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Single layer Spiral coil

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  • #16
    I played with the coil today. It's really sensitive. I was able to find a lead bb at 3 inches. It's also deep. I'm able to find bronze targets 1/2cmx1cmx2mm at 8 inches. It's also a very quiet coil. If I keep the coil at a consistent level off the ground (which can be difficult since ground isn't naturally leveled), I notice a very stable threshold every time, which surprised me. Ground balancing helped out a lot with stabilizing this coil.

    The only problem I'm having is the aforementioned issue with lowering the coil. As it approaches the ground, the machine sounds like it hits a large target every time. it's really annoying, and I know I could improve the coil by shielding it. I just don't want to ruin the coil by using the wrong shielding material. I really need your advice on this issue guys. My SD 2100 v2 should not be that different from the SD 2000 in terms of components.

    Could it be due to the coil having a larger number of windings and therefore more capacitance than the machine could handle? I measured capacitance at 18.98 uF.

    I also read some where that the coil needs to be grounded. What does that mean? How would you go about grounding a coil housing? where would you ground the wire? right now, the two sole wires from the microphone cable I'm using are connected to the RX and TX terminals. I striped the copper ground wire mesh that was inside the cable to stabilize the coil.

    By the way, some more coil specifications in case you don't have a LCR meter, and would like to construct the coil:

    Windings: 39 turns
    Outer Diameter: 7 and 1/2 inch (exact)
    Wire type: Litz 330/46 with nylon shielding
    Inductance: 301 uH
    Resistance: .898 OHM
    Capacitance: 18.98 uF (measured at connector)

    I was able to attain a better inner circle by winding the coil to 7 1/2 inches (The eight .10 US dimes were placed at 2 inches from the center), and then removing 5 turns from the inner circle. It allowed me to remain very close to the required 300 uH inductance. However, resistance is close to .9 OHM and is not effecting performance as I expected (Could the coil be even more sensitive if its resistance was at .5 OHM?). The coil remains quiet sensitive.

    Last issue that I'm having is the coil's sensitivity to hot rocks. I dug faint signals thinking they were small lead shards. Once I dug faint signals, I noticed they were hot rocks. It frustrated me since I particularly got rid of my old vlf metal detector because I got tired of digging the hot rocks. Is the spiral coil design not suitable for hot rocks?!?!? I was told PI was better at avoiding them.

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    • #17
      Originally posted by mario View Post
      Could it be due to the coil having a larger number of windings and therefore more capacitance than the machine could handle? I measured capacitance at 18.98 uF.

      By the way, some more coil specifications in case you don't have a LCR meter, and would like to construct the coil:

      Windings: 39 turns
      Outer Diameter: 7 and 1/2 inch (exact)
      Wire type: Litz 330/46 with nylon shielding
      Inductance: 301 uH
      Resistance: .898 OHM
      Capacitance: 18.98 uF (measured at connector)


      Last issue that I'm having is the coil's sensitivity to hot rocks. I dug faint signals thinking they were small lead shards. Once I dug faint signals, I noticed they were hot rocks. It frustrated me since I particularly got rid of my old vlf metal detector because I got tired of digging the hot rocks. Is the spiral coil design not suitable for hot rocks?!?!? I was told PI was better at avoiding them.
      Mario, the equivalent capacitance of the search head can not be measured by C-meter.
      It is calculated after measurement of L and the resonant frequency. Even with a long microphone cable, the real value of calculated capacitance is several thousand times less than yours 19uF.
      If you find some hot ricks placed in shallow, some of them are meteorites. Search WEB for details because some meteorites are very expensive. You can get good money for them if offer to the jewelers, collectioners, museums and researching institutions.

      Minelab uses graphite paper for coil shielding. You can see this in video at:
      http://www.youtube.com/user/woodypoopoo
      See also:
      http://detectormods.com

      Comment


      • #18
        Thank you!

        After watching th[FONT=verdana,geneva,lucida,'lucida grande',arial,helvetica,sans-serif]is[/FONT] video I see my mistake. I tried a quick fix by enclosing the coil in some aluminum kitchen foil shielding and grounding the shield to the detector.



        Unfortunately, the detector was able to detect the shielding and all I got was a persistent hum. I'm going to try next conductive paint if I find any.

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        • #19
          Originally posted by mario View Post
          Unfortunately, the detector was able to detect the shielding and all I got was a persistent hum. I'm going to try next conductive paint if I find any.
          Mario, try to cut alu foil as shown below
          Attached Files

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          • #20
            I tried it. No luck

            Comment


            • #21
              depth comparisons

              I would like to know if any one could direct me to a site showing depth results of different coil sizes for a minelab SD,GP, GPX. I would like to see how well my coil stacks up against other coils.

              This coil is relatively fast, but I noticed that after 8 inches, it looses sensitivity dramatically. A dime or bullet won't register at greater than 8 inches - Which is really strange considering that I can find small metal targets easily at the less than 8 inch threshold. I'm able to get a BB (1/6 gram) sized lead object at about 2-3 inches... an iron BB comes in at 5 inches.

              Is it normal for PI to loose dramatic sensitivity in this non linear fashion?

              Also, Air tests on a soda can come in at 2 feet.

              Comment


              • #22
                Originally posted by mario View Post
                I would like to know if any one could direct me to a site showing depth results of different coil sizes for a minelab SD,GP, GPX. I would like to see how well my coil stacks up against other coils.

                This coil is relatively fast, but I noticed that after 8 inches, it looses sensitivity dramatically. A dime or bullet won't register at greater than 8 inches - Which is really strange considering that I can find small metal targets easily at the less than 8 inch threshold. I'm able to get a BB (1/6 gram) sized lead object at about 2-3 inches... an iron BB comes in at 5 inches.

                Is it normal for PI to loose dramatic sensitivity in this non linear fashion?

                Also, Air tests on a soda can come in at 2 feet.
                A Lot depends on the Type of Metal. I Believe your US Dimes are mainly composed of Zinc.

                Comment


                • #23
                  I'm still playing with this coil. After testing it further, I'm inclined to think it performs much like a 4 inch coil I had for my tesoro, not an 7.5 inch coil.

                  Am I wrong to say that performance depends primarily on the Inner Diameter of a coil, not it's Outer Diameter? If so, it would explain it's similar performance to a smaller coil.

                  It's a good coil....still, it's got me digging hot rocks. I'm going to have to research a mod for my minelab. I remember reading something about making the SD's ignore them...

                  Comment


                  • #24
                    Originally posted by mario View Post
                    I'm still playing with this coil. After testing it further, I'm inclined to think it performs much like a 4 inch coil I had for my tesoro, not an 7.5 inch coil.

                    Am I wrong to say that performance depends primarily on the Inner Diameter of a coil, not it's Outer Diameter? If so, it would explain it's similar performance to a smaller coil.

                    It's a good coil....still, it's got me digging hot rocks. I'm going to have to research a mod for my minelab. I remember reading something about making the SD's ignore them...
                    Mario, My Observations:

                    Greatest Sensitivity on SMALL Objects Occurs at the Inner Diameter Point of the coil.
                    (Like at the Start Turn.)
                    And Greatest Sensitivity of Larger Objects occurs at the Center of the coil.

                    I Find this true for all sizes of these Flat wound Radial Coils.

                    But Bigger coils also become Less Sensitive to SMALL Objects, even at this Start Turn.

                    HOT ROCKS: You might try winding Two Conventional Round or OVAL coils, overlapping each other and using the Cancelling Circuit configuration.
                    It Should help, But I have one area near me where EVERY ROCK is like a Big Piece of IRON. (Both the Small Rocks and HUGE SIZE BOLDERS.)
                    I would really like to know What Exactially is in them to cause this?

                    Does ANY ONE KNOW?

                    Comment


                    • #25
                      I am interested in spiral coil also. We are building pulse induction detector and how do this coil type perform in metal detecting. I assume that you cannot make the spiral as toroide so it will not have magnetism outside coil at all. So you wire the sipral coil as a horse shoe so that it generates magnetic field to outside of the coil trying to go from minus to plus side. This field can be made stronger by adding metal inside the spiral loop to make the magntic field outside spiral stronger I believe. Am I assuming right?

                      What is good calculation equation for inductanse and magnetic field for spiral coil in metric measures?

                      I have found that there is so many equations arround net and all produce a bit varied results in micro henries so I need a trust worthy equations for them.

                      Comment


                      • #26
                        Originally posted by Jeroi View Post
                        I assume that you cannot make the spiral as toroide so it will not have magnetism outside coil at all.
                        Actually, you can have a toroid cross section if you use two coils in opposite polarity placed one next to the other. They'll encompass nice portion of the ground beneath them, and will behave as any other coil for MD purposes, but will cancel out any earth field, and far field RF. Just like humbucker pickups do with electric guitars.

                        Comment


                        • #27
                          Is it possible to use a thin alu-plate as some kind of reflector?

                          If yes, how much distance should it have to the coil?

                          Comment


                          • #28
                            Originally posted by Davor View Post
                            Just like humbucker pickups do with electric guitars.
                            So it should be good for "Heavy Metal" then.

                            Rock on.

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                            • #29
                              Exactly
                              Originally posted by Funfinder View Post
                              Is it possible to use a thin alu-plate as some kind of reflector?

                              If yes, how much distance should it have to the coil?
                              You need far field conditions for reflection, and the size of a reflector must be a wavelength or bigger.

                              Comment


                              • #30
                                Originally posted by Davor View Post
                                Actually, you can have a toroid cross section if you use two coils in opposite polarity placed one next to the other. They'll encompass nice portion of the ground beneath them, and will behave as any other coil for MD purposes, but will cancel out any earth field, and far field RF. Just like humbucker pickups do with electric guitars.
                                What do you mean? Could you produce image about the polaritities and so on. Atm our desing is singel coil type, so can one sipral horseshoe desing work as metal detector as in demos the siparal generates weaker magnetic field outside of coil that can be made bigger placing ferromagnetic material inside the spiral. So with one coil desing, I would wire one horseshoe sipral coil on to construction yard concreat metal pipe and get quite good magnetic field arround the horseshoe?

                                Can somebody veryfy if this is true?

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