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Another dumb question..but you guys are used to them

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  • #16
    Hey, Guys...I'm still hanging around, not saying too much, because I'm too ignorant to jump in with something intelligent.
    Davor said... I'm more interested how you locate the approximate meteorite fall site. By asking the locals?
    No..I know the exact location where a meteorite was recovered. I'm just looking for spare pieces that might have fallen off on the way down.
    But, you guys keep talking, and maybe I'll learn something.
    And Mikebg...What are you using for the coils? And how big are they? Has a setup like this been made for a TF-900? What changes can be expected?

    Comment


    • #17
      Hi Jim.
      I used for coils plastic shells made in Bulgaria. I can't remember the exact diameter of RX coils (23 or 25cm). The shell for TX coil has external diameter 37cm. May be for RX coils is more suitable 15cm.

      Hi WM6,
      I use term TWIN LOOP instead to explain that signals from both RX coils are subtracted. The term is taken from PI because twin RX loops are used by Clone and Chemelec in their PI projects.
      As mentioned in above post, I know that the owners of Lobo made modifications of RX in Russia (or Ukraine). They know how to improve the block diagram of RX because they are radio amateurs. Below is attached the block diagram of a competent designed VLF metal detector. I know their "low noise" circuit of preamp because we have analysed this. When I have time, I will post the information in the forum (There is Lobo thread).

      I think they have not increased TX frequency of Lobo because 18 - 20kHz is suitable for gold nuggets. However increasing is very likely because they know that one gram jevelry meteorite is more expensive than one gram gold nugget.
      Attached Files

      Comment


      • #18
        Mike, do not forget there is a velosiped (bicycle in English) class of two box design. it is also perspective. Anker makes them, but does not sell overseas.
        Attached Files

        Comment


        • #19
          Originally posted by mikebg View Post

          Hi WM6,
          I use term TWIN LOOP instead to explain that signals from both RX coils are subtracted.
          Thanks mikebg. Can you explain graphically how those twin RX loops are connected?
          Attached Files

          Comment


          • #20
            Originally posted by kt315 View Post
            Mike, do not forget there is a velosiped (bicycle in English) class of two box design. it is also perspective. Anker makes them, but does not sell overseas.
            Anatoly, this is an incompetent design because there is no induction balance. Note the parallel excentric coil configuration in the right side of post #3. Both coils are in induction balance. Fisher - Gemini and CEIA have fine adjustment for induction balance. Only a precise induction balance can increase modulation index of target signal. This is valid also for pulse induction (wide band) metal detectors.

            Comment


            • #21
              Referring to post #18 (the "bicycle") -

              I don't actually know how this detector works but I wonder if one ring contains a transmit coil and the other contains a receive coil plus a nulling coil connected in series with the transmit coil. This would work in the same way as a conventional concentric search head, but with the transmit and receive coils separated. You could get a good null this way but I don't know how good its detection performance would be in practice.

              Comment


              • #22
                Gwil, I guess that balance was just made that way, but there is no an option the operator to fine tune balance as this is designed in Fisher - Gemini.

                Dear WM6,
                I understood your question, even without specifying circuit diagrams and answered in post # 17
                "that signals from both RX coils are subtracted". That means they are connected in series with opposite phase dots
                or are connected with equal phase dots to inputs of a voltage subtractor (known as difference or differential amplifier).
                To avoid shielding, we need preamp circuits with input symmetry as shown below. The circuit in left side is known as in-amp (instrumentation amplifier).
                The input symmetry is tradition in
                amateur RX design. To make input symmetry with a single RX coil, the coil should have center tap connected to ground.
                P.S. Sorry, I posted the wide band variant for PI. For narrow band, connect a capacitor between both inputs.
                Attached Files

                Comment


                • #23
                  Originally posted by mikebg View Post
                  Anatoly, this is an incompetent design because there is no induction balance. Note the parallel excentric coil configuration in the right side of post #3. Both coils are in induction balance. Fisher - Gemini and CEIA have fine adjustment for induction balance. Only a precise induction balance can increase modulation index of target signal. This is valid also for pulse induction (wide band) metal detectors.
                  seems you are not in course while I have wrote about Anker-pulse detector. it is only prototype of a military deeper IMB for mine and bombs detecting, Anker took a principe from it. you are NOT needed in the balance in the case
                  Attached Files

                  Comment


                  • #24
                    Judging by the number of cables going from the "wheels" the balance is the simplest there is: Rx and Tx are running parallel in one wheel, and antiparallel in the other. So both Tx-es are running in a same phase, while Rx-es are running in counter-phase, so mikebg-s drawings are correct (with addition that the Tx coils are split in two). As a bonus, this configuration must be resilient against the far field EMI.

                    Comment


                    • #25
                      Davor ... Is that what you say?
                      Jose
                      Attached Files

                      Comment


                      • #26
                        Originally posted by Jose View Post
                        Davor ... Is that what you say?
                        Jose
                        People, this is a stupid coil configuration like the monocoil which incompetent designers use in PI and BFO metal detectors! Below is illustrated what happens at pulse induction when N2 = N1.
                        Only induction balance can increase modulation index of target signal. The balance is used even in metal detectors designed in 19th century.
                        A competent designed coil configuration should suppress the AIR&GND signal in input of preamp. The AIR&GND signal operates as carrier wave for target signal. When it is small, we can increase the gain of opamp. That means more sensitivity to deep and small targets.
                        Attached Files

                        Comment


                        • #27
                          Actually the resulting voltages are +200 and +200, hence with differential input it is zero output. In case there is no ground symbol in between Rx coils, a voltage at Rx input would be zero, ergo balance. The same principle is employed with concentric coils where a bucking coil lends its flux to cancel the main Tx coil flux, ergo balance.

                          I think this configuration has a potential for a stealth bag-carried detector for casual strolls just about anywhere. In case of two quadrant discriminating detectors one coil would be in phase, but the other one in counterphase, so only one would give meaningful response.

                          See corrected image
                          Attached Files

                          Comment


                          • #28
                            Originally posted by Davor View Post
                            Actually the resulting voltages are +200 and +200, hence with differential input it is zero output. In case there is no ground symbol in between Rx coils, a voltage at Rx input would be zero, ergo balance. The same principle is employed with concentric coils where a bucking coil lends its flux to cancel the main Tx coil flux, ergo balance.

                            I think this configuration has a potential for a stealth bag-carried detector for casual strolls just about anywhere. In case of two quadrant discriminating detectors one coil would be in phase, but the other one in counterphase, so only one would give meaningful response.

                            See corrected image
                            I've been thinking about something very similar. I agree it would balance air and ground quite nicely in the right conditions. It would also be very good at EMI rejection, but its not without its drawbacks. To ground balance it would need the ground type to be quite uniform. Any variation in the ground under each coil would create a strong signal. Obviously any tilting would have the same effect. But the main drawback is that your diluting your TX and RX across two coils when only one can be detecting at any one time. So half the power, half the sensitivity (maybe you'll get some of this back with the reduced noise) but potentially it only has 1/4 of the depth of an equivalent single loop design. Still, it may give the figure of 8 coil a run for its money in very noisy areas.

                            Midas

                            Comment


                            • #29
                              Originally posted by Midas View Post
                              I've been thinking about something very similar. I agree it would balance air and ground quite nicely in the right conditions. It would also be very good at EMI rejection, but its not without its drawbacks. To ground balance it would need the ground type to be quite uniform. Any variation in the ground under each coil would create a strong signal. Obviously any tilting would have the same effect. But the main drawback is that your diluting your TX and RX across two coils when only one can be detecting at any one time. So half the power, half the sensitivity (maybe you'll get some of this back with the reduced noise) but potentially it only has 1/4 of the depth of an equivalent single loop design. Still, it may give the figure of 8 coil a run for its money in very noisy areas.

                              Midas
                              About diluting the TX field strength:

                              With the fields of coil 1 and coil 2 aiding, how does this affect the field strength? It should compensate for some of the dilution.

                              With the fields aiding, there is also a large area of enhanced angled field vectors that increase the response of coins in the vertical position.

                              Looking at the ergonomics, it gives space for the feet of the operator and the weight is closer to the body.

                              Tinkerer

                              Comment


                              • #30
                                Originally posted by Midas View Post
                                ... diluting your TX and RX across two coils when only one can be detecting at any one time...
                                That's in a case of using 2 quadrant detection with discrimination. In case you go a step forward and make a full 4 quadrant Rx, you'll have both coils giving sensible output, discrimination, and even pinpointing by means of a sharp minimum right in between the coils.

                                I'd say this construction deserves making a 4 quadrant Rx.

                                BTW, a regular 2 box uses a Tx at nearly orthogonal position, and quite away from the business area of the Rx coil. That would account for much more dilution than this one. Besides, you can also introduce some slow motion compensation, GEB, doodahs and doohickeys of nowadays rigs, and get a very decent deep scanning device - with pinpointing.

                                Maybe I'll have to play a bit with 4 quadrant discriminator. The rest is easy.

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