An improved metal detector

Abstract

A coaxial cable to connect a magnetic field receiver of a hand-held metal detector to receive electronics of the metal detector, the coaxial cable comprising: an inner conductor to connect to an inductive winding of the magnetic field receiver; a conductive electrostatic shield surrounding the inner conductor; a dielectric layer between the inner conductor and the conductive electrostatic shield;wherein the conductive electrostatic shield is configured and arranged such that a late-time time constant of eddy currents induced in the conductive electrostatic shield, due to changing magnetic fields generated from changing transmit signal currents flowing from transmit electronics of the metal detector to a magnetic field transmitter of the metal detector, is less than 1.2us. 20 24 28 71 01 20 D ec 2 02 4 A B S T R A C T 2 0 D e c 2 0 2 4 A c o a x i a l c a b l e t o c o n n e c t a m a g n e t i c f i e l d r e c e i v e r o f a h a n d - h e l d m e t a l d e t e c t o r t o r e c e i v e e l e c t r o n i c s o f t h e m e t a l d e t e c t o r , t h e c o a x i a l c a b l e c o m p r i s i n g : a n i n n e r c o n d u c t o r t o c o n n e c t t o a n i n d u c t i v e w i n d i n g o f t h e m a g n e t i c f i e l d r e c e i v e r ; a c o n d u c t i v e e l e c t r o s t a t i c s h i e l d s u r r o u n d i n g t h e i n n e r c o n d u c t o r ; a d i e l e c t r i c l a y e r b e t w e e n t h e i n n e r c o n d u c t o r a n d t h e c o n d u c t i v e e l e c t r o s t a t i c s h i e l d ; w h e r e i n t h e c o n d u c t i v e e l e c t r o s t a t i c s h i e l d i s c o n f i g u r e d a n d a r r a n g e d s u c h t h a t a l a t e - t i m e t i m e c o n s t a n t o f e d d y c u r r e n t s i n d u c e d i n t h e c o n d u c t i v e e l e c t r o s t a t i c s h i e l d , d u e t o c h a n g i n g m a g n e t i c f i e l d s g e n e r a t e d f r o m c h a n g i n g t r a n s m i t s i g n a l c u r r e n t s f l o w i n g f r o m t r a n s m i t e l e c t r o n i c s o f t h e m e t a l d e t e c t o r t o a m a g n e t i c 2 0 2 4 2 8 7 1 0 1 f i e l d t r a n s m i t t e r o f t h e m e t a l d e t e c t o r , i s l e s s t h a n 1 . 2 µ s . 2 0 D e c 2 0 2 4 2 0 2 4 2 8 7 1 0 1 Figure 1 1 / 5 5 17 18 13 14 15 16 1011 6 3 1 2 8 7 9 12 4 1 / 5 17 4 18 5 6 13 14 3 1 15 2 16 8 7 9 12 11 10 Figure 1 20 24 28 71 01 20 D ec 2 02 4 2 0 2 4 1 / 5 2 0 2 4 2 8 7 1 0 1 2 0 D e c 1 7 4 1 8 5 6 * 1 3 1 4 3 1 1 5 2 1 6 8 7 9 1 2 1 1 1 0 F i g u r e 1 2 0 2 4 2 0 2 4 2 8 7 1 0 1 2 0 D e c 1 8 5 * 1 3 1 4 1 5 8 7 9 1 2

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a metal detector. BACKGROUND

[0002] Most hand-held metal detectors have a flexible cable that connects between an electronics housing and a magnetic antenna used for transmitting a transmit magnetic field into an environment, usually soils, and the metal detector receives a receive magnetic field from the said environment via the magnetic antenna. A receive signal from the antenna responsive to the receive magnetic field is usually amplified by an electronics amplifier within receive electronics within the electronics housing, and a signal output of the electronics amplifier is processed, wherein the processing includes synchronously demodulating the receive signal relative to the transmitted signal. Outputs of the synchronous demodulators are further processed, including at least low-pass filtering to remove transmit signal related frequency components, and to reduce noise. Usually high-pass filtering is applied to reduce D.C. drift and very low frequency 1 / f noise, and also varying D.C. offsets owing to varying soil conditions. Following the filtering, is usually further signal processing that feeds an indicator output, typically in the form of modulated audio, that an operator listens to in order to detect and locate metal targets.

[0003] This disclosure presents a new type of a flexible cable that connects between an electronics housing and a magnetic antenna. SUMMARY

[0004] According to a first aspect of the present disclosure, there is provided a coaxial cable to connect a magnetic field receiver of a hand-held metal detector to receive electronics of the metal detector, the coaxial cable comprising: an inner conductor to connect to an inductive winding of the magnetic field receiver; a conductive electrostatic shield surrounding the inner conductor; a dielectric layer between the inner conductor and the conductive electrostatic shield; wherein the conductive electrostatic shield is configured and arranged such that a late-time time constant of eddy currents induced in the conductive electrostatic shield, due to changing magnetic fields generated from changing transmit signal currents flowing from transmit electronics of the metal detector to a magnetic field transmitter of the metal detector, is less than 1.2gs.

[0005] In one form, the magnetic field receiver is also the magnetic field transmitter; and wherein the changing transmit signal currents flow within the coaxial cable. In one form, the changing transmit signal 2024287101   20 Dec 2024 currents flow within at least one transmit cable in close proximity of the coaxial cable. In one form, the conducting electrostatic shield comprises insulated non-ferromagnetic metal strands of resistivity at least 2.5 times greater than a resistivity of copper. In one form, the conducting electrostatic shield comprises non-magnetic stainless-steel strands. In one form, the conducting electrostatic shield is formed by spirally wound strands; and wherein a ratio of a pitch of the conducting electrostatic shield to a mean radius of the conducting electrostatic shield is less than 2.7. In one form, the insulated metal strands are connected together at one end of the cable but each of the strands remains open and not connected at another end of the cable. In one form, the insulated metal strands comprise at least two different types of strands, each with a different resistivity. In one form, the insulated non-ferromagnetic metal strands are formed by combining two or more metals through electroplating.

[0006] According to a second aspect of the present disclosure, there is provided a coil cable of a handheld metal detector, comprising: a coaxial cable according to the first aspect; an outer sheath surrounding the coaxial cable.

[0007] In one from, the magnetic field receiver is also a magnetic field transmitter; and wherein the changing transmit signal currents flow within the coaxial cable; and wherein the coil cable further comprises an external low resistance cable along the coaxial cable; wherein at least part of the changing transmit signal currents flow within the external low resistance cable along the coaxial cable. In one form, the coil cable further comprises at least a transmit cable to conduct the transmit signal currents. In one form, the conducting electrostatic shield comprises insulated non-ferromagnetic metal strands of resistivity at least 2.5 times greater than a resistivity of copper. In one form, the conducting electrostatic shield comprises non-magnetic stainless-steel strands. In one form, the conducting electrostatic shield is formed by spirally wound strands; and wherein a ratio of a pitch of the conducting electrostatic shield to a mean radius of the conducting electrostatic shield is less than 2.7. In one form, the insulated metal strands are connected together at one end of the cable but each of the strands remains open and not connected at another end of the cable; and an external cable along the coaxial cable is connected to the receive inductor of the magnetic field receiver. In one form, the insulated metal strands comprise at least two different types of strands, each with a different resistivity. In one form, the insulated non-ferromagnetic metal strands are formed by combining two or more metals through electroplating. BRIEF DESCRIPTION OF DRAWINGS

[0008] Embodiments of the present disclosure will be discussed with reference to the accompanying drawings wherein:

[0009] Figure 1 depicts a typical coaxial cable cross section; 2024287101   20 Dec 2024

[0010] Figure 2 depicts an example basic block diagram of a time-domain metal detector;

[0011] Figure 3 depicts example signal waveforms of a pulse-induction-like time-domain metal detector;

[0012] Figure 4 depicts a coaxial cable according to one embodiment of the present disclosure; and

[0013] Figure 5 depicts connections of a coaxial cable according to one embodiment of the present disclosure. DESCRIPTION OF EMBODIMENTS

[0014] The metal detector industry usually refers to the antenna as a “coil”, and the flexible cable connecting the coil to the electronics within the electronics housing as a “coil cable”.

[0015] The magnetic receiver usually consists of at least one receive inductive winding. The receive inductive winding is fed to the receive electronics via a cable within the coil cable, usually in the form of an electrostatically shielded coaxial cable, with the outer screen of the coaxial cable in effect connected to the receive electronics reference “ground”.

[0016] While the description uses PI or PI-like metal detectors for discussion purposes, it is envisaged that the present disclosure may be applied to other types of metal detectors such as Continuous Wave (CW) or frequency-domain metal detectors. With reference to Figures 1 and 2, Figure 1 shows an example radial cross-section of a type of typical state-of-the-art metal detector coil cable used for PI-like metal detectors, that for example, may have periods of constant current transmission during which receive processing occurs. This coil cable is covered by a plastic (or rubber-like) sleeve 2 with an outer circumference 1, and enclosed 3 within the plastic sleeve 2 are various conductors connecting elements within the coil 52 to the electronics within the electronics housing 84. One of these conductors is a coaxial cable 13 connecting a magnetic receive sensor (invariably a receive inductive winding, or windings 53) to an input amplifier 64 of receive electronics. This coaxial cable 13 includes an inner conductive core (also known as inner conductor) 17 consisting of many strands of insulated copper wire 18. This inner conductive core 17 (which is 58 in Figure 2) is surrounded by a low dielectric permittivity plastic insulator 16. Layered upon the outside of the low dielectric permittivity plastic insulator 16, is the coaxial cable shield (also known as conductive shield) 59, consisting of many tightly packed individual insulated conductor strands (also known as shield strands) 15 that all are touching their respective adjacent shield conductors as a monolayer. These completely cover the low dielectric permittivity plastic insulator 16 such that no “electrostatic gaps” appear. The coaxial cable shield 59 acts as an electrostatic shield to the inner conductive core 17. The coaxial shield strands 15 are spirally wound around the low 2024287101   20 Dec 2024 dielectric permittivity plastic insulator 16. Surrounding the coaxial cable strands is a plastic sheath 14. Also enclosed within the plastic sleeve 2 could be a conductor 10 with inner insulated copper strands 12 surrounded by an insulator 11 to supply some element within the coil housing (such as, for example, the transmit inductive winding 54, electrostatic screen 56), or an alternate ground path 85 for receive inductive winding 53. Yet further additional internal conductors could be added to service other elements within the coil housing. For example, to connect to various electrostatic screens, or an alternative connection to various electrostatic screens. Two transmit cables 4 and 7 feeding the magnetic transmitter within the coil housing consist of insulated copper multi-strands 6 and 9 respectively, which are surrounded by plastic insulators 5 and 8 respectively. These two transmit cables 4 and 7 are of low resistance.

[0017] Figure 2 shows an example basic block diagram of a time-domain metal detector. A coil housing 52 contains a transmit inductive winding 54 and a receive inductive winding 53. Both of these may be in the form of multiple windings (usually connected in series), but for simplicity showing as only one each. They are geometrically arranged to solicit an approximate null when the transmitter is generating a changing magnetic field, and coil is exposed to just air (no targets or soil nearby). The coil is connected to the electronics housing 84 by a flexible coil cable 51. An electrostatic screen 56 consisting of a conductive material (for example Aquadag etc.), electrostatically screens the transmit inductive winding 54 via conductor 62 that is connected to an electronics ground 57 (or at least a receive electronics reference ground). Conductors 61 and 60 connect the metal detector transmit electronics 63 to the transmit inductive winding 54. A receive coaxial cable consisting of an inner core 58 that is surrounded by coaxial cable shield 59, that is usually in the form of strands of copper wire covering an insulator surrounding the coaxial cable inner core 58, connects the receive inductive winding 53 to the receive electronics within the electronics housing 84, at an input of an electronics receive preamplifier 64. The receive inductive winding 53 is screened by an electrostatic screen 55 that is connected to one end of the receive inductive winding 53, which is connected to the electronics ground 57 via the coaxial cable shield 59. The screens 55 and 56 may be connected together or be a single screen. The coil cable may include a receive parallel ground conductor as ground path 85, or a ground connection between the receive inductive winding 53 and the electronics ground 57 as an alternate connection to the coaxial cable shield 59. An outer sheath 50 of the coil cable encloses all the coil cable conductors 58, 59, 60, 61, 62, 85. An output 68 of the preamplifier 64 is connected to inputs of synchronous demodulators 66 and 67. Usually there are several synchronous demodulators, but only two are shown for simplicity. The forward gains of these synchronous demodulators 66 and 67 are controlled by signals at connections 69 and 70 respectively that are supplied by a master clock and timing electronics 65. This master clock and timing electronics 65 also supplies control signals via 71 to the transmit electronics 63. Outputs 72 and 73 of the synchronous demodulators 66 and 67 respectively, are fed to inputs of filters 74 and 75 respectively. These filters 74 and 75 at least contain low-pass filtering to remove transmit signal related frequencies, 2024287101   20 Dec 2024 and also to limit the noise bandwidth. They also usually include high-pass filtering to remove 1 / f noise and DC offsets and DC drift. Outputs 76 and 77 of the filters 74 and 75 respectively feed further processing electronics 78, that may also be connected 79 to the master clock and timing electronics 65. This further processing electronics 78 may include signal processing such as “ground balancing” for example, which is a process known to those skilled in the art. An output of the further processing electronics 78 feeds an indicator output, shown here as headphones 80, via connections 81, 82 and 83. The electronics is controlled by the user via a user interface, but this is not shown for reasons of simplicity.

[0018] Figure 3 shows example signal waveforms of a pulse-induction-like time-domain metal detector, that transmits periods of approximately constant magnetic field (proportional to transmitted current) that are separated by periods of rapid change in transmitted magnetic field, or in other words, periods of rapid change in transmitted current in transmit inductive winding 54. Pertinent to understanding this disclosure is a delay between the cessation 91 and 93 of rapid change of transmit current periods, shown as 97 and 99 in Figure 3, and commencement 94 and 95 of the receive synchronous demodulation process 108 and 112 during periods of approximately constant transmitted magnetic fields 96, 98 and 100, shown as delay periods 121 and 120. The current flowing in the transmit inductive winding 54 is shown in the example waveforms of Figure 3 as positive current periods of approximately constant current 98 and negative current periods of approximately constant current 96 and 100. Transitions between these alternating sign constant current periods 96, 98 and 100, are the rapid changes in currents 97 and 99, that are approximate linear current ramps. The inductive reactive transmit voltage associated with the periods of rapid changes in currents 97 and 99 are high voltage periods 103 and 105 respectively, with 103 being a positive high voltage period and 105 being a negative high voltage period (because of the well-known equation v = L^). Zero reactive EMF is shown as 102, 104 and 106, and the positive reactive high voltage period 103 commences at time 90, whilst the negative reactive high voltage period 105 commences at time 92. The synchronous demodulation waveforms 108, 109, 111, 112, show an example of the forward gains of the synchronous demodulation (controlled via 69 and 70 in Figure 2), relative to zero gain 107, 101. For the purposes of understanding, example scales may be: the net transmit inductive winding 54 may be 300gH, and that of the receive inductive winding 53 similar, and the constant current periods 96, 98, 100 may have an absolute magnitude of say 1.5A, and the absolute reactive high voltages 103, 105 during the periods of rapid change in transmit inductive winding current 97, 99 about 180V. Thus, the period of reactive high voltages 103, 105 and thus periods of rapid change in transmitted current 97, 99 are 5gs. Round coil housing diameters typically used in high-end detectors purpose-built for gold nugget prospecting vary from about 25 to 50cm in diameter, and coil cable lengths are about 1.4m. The preamplifier 64 may have a bandwidth of the order of 1MHz, with a gain of the order of 100, and an equivalent input noise of the order of 1nV / ^Hz . The low-pass filter cutoff of the filters 74 and 75 may be several Hz. The preamplifier demodulated random noise (from the 2024287101   20 Dec 2024 equivalent input noise, Johnson noise and equivalent input current noise) is usually clearly heard by an operator. Thus, the state-of-the-art metal detectors are indeed very sensitive to minute imperfections such as spurious cable movement induced signals, which is a problem addressed herein.

[0019] Induced EMFs in the receive inductive winding directly proportional to the rate of change of the transmitted magnetic field are called the (received) reactive component, usually called the “X” component, and in terms of targets, this arises due to the energy lossless components of ferro- or ferrimagnetism, and the “inductive” component of eddy currents, whereas the “R” component is manifest due to energy loss associated received signals, such as the energy associated loss component of soil viscous remanent magnetism (VRM) and eddy currents.

[0020] In historically earlier generation metal detectors, the receive coaxial cable was typical of the form of “RF coaxial cables” or “audio coaxial cables”, with outer shields consisting of multiple bare or tinned copper strands. Typical RF coaxial cables have tinned copper strands involving left-hand and righthand interwoven plaited spiral braids to ensure good “continuous” shield electrical contacts and shielding coverage along the length of the coaxial cable. This ensures both high electrostatic field shielding and high common-mode rate of change of magnetic field rejection. However, when these RF or audio coaxial cables are moved relative to their metal detector coils, e.g. due to inertia of the flexible cable when a coil is bumped against the soil’s surface, or bumped against foliage, a spurious signal is produced due to eddy currents flowing between the continuum of short-circuited uninsulated strands, that are induced by a varying transmit magnetic field in the vicinity of the coil, wherein the said coaxial cable shield eddy currents move relative to the metal detector coil (because of the coil cable movement), and thus this coil cable movement manifests like a conductive “metal target” moving relative to the coil. As the sensitivity of metal detectors improved with time, this spurious signal coil cable movement source became a significant problem. Hence, these uninsulated copper strands of the shields of these receive coaxial cables were replaced with insulated copper wire strands to remove the continuum of short circuits along the whole length of the coaxial cable shield. This solution vastly reduced this coil cable movement spurious signal source, and in many cases, effectively eliminated this problem. These extant metal detector receive coaxial cable insulated copper wire screens are typically wound as a spiralled monolayer around an insulator surrounding the coaxial cable’s central conductor. These Litz wire-like spiral woven coaxial cables have relatively good common-mode rejection of external changing magnetic fields.

[0021] Extant flexible coil cables of state-of-the-art high-end hand-held metal detectors that are purpose-designed to search for gold and / or land-mines, mostly consist of three types:

[0022] (a) The first type is a single coaxial cable used for Pulse Induction (PI) monoloop windings, that acts both to transmit a transmit current during transmit periods and receive a low-level receive signal during receive periods. Such a PI metal detector cannot support a high-sensitivity ferrous / non-ferrous 2024287101   20 Dec 2024 target discrimination capability. That is, this type of said PI detector cannot have a requirement for high sensitivity to X. The resistance of the coil coaxial cable should be low in order to reduce transmit power consumption (ideally <<1 Ohm).

[0023] (b) The second type are dual coaxial cables used for PI with coils containing a separate transmit inductive winding, that may also act as a receive winding, and a purely receive inductive winding, with each winding connected to its own coaxial cable, and, with each coaxial cable being part of, or, within a common sheath, of the “coil cable”. The coaxial cable that is connected to the transmit inductive winding (that may also act as a receive winding) has low resistance to reduce transmit power consumption, whereas this requirement is not necessary for the purely receive inductive winding coaxial cable (where an order of an Ohm will suffice). Such a metal detector can support high-sensitivity ferrous / non-ferrous target discrimination capability when one inductive winding is used as transmitter and the other as a receiver, wherein such a metal detector requires high sensitivity to X. However, the metal detector need not include this metal type of discrimination feature, and indeed many high-end detectors aimed at gold prospecting do not. In summary, regarding cable resistances and ferrous / non-ferrous discrimination: conductors within the coil cable supplying the transmitter should be of low resistance in order to reduce transmitter power consumption, and monoloop PI windings cannot be used for ferrous / non-ferrous discrimination. Coils with separate transmit and receive inductive windings, can support ferrous / non-ferrous discrimination and the receive coaxial cable within the coil cable need not be of very low resistance.

[0024] (c) The third type is the coil cable as described with reference to Figure 1.

[0025] However, it was discovered that even this coaxial cable Litz-wire-like solution has become insufficient, due to the high-end detectors becoming more and more sensitive, and, that these coil cables cause spurious movement signals via the mechanisms that follow below.

[0026] With reference to Figures 1 and 2, in the extant newer generation of coil cables, eddy currents in fact are generated within the insulated stand Litz wire-like receive coaxial cable shields, and are induced from a rate-of-change of a magnetic field within the coil cable generated by transmitted (changing) current flowing in transmit conductors 4, 7, 60, 61 within the coil cable sheath 2 that feed the transmit inductive winding 54. These coaxial cable shield inter-strand eddy current changing magnetic fields in turn induce a differential EMF between the coaxial cable inner conductive core 17, 58 and shield 59 that is manifest at the receive electronics input (of amplifier 64). In this case these coaxial shield eddy current paths consist of circuits between the insulated shield strands due to the short circuits at either end of the receive coaxial shield, that is, not via shorting along the whole cable length as is the case for uninsulated strands. In other words, the eddy current paths between the insulated coaxial shield strands circulate via the cable’s shield’s short-circuited ends in a “there-and-back” fashion. Each strand may be 2024287101   20 Dec 2024 considered as a low impedance transmission line relative to the others, or groups of strands near each other may be considered as transmission lines. These transmission lines are of low characteristic impedance and exhibit frequency dependent loss. That is, the eddy currents may be characterised as having distributed “time-constants”, with the late asymptotic (late-time) time-constant range being the most relevant to the coil cable movement spurious signal, due to coil cable movement affecting these inter-strand shield eddy currents, and changing coupling between the resulting eddy current magnetic field and coaxial cable’s inner conductive core 17, 58.

[0027] In practice, metal detectors that ideally are supposed to transmit constant magnetic fields during the receive demodulation periods (such as 96, 98, 100), do not in fact transmit ideal constant currents because of non-ideal electronics behaviour, and because of soil and target “R” components that alter the “ideal constant transmitted field”. Thus, herein the term constant magnetic field or constant transmit(ted) current should be understood to have such limitations.

[0028] Herein and within the metal detector industry, the term “Litz wire” has two meanings. In the metal detector industry, it mostly means bundled insulated (copper) multi-strands, with the said multistrands being connected together at the ends of the cable. But it can also mean a cable with insulated strands connected to each other at each end, but also woven together such that the distributed radial location of each strand within the Litz wire is democratic when averaged along the length of the cable: That is, each strand’s radial density distribution within the Litz wire is approximately the same at least for a long length of said Litz wire. Hence for example, the transmit cables 6 and 9 could be merely insulated strands bundled together, but not deliberately woven together, or deliberately woven in a manner to ensure that each strand’s radial density distribution within each of the transmitted cables is approximately the same.

[0029] Below is a summary of the process causing the coil cable movement spurious signal problem in the latest generation of insulated strand coaxial cable shield at the time of writing, wherein the coil cable includes separate conductors 4, 7, 60, 61 to feed the transmit inductive winding 54 within the coil. Note, for the purpose of understanding, it is important to think of the below description mathematics applying literally along the whole length of the cable.

[0030] (i) Varying currents subject to the transmit signal flowing in conductors 4, 7, 60, 61 enclosed within the coil cable sheath 2, 50 that supply the coil’s transmit inductive winding 54 generate a varying magnetic field within the coil cable (and externally to the cable too), according to Ampere’s Law SIX yH. dl = ^(J + —). dS. Mostly, these transmitted field directional components are approximately in a plane radial to an axially straight section of coil cable. 2024287101   20 Dec 2024

[0031] (ii) This varying magnetic field within the coil cable couples into coaxial cable shield strands 15, thus inducing EMF differentials between the coaxial shield strands depending on Faraday’s Law 0.dl = -#f.dS .

[0032] (iii) Typically the shield strands 15 are spirally wound about an insulator 16 surrounding the inner conductive core 17 of the coaxial cable, such that the spiral pitch is defined by P= 2^rnd ^(2rnr)2-(nd)2 where n is the number of shield strands 15 of the coaxial cable shield, that are woven touching each other in a mono-layer, and d the diameter of each insulated strand, and r is the radius of the insulator 16 plus d / 2, hence ensuring no “electrostatic gaps” in the coaxial shield. If the coil cable length is L, then the coaxial shield length (= each strand length) is 2^p and the number of spiralled turns is ^ = L^(2rer)2-(nd)2 2nrnd . Thus, during metal detector production when preparing the coaxial cable shield to be soldered (or a lug connected to it), it is not possible to tell which strands were the outmost strands of the original weave when the cable was manufactured. For the benefit of insight into typical scales, r may be about 1mm or so, and the spiralled shield pitch a few centimetres.

[0033] (iv) The difference in EMF potentials induced in the coaxial cable (Litz wire-like) shield strands causes eddy currents to flow in each insulated strand.

[0034] (v) These eddy currents then generate a changing coaxial cable shield eddy current magnetic field. Typically, the coaxial cable shield eddy current magnetic field directional components are also mostly in a plane radial to an axial straight section of coil cable.

[0035] (vi) The changing coaxial cable shield eddy current magnetic field induces an EMF at the coaxial cable output relative to its input, due to the coaxial cable shield eddy current magnetic field coupling to the enclosed magnetic circuit of central core and shield, according to Faraday’s Law 0.dl = -#ff.dS.

[0036] (vii) Moving the flexible coil cable, such as for example bending and / or squashing, then causes the local coaxial cable spatial eddy current distributions relative to the cable to move as the individual coaxial cable shield strands 15 move relative to the transmit conductors 4 and 7, and, the transmit cables 4, and 7 to move relative to each other (locally), and yet further, the coaxial cable shield eddy current magnetic field to move relative to the coaxial cable inner conductive core 17. This causes a coil cable movement induced changing EMF signal waveform at the preamplifier input (relative to a commonmode). 2024287101   20 Dec 2024

[0037] (viii) This coil cable movement induced changing EMF signal waveform is demodulated by the receive electronics 66, 67 and then manifest as a spurious indicator output signal (such as at audio indicator 80), that interferes with the capability of detecting the weakest perceptible metal target indicator output signals.

[0038] (ix) Note that the transmit signal voltages 103 and 105 relative to the coaxial cable’s shield ground potential induces an EMF within the coaxial cable shield strands 15 via capacitive coupling between the coaxial cable and transmit conductors 4 and 7. Whilst these transient capacitively coupled EMFs do cause yet further eddy-like currents to flow, the magnitudes are relatively low, and the relative movement of this capacitive source plays effectively no role in the spurious movement signals.

[0039] (x) Note that the longest of the coaxial cable shield eddy current distributed late-time time constant is typically relatively short (e.g. 2 to 4 microseconds), because of the effective inductances and mutual inductances associated with the insulated strands of the shield, and resistances of each strand. Hence, the spurious movement signal is only generated by receive signal processing shortly after changes in the transmit field 97, 99 in time-domain metal detectors, with an associated “back-EMF” (high rate of change in transmit magnetic field) in pulse-induction or pulse-induction-like metal detectors 103, 105. Thus to clarify, if for example, the receive demodulation commenced 120, 121 at say 10 microseconds following a transmit rapid rate-of-change of magnetic field 97, 99, during a period of constant or zero transmitted magnetic field 96, 98, 100, typically the coaxial shield eddy currents would have decayed substantially so that subdued spurious signals would occur due to coil cable movement, but, this would not be the case if the receive synchronous demodulation instead commenced with say a 4 microsecond delay period 120, 121, when a coil cable movement spurious signal may be obvious to the metal detector operator.

[0040] (xi) This spurious movement induced spurious signal may occur anywhere along the length of the coil cable.

[0041] (xii) Besides electrostatically screening the highly sensitive receive signal supplied to the receive electronics input via the coaxial cable inner conductive core 17 and 58, as said above, the coaxial cable also has an advantage of producing a high common-mode rejection ratio to external magnetic fields. This high common-mode rejection ratio includes parts of the coaxial cable within the influence of the stronger transmitted magnetic field in practice. Hence effectively a low X-component (via the transmitted magnetic field) is induced into a coaxial cable, but this is not the case for receive cables with radial magnetic dipole sensitivity: In particular, cables with a dominant grounding path offset radially from the inner conductive core 17, 58, that may detect significant levels of X from rates of change in the coil’s or cable’s transmitted magnetic field. 2024287101   20 Dec 2024

[0042] Note that herein the “delay period” of commencement of the receive demodulation relative to a termination of a rate of change of transmit magnetic field for time-domain metal detectors, includes the group delay of a receive amplifier plus that of the receive sensor (invariably an inductive receive winding or windings), not merely the delay periods shown as 120 and 121 in Figure 3. That is, in effect, these periods 120, 121 are less than those superficially indicated by 120 and 121, by for example, round about half to one microsecond, depending on the receive amplifier bandwidth and natural resonant frequency of the receive inductive winding connected to the coil cable and electronics (with damping removed).

[0043] Further, when a coaxial cable is used to supply both a transmitting and receiving inductive winding as in types (a) and (b) presented earlier, the above principles are still applicable due to two reasons:

[0044] (i) Any coil cable movement causing a change in the coaxial cable shield strand-to-strand, or group of strands to group of strands mutual inductance coupling coefficients alters the changing transmit current induced eddy current pattern within the shield insulated strands, and the resulting changing eddy current magnetic field induces a corresponding change in the waveform of the induced EMF at the output of the inner cable of the coaxial cable relative to the shield, thus producing a coil cable movement induced spurious signal.

[0045] (ii) If the mutual inductance coupling coefficient between the shield’s inter-strand eddy current paths and the coaxial cable’s central core changes due to cable movement, that then too causes a change in the received induced EMF waveform in response to the cable movement and hence induces a spurious signal when the coil cable is moved.

[0046] The problem to be solved in this disclosure is how to eliminate these coil cable movement spurious signals effectively, other than having two substantially separated coil cables, one for receive, the other for transmit, or having a significant gap between them, and this problem further includes the dual purpose coaxial cable for a PI monoloop inductive winding.

[0047] The following requirements overcome the coil cable spurious movement signal problems:

[0048] (i) The late-time (asymptotic) time-constant of the coaxial cable electrostatic shield eddy currents must be short (less or equal to about a microsecond, e.g. <1.2 microseconds works well in practice) so that the decaying eddy currents during periods of zero or constant transmitted magnetic field 96, 98, 100, immediately following a rapid change in transmit magnetic field 97, 99, decay sufficiently significantly before the commencement of receive synchronous demodulation 120, 121 such that the coil cable movement that changes an induced EMF receive signal waveform in the coaxial cables inner conductive core 17, 58 output relative to common mode, is thus relatively insignificant. This means that 2024287101   20 Dec 2024 the “bulk” coaxial cable shield radial cross-section supporting the offending eddy currents must be associated with effective high “series” resistance compared to the industrial standard insulated copper coaxial spiral weave, all else being equal.

[0049] (ii) The resistance of the “ground” path (provided by 59, 85) connecting the coaxial cable between the receive sensor (receive inductive winding 53) and the electronics receive amplifier 64 “ground” 57 must be low; e.g. less than a few Ohms for a receive only inductive winding, and <<1 Ohm ideally for a PI common transmit / receive coaxial coil cable (required for a monoloop winding).

[0050] There are several embodiments that could be considered to achieve a desirable outcome:

[0051] Embodiment A: Consider a typical state-of-the-art industrial standard coil cable used for coils with separate transmit and receive inductive windings, for example Figure 1, wherein the receive coaxial cable has say r = 1.05mm with a shield spiral pitch of say P=23mm. This requires that nd ~ 6.34 mm, and thus the pitch to shield mean radius ratio is ^- 21.9. Suppose that nd were reduced by say a factor of 2.5 to 2.537mm, with thus P ~ 2.75mm so the pitch to mean shield radius ratio is ^- 2.75 (viz decreases about 8.4 times), in order to decrease the shield late-times distributed time-constant. In this instance, the shield resistance will increase by a factor of 2.52 = 6.25 times, but the shield eddy current late-time effective inductance will increase too, by roughly 2.5 times, and hence the said shield eddy current late-time time-constant decreases by about 2.5 times. However, the typical change in mutual coupling coefficient between the transmit conductors (6 and 9) within the coil cable (1) and inter-strand eddy current shield paths due to coil cable movement (bending, squashing etc) tends to be reduced intrinsically too, because of the relatively smaller shield spiral pitch compared to the coil cable dimensional changes (bending radii etc) causing the spurious movement signal. However, the higher effective series inductance of the coaxial cable could cause undesirable RF effects (e.g. when the receive inductive winding’s electrostatic screen 55 capacitively couples to a wet soil surface), and production may have yield issues due to just one or two of the few shield strands not being soldered at each end resulting in a relatively high percentage change in electrostatic screening (although capacitive coupling between the unsoldered and adjacent strands may render this electrostatic shielding loss insignificant). To highlight this substantial increase in receive ground inductance, it should be noted that altering nd from 6.342 to 2.537mm (factor of 2.5) increases the coaxial cable shield length (length of each strand) from about 1.456m to 3.641m for a 1.4m length coil cable, and the number of spiralled turns increases from 61 to 509 turns (8.4 times more), and inductance increases with reduced effective conductor width (approximately logarithmically all else being equal). Further, this coaxial cable design is quite different to coaxial cables found in extant metal detector coil cables whose (Litz-wire) shields consist of insulated strands with pitches more conducive to low series resistance and series inductance and production-line efficiency. That is, such a proposed design change moves away from the industrial standard high-end 2024287101   20 Dec 2024 metal detector coaxial cable designs with relatively long spiral pitches compared to coaxial shield diameter. In other words, the extant designs attempt to fill the shield to near full strand capacity, but maintain a reasonably practical (long) spiral pitch. In summary, this embodiment where the pitch to radius ratio is say less than say 2.75, may satisfy the spurious movement problem, but may generate other unrelated problems.

[0052] Embodiment B: Connect the coaxial shield’s copper strands all together only at one end of the coil cable, such as the electronics connector end, but leave all of the shield’s strands open-circuit at the coil housing end, and, also include within the coil cable a separate low resistance receive “ground” conductor as ground path 85, to connect the receive inductive winding 53 to the electronics ground 57, e.g. a Litz wire conductor. As no inter-strand coaxial shield eddy currents are able to be supported with no insulated shield strand connections at one end, this method eliminates the eddy current shield movement problem, but yet the shield continues to act as an effective electrostatic screen for the very low level receive signals. This solution may be associated with a production issue; namely, no shield strands must short to another strand at the shield strand-open coil cable end, and very especially, no strands should make intermittent shorting and open circuits with each other, because this will cause changing eddy currents to flow in the shield. This could be overcome by oxidising the insulated copper strands at the open ends, or some other insulating process. But this could be challenging in production. In the case of having separate transmit and receive inductive windings requiring conductors 4, 7 feeding the transmit inductive winding as per Figure 1, where cable 10 acts as the said separate receive ground cable (for ground path 85 in Figure 2), and the coil end of the coaxial shield strands 15 are all open, spurious X cable movement signals are inevitable from coil cable movement, especially when moved within the stronger transmitted magnetic field. Thus, this solution of a non-coaxial cable shield ground path may be problematic for separate transmit and receive winding cables if the metal detector is required to be sensitive to changes in X, such as for a ferrous / non-ferrous discrimination capability. In summary, even though this method is potentially satisfactory in terms of spurious coil movement signals, this solution may present production challenges, and cause problems for metal target ferrous / non-ferrous discrimination if applicable.

[0053] Embodiment C: Swap the coaxial cable shield strands for a conductive plastic sheath. Further, a separate copper conductor, without insulation, could be embedded within the conductive plastic along its length to maintain low local shield resistance relative to the receive electronics “ground.” This solution assumes the resistivity of the conducting plastic is much higher than copper, and hence the plastic conductive shield induced eddy currents decay very rapidly (compared to copper). However, this solution also has the same X problem in Embodiment B above, even if an alternate low resistance path (<<1 Ohm) is not implemented via a separate “ground” return conductor as ground path 85, again because of (radial) magnetic dipole moment between the coaxial cable inner core and relatively low resistance cable 2024287101   20 Dec 2024 embedded in the plastic. Note that as said, the embedded conductor must not be insulated, and thus it cannot be of (very) low resistance in order to avoid eddy currents between the continuum connections of the uninsulated strands. Further, note that if the embedded plastic ground conductor is connected in parallel with a separate low resistance ground conductor as ground path 85 to lower the grounding resistance path, then an eddy current ground-loop path is set up between the low resistance conductor and the embedded conductor. However, if the conductor embedded in the conductive plastic is of relatively “high” resistance (of the order of an Ohm which it should be at least), then this eddy current loop may have a sufficiently fast time-constant not to cause a spurious eddy current cable movement problem. However, there is no actual need to connect them in parallel anyway; one end of the embedded conductor may be left open circuit. Thus, this too could be a satisfactory solution (for metal detectors that are not required to be sensitive to X). Obviously, such a coil cable would be highly unorthodox, and probably relatively expensive.

[0054] Embodiment D: Connect just a few of the coaxial shield’s copper strands at one end to each other and leave all the others open-circuit, but connect all of the shield’s copper strands together at the other end (e.g. the electronics end). As only a few strands are connected, the late-time time constant will be rapid due to relatively higher eddy current path resistance, and hence this overcomes the coil cable movement problem. However, this arrangement may be mechanically fragile, and then there is the production problems mentioned above. Again, in this case a separate low resistance “ground” conductor as in Embodiment B and C above could be included; e.g. a Litz wire conductor (<<1 Ohm for PI monoloop) in parallel with the shield (when X plays no or little role in the signal processing). However, rather than include the parallel low resistance ground path 85, Embodiment B is probably better than this Embodiment D. Use a dual coaxial cable as already mentioned above, one for the transmit path, that should be of low resistance (ideally << 1 Ohm), and the other for receive, and both with Litz wire-like insulated copper spiralled monolayer shields as described above. However, in order to maintain a low resistance and capacitance of the transmit coaxial cable, both to maintain low transmit power consumption; this coaxial cable needs to be relatively large. The said extant versions of this dual coaxial cable, perhaps have too high transmit cable resistances (and capacitances), and too high receive capacitance for the best results with the latest generation of state-of-the-art metal detectors. When bulkier versions of these cables are housed within the coil cable, together with other cables required for shielding the transmit winding for example, the coil cable becomes thick, heavy and inflexible, unlike that of the example cable in Figure 1. Note that eddy currents are generated in the transmit coaxial cable shield, but when these couple into the receive signal via the receive coaxial cable, they are too small to cause a coil cable movement signal. Also, spurious X movement signals are low, again due to reasons given above. Thus, even though this Embodiment D may subdue the spurious signal coil movement problems, the cable may have other undesirable features. 2024287101   20 Dec 2024

[0055] Embodiment E: Swap the coaxial shield strands to a higher resistivity conductor compared to the universally industrial-standard copper, by say at least 2.5 times higher; such as for example, insulated copper coated non-magnetic stainless steel (viz, non-ferromagnetic), or insulated non-magnetic stainless-steel strands with suitable strand connections at each end of the coil cable. Such an unorthodox coaxial cable is obviously not an “off-the-shelf” available commercial item and would have to be especially commissioned to be manufactured by a metal detector producer. The strands need to be non-magnetic because a (mechanically stronger) magnetic stainless steel version would have longer (distributed) timeconstants resulting from an associated higher inductive component, than the non-magnetic version. This is a satisfactory solution for separate transmitter and receiver coils, and indeed works well in practice, and overcomes the coil cable movement spurious problem, and displays no “RF antenna” issues in practice due to relatively low series coaxial cable shield inductance (for spiral pitches about 20mm as described above). Further, this solution produces low spurious X cable movement problems, if relevant, due to the nature of the coaxial cable as stated above. However, if such a shield were to be employed for a PI monoloop coil cable, then a low resistance conductor external to the coaxial cable’s shield (within the coil cable sheath) connected in parallel with the coaxial cable’s shield is required to provide the dominant low resistance “grounding path” as in Embodiment B, C, and D above. A “ground-loop” eddy current path between the low resistance added receive cable ground path and coaxial cable ground path will have a short late-time time-constant due to the relatively high resistance of the said relatively high resistance non-magnetic metal employed in the coaxial cable shield. Thus, this solution is suitable for both PI monoloop with the added low resistance receive grounding path, and metal detectors with separate transmit and receive inductive coil windings, with just the coaxial cable shield providing the receive grounding path. Bronze or brass may be suitable alternatives to stainless steel (depending on the particular resistivity).

[0056] Embodiment F: The inner core of the receive coaxial cable may be replaced with twin differential inner cores, commonly known as a “twinax” (coaxial) cable. With this arrangement, both ends of the receive inductive winding 53 are connected only to the twin central core, and not directly to the twinax shield within the coil housing. The twinax shield is then only connected to the receive electrostatic screen 55 and the electronics ground 57. Assuming that the centres of the transmit cable pair 4, 7, 60, 61 are placed parallel to those of the central cores of the twinax, and the transmit cables are close to the outer shield of the twinax, then the coupling of the magnetic field from the currents in the transmit cable pair 4, 7, 60, 61 into the twinax shield is increased relative to that of Figure 1, assuming the twinax insulation around each central core is similar to that of the single coaxial cable core of that of Figure 1. Thus, on the one hand the coil cable movement signal may be reduced due to the differential nature of the twin central cores, but, on the other hand, greater shield eddy currents are produced. So unless the twinax also uses e.g. high shield strand resistances, this will not eliminate the coil cable movement problem. Furthermore, this would add undesirable bulk and expense to the coil cable. Yet further, magnetic coupling between the 2024287101   20 Dec 2024 transmit pair of cables in the coil cable and twinax twin inner cores of the above described twinax would be relatively large (measured as X offset), unless the twinax core pair were twisted, but then that would add yet more bulk. Obviously, this arrangement would cause a spurious X movement signal. It thus would be better if any extra bulk and expense were added via the use of other elements to reduce the coil cable movement problem more successfully, such as for example changing the transmit pair of cables (60, 61) to a coaxial cable as in Embodiment E above.

[0057] Embodiment G: The coaxial shield may consist of a mixture of differing resistivity metal insulated non-magnetic strands such that the late-time time-constant of the receive coaxial cable eddy currents decay fast enough not to be detected at the commencement of synchronous demodulation. For example, the shield may consist of say 85% non-magnetic stainless steel insulated strands, and 15% copper insulated strands, or, some other ratio, where both varieties of metal strands have the same diameter (designated as “d” above), such that the total number of strands (designated as “n” above), and the pitch are similar to the example above (P=23mm). Alternatively thick films of a more conductive metal, like copper, could be electroplated onto a less conductive stands of the coaxial shield, such as nonmagnetic stainless steel, but this would mean something like several grams (e.g. 10g) being electroplated per coil cable and hence would be more expensive than the mixture of copper stands plus stainless steel strands solution to provide the same late-period coaxial shield eddy current time constant.

[0058] Embodiment H: Combinations of Embodiment A and Embodiment G inclusive above.

[0059] Figure 4 depicts a coaxial cable according to one embodiment of the present disclosure. The coaxial cable 133 comprising an inner conductor 138 to connect to an inductive winding of the magnetic field receiver (not shown), a conductive electrostatic shield 135 surrounding the inner conductor 138, and a dielectric layer 136 between the inner conductor 138 and the conductive electrostatic shield 135. Surrounding the conductive electrostatic shield 135 is a plastic sheath 134. While in Figure 4, the conductive electrostatic shield 135 is formed by spirally wound strands, other winding forms are possible as long as a late-time time constant of eddy currents induced in the conductive electrostatic shield 135, due to changing magnetic fields generated from changing transmit signal currents flowing from transmit electronics of the metal detector to a magnetic field transmitter of the metal detector, is less than 1,2^s. In this embodiment, the conductive electrostatic shield 135 is stainless-steel strands. In another embodiment, the conductive electrostatic shield 135 may be a combination of different materials. For example, in one embodiment, the conductive electrostatic shield 135 comprised of 85% insulated stainless-steel strands and 15% percent insulated copper strands. Other combinations are also possible such as 80% insulated stainless-steel strands and 20% insulated copper strands. 2024287101   20 Dec 2024

[0060] While not shown in Figure 4, the strands may be connected together and connected to the electronics ground 57 at the electronics end; and at the magnetic field sensor end, the strands may be connected together and to the electrostatic screen 55 and the receive inductive winding 53.

[0061] Figure 5 depicts connections of a coaxial cable according to one embodiment of the present disclosure. In this embodiment, a coaxial cable, comprising an inner conductor 158 and a conductive electrostatic shield 200 surrounding the inner conductor, connects a magnetic field receiver comprising winding 153 of a hand-held metal detector to receive electronics of the metal detector. The input of the received electronics in this embodiment is an amplifier 164. The conductive electrostatic shield 200 at the end of the coaxial cable closer to the receive electronics is grounded to ground 157. The conductive electrostatic shield 200 at the end of the coaxial cable closer to the magnetic field receiver is open ended (i.e. open circuit). A ground cable 185 is provided along the coaxial cable to ground one end of the winding 153 to ground 157. The inner conductor 158 at the end of the coaxial cable closer to the receive electronics is connected to amplifier 164. The inner conductor 158 at the end of the coaxial cable closer to the magnetic field receiver is connected to another end of the winding 153.

[0062] A simple test was set up to obtain test results of cables of various embodiments versus a typical coaxial cable. In this test, force was applied to the typical coaxial cable and the cables of various embodiments, when they were used in turn to connect a magnetic field receiver of a hand-held metal detector to receive electronics of the metal detector, while the output of filters (for example filter 74 and 75 of Figure 2) was monitored. It was found that when the typical coaxial cable was used, a clear signal above background noise was observed. When cables of various embodiments were used instead, there is no observable signal above background noise.

[0063] While the examples provided in this disclosure are in relation to time-domain hand held detectors, the new coaxial cable of the present disclosure can also be used with frequency-domain handheld detectors, but without the advantages described herein for time-domain hand held detectors.

[0064] Those of skill in the art would understand that information and signals may be represented using any of a variety of technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof.

[0065] Those of skill in the art would further appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, 2024287101   20 Dec 2024 modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present disclosure.

[0066] The steps of a method or algorithm described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. For a hardware implementation, processing may be implemented within one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof. Software modules, also known as computer programs, computer codes, or instructions, may contain a number of source code or object code segments or instructions, and may reside in any computer readable medium such as a RAM memory, flash memory, ROM memory, EPROM memory, registers, hard disk, a removable disk, a CD-ROM, a DVD-ROM or any other form of computer readable medium. In the alternative, the computer readable medium may be integral to the processor. The processor and the computer readable medium may reside in an ASIC or related device. The software codes may be stored in a memory unit and executed by a processor. The memory unit may be implemented within the processor or external to the processor, in which case it can be communicatively coupled to the processor via various means as is known in the art.

[0067] Throughout the specification and the claims that follow, unless the context requires otherwise, the words “comprise” and “include” and variations such as “comprising” and “including” will be understood to imply the inclusion of a stated integer or group of integers, but not the exclusion of any other integer or group of integers.

[0068] The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement of any form of suggestion that such prior art forms part of the common general knowledge.

[0069] It will be appreciated by those skilled in the art that the disclosure is not restricted in its use to the particular application described. Neither is the present disclosure restricted in its preferred embodiment with regard to the particular elements and / or features described or depicted herein. It will be appreciated that the disclosure is not limited to the embodiment or embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the scope of the disclosure as set forth and defined by the following claims.

Claims

1. A coaxial cable to connect a magnetic field receiver of a hand-held metal detector to receive electronics of the metal detector, the coaxial cable comprising:an inner conductor to connect to an inductive winding of the magnetic field receiver;a conductive electrostatic shield surrounding the inner conductor;a dielectric layer between the inner conductor and the conductive electrostatic shield;wherein the conductive electrostatic shield is configured and arranged such that a late-time time constant of eddy currents induced in the conductive electrostatic shield, due to changing magnetic fields generated from changing transmit signal currents flowing from transmit electronics of the metal detector to a magnetic field transmitter of the metal detector, is less than 1,2^s.

2. The coaxial cable of claim 1, wherein the magnetic field receiver is also the magnetic field transmitter; and wherein the changing transmit signal currents flows within the coaxial cable.

3. The coaxial cable of claim 1, wherein the changing transmit signal currents flows within at least one transmit cable in close proximity of the coaxial cable.

4. The coaxial cable of claim 1, wherein the conducting electrostatic shield comprises insulated nonferromagnetic metal strands of resistivity at least 2.5 times greater than a resistivity of copper.

5. The coaxial cable of claim 1, wherein the conducting electrostatic shield comprises non-magnetic stainless-steel strands.

6. The coaxial cable of claim 1, wherein the conducting electrostatic shield is formed by spirally wound strands; and wherein a ratio of a pitch of the conducting electrostatic shield to a mean radius of the conducting electrostatic shield is less than 2.7.

7. The coaxial cable of claim 1, wherein the insulated metal strands are connected together at one end of the cable but each of the strands remains open and not connected at another end of the cable.

8. The coaxial cable of claim 1, wherein the insulated metal strands comprise at least two different types of strands, each with a different resistivity.

9. The coaxial cable of claim 4, wherein the insulated non-ferromagnetic metal strands are formed by combining two or more metals through electroplating.2024287101   20 Dec 202410. A coil cable of a hand-held metal detector, comprising:a coaxial cable according to claim 1; and an outer sheath surrounding the coaxial cable.

11. The coil cable of claim 10, wherein the magnetic field receiver is also a magnetic field transmitter; wherein the changing transmit signal currents flows within the coaxial cable; wherein the coil cable further comprising an external low resistance cable along the coaxial cable; and wherein at least part of the changing transmit signal currents flows within the external low resistance cable along the coaxial cable.

12. The coil cable of claim 10, wherein the coil cable further comprising at least a transmit cable to conduct the transmit signal currents.

13. The coil cable of claim 10, wherein the conducting electrostatic shield comprises insulated nonferromagnetic metal strands of resistivity at least 2.5 times greater than a resistivity of copper.

14. The coil cable of claim 10, wherein the conducting electrostatic shield comprises non-magnetic stainless-steel strands.

15. The coil cable of claim 10, wherein the conducting electrostatic shield is formed by spirally wound strands; and wherein a ratio of a pitch of the conducting electrostatic shield to a mean radius of the conducting electrostatic shield is less than 2.7.

16. The coil cable of claim 10, wherein the insulated metal strands are connected together at one end of the cable but each of the strands remains open and not connected at another end of the cable; and an external cable along the coaxial cable is connected to the receive inductor of the magnetic field receiver.

17. The coil cable of claim 10, wherein the insulated metal strands comprise at least two different types of strands, each with a different resistivity.

18. The coaxial cable of claim 4, wherein the insulated non-ferromagnetic metal strands are formed by combining two or more metals through electroplating.

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