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How to judge shield effectiveness?

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  • #76
    I think any of us who has a garden or access to land with test bed.
    For instance any coils that Ive constructed for customers or coils for builds from here and else where have been useless before shielding or if the manufactured coils have been faulty its shown up.
    Have a Arado 120 here that's been botched to hell, original fault is in the search head itself.
    Symptoms are poor depth , chirpy when sense anything up from Min, discrimination OK which tells me with the Arado that the coils are still in statue as from when they were made.
    Customer gave me the go ahead to attempt repair or rebuild new coils for it.
    Its only me knowing what results to expect on my land that gave me the ability to give the customer a accurate and honest estimate of repairs to his detector.
    Trying to do this else where or in a workshop would have been guess work and a wast of time because not knowing condition of the ground and everything else above and beyond.

    So my way of testing for shielding properties is 1st test bed in real ground, 2nd if all worked to my satisfaction then it goes out for a couple of hours on pasture land.
    Then and only then would I say the shielding is doing Its job, and doing it that way would be more than willing to participate in any experiments with you.
    As have quoted so many times you cannot simulate search head shielding in a false environment like other types of shielding because your dealing with forces that cannot be reproduced else where, if it was a case of just interference from phone,TV,Radio,electric,in fact anything that's man made we can deal with reproduce and simulate it but not mother nature.
    Regards

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    • #77
      Member TEC uses 2-3 wires only connected to earth at one end, and wrapped around coil , there is a picture on GT somewhere.

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      • #78
        I'm not 100% sure we are talking about the same! but If - to 6666 I just made two coil with this type of wires.
        1. Picture Vista Gold round Stock coil. You can still see there wire they used.
        2. Picture a 13" Vista Gold coil with 3 wires every 1-1.5". One left, one right and one in the middle.
        3. picture a 15" Deus coil with wire every 1.5"-2" allso 3 wires.
        Both Home made coils got 3 wires going from a soldering place (ground) and ends with some distance 0.5" at the same area.
        Henrik.
        Attached Files

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        • #79
          Originally posted by Henrikras View Post
          I'm not 100% sure we are talking about the same! but If - to 6666 I just made two coil with this type of wires.
          1. Picture Vista Gold round Stock coil. You can still see there wire they used.
          2. Picture a 13" Vista Gold coil with 3 wires every 1-1.5". One left, one right and one in the middle.
          3. picture a 15" Deus coil with wire every 1.5"-2" allso 3 wires.
          Both Home made coils got 3 wires going from a soldering place (ground) and ends with some distance 0.5" at the same area.
          Henrik.
          As I understand it, 6666 is talking about using only the drain wire (without either a graphite or "wrap-around" shield). This thin wire is sufficient to provide shielding on its own. The question is: "What is the maximum spacing you can get away with before it becomes ineffective?".

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          • #80
            Yes only the drain wire, in this picture is a description, the winding are sparse already.


            I dont know the answer to the question : "What is the maximum spacing you can get away with before it becomes ineffective?".
            Attached Files

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            • #81
              Originally posted by 6666 View Post
              Yes only the drain wire, in this picture is a description, the winding are sparse already.


              I dont know the answer to the question : "What is the maximum spacing you can get away with before it becomes ineffective?".

              Just tried this not totally successful

              Seems fine indoors. Falsing every time the coil touches the grass or ground. Wound separate drain for TX and RX insulated from each other. 5 to 10 mm spaceing

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              • #82
                Are you trying this on VLF ?
                Info was for PI.

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                • #83
                  tried a similar approach on a pi coil, a single wrap of thin equipment wire connected to - at one end then wrapped around the coil in a spiral spaced about 1/2 an inch apart, it seems to work though can be temperamental in some environments, my final version uses the same principle except the wire is 0.1 mm uncoated copper, applied to the final plastic tape on coil and then painted with graphite paint as satdaveuk's recipe its basically a drain wire but it worked the best, no problems and that coil does 12us(measured at exit of preamp).
                  my guess is lots of things work its a matter of finding the method that works best for your coil builds.
                  i recently tested a couple of coils that silverdog made, his shielded coil is made with a foil wrap, but it works fine on my barracuda, it even goes down to 11us, but some users here claim foil is no good for pi.

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                  • #84
                    Originally posted by 6666 View Post
                    Are you trying this on VLF ?
                    Info was for PI.
                    Yes VLF thought I would try it on a IDX coil build at least now I know the answer.

                    No big deal easy to take off and start again.

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                    • #85
                      I find this thread a highly interesting read. Especially since I'm dealing with PI shielding right now.

                      Am now using silver coated polyester mesh-cloth spiral wound around coil as my MPP squealed when getting near the ground outside -it started at 2 feet-

                      Indoor tests are good so far, no appreciable loss of sensitivity, will be testing for real outside the next days.

                      The SRF drops by about 3.5% with the shield.
                      Cable with plug, coil and shield comes in at around 591 kHz now.

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                      • #86
                        Am now using silver coated polyester mesh-cloth spiral wound around coil
                        Is it working for you ?

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                        • #87
                          Originally posted by 6666 View Post
                          Is it working for you ?
                          Can't say for real outside yet - weather is still crappy. Should be better in a few days.

                          The material likes curling up at the edges, so I wound it with thick edges anyway to see if it had any detrimental loop effects.
                          There are none that I can see with the inside tests. Trying to wind it cleanly is much more work.

                          In case you're wondering why the shield is on the outside, well, I thought (hoped) I wouldn't need one.
                          I found the information about shielding I have read quite ambiguous. Well, now I know. Alas, I definitely need one.

                          So I did this quick job to see if it is a viable option, without having to get into the graphite paint business
                          and having to wait for stuff to arrive. The two spots with masking tape are the gaps. They are narrower underneath.

                          Three pics:

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                          • #88
                            @6666

                            Have tested outside for 2 hrs. now, many people around with cell phones.

                            Results are good for me. No interference to be heard. Waaay better than before.
                            I only get a slight change in threshold when lowering coil from 2ft to ground now.
                            The SAT takes care of that.
                            Waving it in the air with abrupt stops and direction changes is also not to be heard.
                            So earth field cancellation also works fine.
                            Dug up all kinds of small and big stuff.

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                            • #89
                              Thats a good result, glad it is working for you

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                              • #90
                                R25 value reduced to 4k7. Surf pi on a 5.5 inch(140mm) coil unshielded. White gold ring detected in air at 7 inches(178mm). Very pleased thanks for the gen.

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