GB2536440A

GB2536440A - An antenna

Info

Publication number: GB2536440A
Application number: GB1504419.1A
Authority: GB
Inventors: Firth Bradley
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 2015-03-16
Filing date: 2015-03-16
Publication date: 2016-09-21
Also published as: GB201504419D0

Abstract

An antenna for detecting an subsurface object comprising a plurality of coplanar conductive loops 110, 120 for producing an electromagnetic field, wherein a portion 112, 122 of each loop is coupled with a portion of at least one other loop. The loops are fed in phase so that a current 117, 127 in each loop flows in the same direction along the coupled portions. The loops may be a semi-circular or rectangular shape, and be two identical loops forming a dual-loop structure. There may be a common connection point (313, Fig 3) for establishing electrical connection between the loops with a matching circuit connected thereto, the circuit comprising a tuning capacitor (812, Fig 6) to match a resonant frequency of the antenna to the frequency of a communication device. The antenna may be coupled-fed, and may comprise a laminar dielectric substrate. An antenna assembly (700, Fig 4) has antennas arranged in different parallel planes, the coupled portions of each antenna at an angle to those of another antenna. The coupled portions may be perpendicular, and may be fed with currents of equal magnitude and phase quadrature. The antennas are used in a metal detector for detecting elongated objects

Description

An Antenna

Field of the Invention

The present invention relates to an antenna, and more particularly though not exclusively, to antennas for producing electromagnetic fields for detecting an elongated object under a surface.

Background of the Invention

Metal detectors arc well known and may he used for detecting metallic objects hidden in the ground or under a surface. Metal detectors operate by transmitting an alternating magnetic field through a transmission coil into the ground or other surface. The reflected magnetic field is detected by a detector coil. Changes in a magnetic field due to the presence of metallic objects can then he detected. Metal detectors for the commercial and domestic markets have been available for many years.

In some cases, a single coil may be employed to transmit and detect magnetic fields when the coil is connected to a circuit capable of switching between a transmission and detection mode. This is usually achieved by a suitable configuration of the metal detector circuit, and details of it will not be further described.

The coil is effectively a small loop antenna having a circumference less than a fraction (typically one tenth) of a wavelength. The current around the circumference of the coil is constant and the voltage induced along the sides of the loop cancel each other, creating a null along the axis of the loop antenna and a radiation peak in the directions lying in the plane of the loop. As a result, one of the disadvantages of a loop antenna is the inaccuracy in determining the exact location of an elongated buried object, such as a pipe or a cable. In particular, when the loop antenna is positioned with its plane parallel to the surface under which the object is buried. In order to overcome this problem, the metal detector is sometimes held in a position so that the plane of the loop antenna is substantially perpendicular to the surface.

The present invention provides an improved antenna particularly suitable for detecting an elongated buried object.

Summary of the Invention

The invention provides a dual loop antenna comprising a first conductive loop and a second conductive loop for producing an electromagnetic field for detecting an object under a surface. The conductive loops are coplanar and are positioned such that a portion of the first conductive loop is coupled with a respective portion of the second conductive loop. In use, the conductive loops are fed in phase so that a peak magnetic field is produced along the coupled portions.

In a first aspect., the present invention provides an antenna for detecting an object under a surface, the antenna comprising a plurality of conductive loops for producing an electromagnetic field, wherein the conductive loops are coplanar and positioned such that a portion of each of the plurality of conductive loops is coupled with a corresponding portion of at least one of the other said plurality of conductive loops, wherein in operation the conductive loops are fed in phase so that a current in each loop flows in the same direction as a current in each of the remaining loops along the coupled portions.

The invention further provides an antenna array comprising at least two dual loop antenna arranged in different parallel planes one above another. Each of the dual loop antennas comprising a first conductive loop and a second conductive loop for producing an electromagnetic field for detecting an object under a surface. The conductive loops are coplanar and are positioned such that a portion of the first conductive loop is coupled with a respective portion of the second conductive loop. In use, the conductive loops arc fed in phase so that a peak magnetic field is produced along the coupled portions. The antennas are arranged such that the coupled portions of each antenna is aligned substantially perpendicular to the coupled portions of another antenna of said antenna array. The antennas are fed with currents of equal magnitude and phase quadrature so that a 360 degrees coverage can be achieved by the antenna array. This provides an advantage in that the sensitivity of the antenna array is increased.

In a second aspect, the present invention provides an antenna assembly for detecting an object under a surface, the antenna assembly comprising two or more antennas, each antenna comprising a plurality of conductive loops for producing an electromagnetic field, the conductive loops being coplanar and positioned such that a portion of each of the plurality of conductive loops is coupled with a corresponding portion of at least one of the other said plurality of conductive loops, wherein in operation the conductive loops are fed in phase so that a current in each loop flows in the same direction as a current in each of the remaining loops along the coupled portions and wherein the antennas are arranged in different parallel planes and such that the coupled portions of each antenna is aligned at an angle relative to the coupled portions of another antenna of said antenna array.

In addition, the invention provides a buried object detector comprising a dual loop antenna and a signal source, the signal source is coupled to the antenna and arranged to feed the antenna with a signal, a detector circuit coupled to the antenna and arranged to detect changes in electromagnetic field detected by the antenna. The dual IS loop antenna comprises a first conductive loop and a second conductive loop for producing an electromagnetic field for detecting an object under a surface. The conductive loops are coplanar and are positioned such that a portion of the first conductive loop is coupled with a respective portion of the second conductive loop. In use, the conductive loops arc fed in phase so that a peak magnetic field is produced along the coupled portions. In operation, the peak magnetic field of the antenna is effectively aligned with the cable or pipe when the plane of the antenna is parallel to the surface of the ground, thereby improving the accuracy of detecting an elongated buried object.

In a third aspect, the present invention provides a buried object detector comprising an antenna and a signal source, the signal source is coupled to the antenna and arranged to feed the antenna with a signal, a detector circuit coupled to the antenna and arranged to detect changes in electromagnetic field detected by the antenna, wherein the antenna comprises a plurality of conductive loops for producing an electromagnetic field, the conductive loops being coplanar and positioned such that a portion of each of the plurality of conductive loops is coupled with a corresponding portion of at least one of the other said plurality of conductive loops, wherein in operation the conductive loops are fed in phase by the signal source so that a current in each loop flows in the same direction as a current in each of the remaining loops along the coupled portions.

Further features of the present invention are defined in the appended claims.

Brief Description of the Drawings

The present invention is illustrated by way of example, and not by limitation, in the figures of the accompany drawings, wherein elements having the same reference numeral designations represent like elements throughout and wherein: Figure 1 is a plan view of an antenna in accordance with a first embodiment of the present invention; Figure 2 is a plan view of an antenna in accordance with a second embodiment of the present invention; Figure 3A shows a plane view of a surface of an antenna in accordance with a third embodiment of the present invention; Figure 3B shows a plane view of a surface of an antenna in accordance with a fourth embodiment of the present invention; Figure 3C shows a plane view of a surface of an antenna in accordance with a fifth embodiment of the present invention; Figure 4 is a perspective view of the an antenna assembly in accordance with a sixth embodiment of the present invention; Figure 5 is a simplified block diagram illustrating a buried object detector incorporating the embodiments of the present invention; and Figure 6 shows a plane view of a surface of an antenna in accordance with an embodiment of the present invention.

Detailed Description

The present invention will he described in further detail on the basis of the attached diagrams.

In the following description, a number of specific details are presented in order to provide a thorough understanding of embodiments of the present invention. It will he apparent, however, to a person skilled in the art that these specific details need not be employed to practice the present invention.

Figure 1 illustrates a plan view of an antenna 100 in accordance with a first embodiment of the present invention. The antenna, in this example, includes two adjacent coplanar conductive loops, a first conductive loop 110 and a second conductive loop 120. As illustrated in Figure 1, the conductive loops 110, 120 are substantially identical and respectively have semi-circular shapes, wherein each of the conductive loops is defined by a diameter portion and an are portion of a semi-circular shape. A diameter portion 112 of the first conductive loop 110 is placed in close proximity with a diameter portion 122 of the second conductive loop 120, such that electromagnetic fields that flow through the respective diameter portions 112, 122 of the conductive loops 110, 120 couple with each other. As illustrated in Figure 1 the overall shape, formed by the two semi-circular conductive loops 110, 120, of the antenna 100 is substantially circular. Each of the conductive loops may be formed using a conductive coil bent to form a semi-circular shape or, as will he described in the forthcoming paragraphs, printing a conductive track on a dielectric substrate. It should be appreciated that the diameter portions 112, 122 of the respective semi-circular loops 110, 120 may or may not be in physical contact with each other for the electromagnetic fields to couple.

In this example, a common feed point 130 is provided at the diameter portions 110, 122 to provide (and to receive) electrical signals to (and from) the conductive loops 110, 120. When antenna 100 is employed as a transmit antenna an electrical signal is supplied to the feed point 130 such that the conductive loops 110, 120 are fed in equal phase.

In Figure 1, the direction of the electric fields 113, 123 of the electrical signal is shown in dotted arrows, and the magnetic fields 115, 125 of the signal is shown in solid arrows.

As shown in Figure 1, the direction of the magnetic field 115 surrounding an arc portion 114 of the first conductive loop 110 is opposite the direction of the magnetic field 125 surrounding an arc portion 124 of the second conductive loop 120. As a result, this causes cancellation of a part or all of both the magnetic fields 115, 125 resulting in the reduction of the magnetic energy along the arc sections 114, 124 of the two conductive loops 110, 120. Conversely, the direction of the magnetic field 131 surrounding the diameter portion 112 of the first conductive loop 110 is in the same direction as the magnetic field 125 surrounding the diameter portion 122 of the second conductive loop 120, thereby creating a peak magnetic field along the diameter portions 112, 122 of the conductive loops 110, 120. As shown in Figure 1, the current 117, 127 in each loop 110, 120 flows in the same direction along the diameter portions, 112, 122.

In another example, each of the conductive loops 110, 120 may he provided with an independent feed point (not shown) through which an electrical signal is supplied independently. The skilled person in the art would appreciate that the antenna would operate in the same manner as described above as long as the two conductive loops 110, 120 are fed in phase.

Figure 2 illustrates a plan view of an alternative antenna configuration 200 in accordance with a second embodiment of the present invention. As shown in Figure 2, the first and second conductive loops 210, 220 are substantially identical and respectively have rectangular shapes. A long portion 212 of the first rectangular conductive loop 210 is positioned in close proximity with a long portion 222 of the second rectangular loop 220. The functional features of the antenna 200 are substantially the same as the corresponding functional features of the antenna 100 illustrated in Figure 1, and for this reason will not be further described. Furthermore, it would be appreciated that the antenna 200 would also operate in the same manner in an arrangement where the short portions 214, 224 of the rectangular loops 210, 220 are placed in close proximity each other.

Figures 3A and 3B respectively illustrate the antennas of Figures 1 and 2 produced on a dielectric substrate metallised on one of its two coplanar surfaces. It is noted that this configuration is advantageous in that the antennas are easy to manufacture and low cost. It is noted in Figure 3A that the antenna 300 is provided with a joint diameter portion 312 for the two conductive loops 310, 320. A common connection point 313 is provided at the diameter portion for providing a common electrical signal to the conductive loops 310, 320.

Similarly, the antenna 400 in Figure 3B is provided with a joint long section 412 for the two rectangular loops 410, 420.

Figure 3C illustrates a printed antenna 500 having structural and functional features similar with the antenna 400 of Figure 3B. As shown in Figure 3C the joint long section 512 of the rectangular conductive loops 510, 520 is wider than the joint long section 412 of the antenna 400 of Figure 3B. It is noted that in this configuration, the electric field of the joint long section can he maximised.

Figure 4 shows an antenna array 700 comprising two antennas 710, 720 arranged one on top of the other in different parallel planes. In this example, each of the antennas 710, 720 has structural and functional similarities to the antenna 300 illustrated in Figure 3A. However, the skilled person in the art would appreciate any one of the antennas illustrated in Figures 1, 2, 3B, and 3C could also be used. As shown in Figure 4, the antennas 710 720 are arranged such that the diameter portion 712 of antenna 710 is aligned substantially perpendicular to the diameter portion 722 of antenna 720. In this arrangement, the antennas 710, 720 are fed with currents of equal magnitude and phase quadrature so that a 360 degrees coverage can he achieved. This provides an advantage in that the sensitivity of the antenna array is increased.

Figure 5 shows a simplified block diagram illustrating the components of a buried object detector 600 in accordance with a further embodiment of the present invention.

The detector 600 includes one of the antennas described in the preceding paragraphs, a transmission circuit 610 and a detection circuit 620, both of which are coupled to the antenna to transmit and receive signals. The detector 600 operates by monitoring the absorption of a radiofrequency magnetic field which is generated by the antenna. In operation, the antenna (e.g. antenna 100) placed above the ground in which an elongated object, such as a cable or pipe, is buried, magnetically couples with the cable or pipe. The peak magnetic field of the antenna is effectively aligned with the cable or pipe when the plane of the antenna is parallel to the surface of the ground. As described above, the peak magnetic field of the antenna is produced along the diameter portions of the conductive loops, which effectively improves the accuracy for detecting an elongated buried object. This is because a maximum signal strength can be detected when the peak magnetic field of the antenna is fully aligned with the elongated buried object, thereby allowing a user to track the orientation of the elongated buried object accurately by monitoring the variation of the signal strengths of the detected signal.

It is appreciated that any suitable transmission and detection circuits can he employed and therefore details of the circuits will not be further described.

It is further noted that the antenna may be connected to the transmission and detection circuits via an impedance matching and tuning circuit. For example, as shown in Figure 6, a tuning capacitor 812 configured for impedance matching is provided along a diameter portion 810 of the antenna 800 in order to match the resonant frequency of the antenna 800 to the operating frequency of the transmission and detection circuits.

Alternatively, rather than connecting the antenna to the transmission and detection circuits, a feed coupling loop may be provided to allow the antenna to be fed without a direct contact with the transmission and detections circuits. This can be achieved by connecting a feed coupling loop to the transmission and detection circuits and arranging the antenna to he coupled-fed by the coupling loop. In another arrangement in which the antenna is coupled-fed, the transmission and detection circuits of the buried object detector may be located remotely and provided with a dipole (or a "pickup coil") arranged to transmit and/or detect signals to/from the feed coupling loop.

It will be appreciated by those already skilled in the art that the present invention can be embodied in other specific forms without departing from the essential character thereof. The presently-disclosed embodiments are therefore considered in all respects to he illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than the foregoing description, and all changes which come within the meaning and range of equivalents therefore are intended to be embraced therein.

Claims

Claims 1. An antenna for detecting an object under a surface, the antenna comprising a plurality of conductive loops for producing an electromagnetic field, wherein the conductive loops arc coplanar and positioned such that a portion of each of the plurality of conductive loops is coupled with a corresponding portion of at least one of the other said plurality of conductive loops, wherein in operation the conductive loops arc fed in phase so that a current in each loop flows in the same direction as a current in each of the remaining loops along the coupled portions.
2. An antenna according to claim 1, wherein each of the conductive loops is substantially semi-circular shape, and said portion is formed by a diameter portion of the semi-circular shaped conductive loop.
3. An antenna according to claim 1, wherein each of the conductive loops is substantially rectangular shaped, and said portion is formed by a long portion of the rectangular shaped conductive loop.
4. An antenna according to any one of the preceding claims, wherein the plurality of conductive loops comprises two substantially identical loops forming a dual-loop antenna structure.
5. An antenna according to any one of the preceding claims, wherein each of said plurality of conductive loops comprises a connection point for establishing electrical connection with each of the conductive loops.
6. An antenna according to any one of claims 1 to 4, further comprising a common connection point for providing a common electrical signal to the conductive loops.
7. An antenna according to claim 5 or claim 6, wherein the connection point comprises a matching circuit connected thereto.
8. An antenna according to claim 7, wherein the matching circuit comprises a tuning capacitor arranged to match a resonant frequency of the antenna to an operating frequency a communication device to which the antenna is connected.
9. An antenna according to any one of claims 1 to 4, wherein the antenna is coupled-fed.
10. An antenna according to any one of the preceding claims, wherein the antenna has a substantially planar form.
11. An antenna according to claim 10, wherein the antenna comprises a laminar dielectric substrate, defining opposing planar surfaces, the conductive loops being formed on one of said planar surfaces.
12. An antenna according to any one of the preceding claims, when in use, the planar surfaces of the laminar dielectric substrate is substantially parallel to the surface under which the object is buried.
13 An antenna assembly for detecting an object under a surface, the antenna assembly comprising a plurality of antennas, each antenna comprising a plurality of conductive loops for producing an electromagnetic field, the conductive loops being coplanar and positioned such that a portion of each of the plurality of conductive loops is coupled with a corresponding portion of at least one of the other said plurality of conductive loops, wherein in operation the conductive loops arc fed in phase so that a current in each loop flows in the same direction as a current in each of the remaining loops along the coupled portions, and wherein the antennas are arranged in different parallel planes and such that the coupled portions of each antenna is aligned at an angle relative to the coupled portions of another antenna of said antenna array.
14. An antenna assembly according to claim 13, wherein the plurality of antennas comprises two suhstantially identical antennas arranged such that the couple portions of one of the antennas is aligned substantially perpendicular to the coupled portions of the other antenna.
15. An antenna assembly according to claim 14, wherein the plurality of antennas are fed with currents of equal magnitude and phase quadrature
16. An antenna assembly according to any one of claims 13 to 15, wherein each of the conductive loops is substantially semi-circular shape, and said portion is formed by a diameter portion of the semi-circular shaped conductive loop.
17. An antenna assembly according to any one of claims 13 to 15, wherein each of the conductive loops is substantially rectangular shaped, and said portion is formed by a long portion of the rectangular shaped conductive loop.
18. An antenna assembly according to any one of claims 13 to 17, wherein the plurality of conductive loops of each antenna comprises two substantially identical loops forming a dual-loop antenna structure.
19. An antenna assembly according to any one of claims 13 to 18, wherein each of said plurality of conductive loops of each antenna comprises a connection point for establishing electrical connection with each of the conductive loops.
20. An antenna assembly according to any one of claims 13 to 18, each antenna further comprising a common connection point for providing a common electrical signal to the conductive loops of the antenna.
21. An antenna assembly according to claim 19 or claim 20, wherein the connection point comprises a matching circuit connected thereto.
22. An antenna assembly according to claim 21, wherein the matching circuit comprises a tuning capacitor arranged to match a resonant frequency of the antenna to an operating frequency a communication device to which the antenna is connected.
23. An antenna assembly according to any one of claims 13 to 18, wherein the antenna is coupled-fed.
24. An antenna assembly according to any one of the preceding claims, wherein each of the antennas has a substantially planar form.
25. An antenna according to claim 24, wherein each of the antennas comprises a laminar dielectric substrate, defining opposing planar surfaces, the conductive loops being formed on one of said planar surfaces.
26. A buried object detector comprising an antenna and a signal source, the signal source is coupled to the antenna and arranged to feed the antenna with a signal, a detector circuit coupled to the antenna and arranged to detect changes in electromagnetic field detected by the antenna, wherein the antenna comprises a plurality of conductive loops for producing an electromagnetic field, the conductive loops being coplanar and positioned such that a portion of each of the plurality of IS conductive loops is coupled with a corresponding portion of at least one of the other said plurality of conductive loops, wherein in operation the conductive loops are fed in phase by the signal source so that a current in each loop flows in the same direction as a current in each of the remaining loops along the coupled portions
27. An antenna substantially as hereinbefore described and as shown in the Figures.