humongous rolls, so I told them I wanted 50 meters each of 3 sizes...no problem, he measured off the amounts of 18, 20, and 22awg, stuck the whole pile on scale and charged me 240 pesos (about USD 18.50 at todays exchange rate)...copper wire by the lb/kilo!!

Now to figure out the best layout of the coil!

Thanks again!

Slick

Hi Slick,

You might try surplus sales of Nebraska for magnet wire. Here is the link;

http://www.surplussales.com/Wire-Cable/Wire6.html

Also, there are others that can be found by using google and search for magnet wire and you can end up with a website like this;

http://www.magnet4less.com/index.php...FRYiagodQylfLA

Then there is ebay which seems to have a wide selection of magnet wire at this time.

Now, I am not sure what you are calling small quantities either, though and you didn't mention the desired size you are wanting.

Hopefully, one of the directions will help you find the wire you want.

Reg

[Slick]

Hi all,

I'm nearly done building a large coil (1meter).

Rather than rewrite my expierence so far here is the link if you care to look. Scroll to bottom of pg2.

http://www.geotech1.com/forums/showt...t=15459&page=2

Terry

Terry,

Looks good! I will be doing something similar, but am going to try a flat, ribbon type design first, and 1m square. Just found a local supplier of magnet wire, so getting ready to start the assembly in the next month or so.

Keep us up to date on how it is going!

Slick in Sonora