GB2458492A

GB2458492A - Antenna array with reduced mutual antenna element coupling

Info

Publication number: GB2458492A
Application number: GB0805161A
Authority: GB
Inventors: Lye Whatt Chua
Original assignee: Thales Holdings UK PLC
Current assignee: Thales Holdings UK PLC
Priority date: 2008-03-19
Filing date: 2008-03-19
Publication date: 2009-09-23
Also published as: GB0805161D0

Abstract

An antenna array 140, a method of making an antenna array or a method of reducing mutual coupling between antenna array elements, comprises: providing at least one conductive element 142, on the same surface of a planar substrate as that of the antenna elements 100. The conductive element 142 being interposed between planar antenna array elements 100. The conductive element 142 may be a printed strip formed on the surface or a wire element mounted on the surface of the substrate and it may form an open-circuit transmission line or a band-stop filter. The antenna array 140 may include a transmission element with a radiating heart-shaped element 100 with a transmission line connected to the lowest pointed part of the element and serrations formed in the outer sides of the said element 100. Also disclosed is a method of reducing mutual coupling between antenna array elements, comprising providing antenna array elements, where each antenna element is formed on a first surface of a respective laminar substrate and at least one conductive isolation element is formed on a further laminar substrate and is interposed between the planar antenna array elements.

Description

1 2458492

A METHOD OF REDUCING MUTUAL COUPLING BETWEEN ANTENNAS

Field of the Invention

The present invention relates to a method of reducing mutual coupling between antennas. In particular it relates to reducing mutual coupling of antenna elements of a planar antenna array for radiating ultra wide bandwidth (UWB) pulses.

Background of the Invention

Antenna arrays are commonly used in radio communication systems. Typically, an antenna array is formed by a set of antenna elements arranged in an array. Figure 1 illustrates a typical planar antenna array 10 having two antenna elements 12, 14 positioned apart from one another. The antenna elements are formed on a surface 16 of a dielectric substrate 18. A ground element 20 is also formed on an opposing surface of the dielectric substrate.

One of the key advantages of employing an antenna array is that hig1er directivity can be achieved in comparison with a single antenna. Moreover, by adjusting the relative amplitude and phase of the signals emitted from each antenna element of an antenna array, the radiation pattern in the far field of the antenna array can be changed. This allows the radiation pattern of the antenna array to be tailored to a particular application without mechanical movement of the antenna array.

In some applications, it is necessary to limit the space required for the antenna array arrangement. In these circumstances, the antenna elements tend to be arranged closely to one another. As a result, the antenna elements interact with one another resulting in mutual coupling that affects the overall effectiveness of the antenna array.

One method of reducing mutual coupling between antenna elements of an array is described in the prior art. In particular, US 2006/0164317 describes a phase array antenna comprising two antenna elements (first and second element) and a dielectric separator being located between the two antenna elements. Essentially, the dielectric separator provides a means of modifying the phase component of the signal received at the second element due to electromagnetic fields emitted by the first element. However, there are a number of drawbacks with this method. Firstly, the effectiveness of the separator is dependent on the dielectric constant of the dielectric separator. Therefore, the dielectric constant of the separator must be selected according to the operational frequency of the antenna elements in order to sufficiently isolate the emission of one antenna element to the other. In addition, the thickness of the dielectric separator also determines its effectiveness. In some applications, the thickness of the dielectric separator must be sufficiently large in order to provide effective isolation. This can result in an overall increase in the size of the antenna array.

Accordingly, there is a need for a method of reducing mutual coupling between antenna elements, without deteriorating the overall performance and the size of the antenna array.

Furthermore, EP 1768211 discloses a flat-plate M[MO array antenna comprising two antenna elements and an isolation element disposed on a substrate. The isolation element is interposed between the two antenna elements, and is connected to a ground plane of the antenna array, via a grounding pin. Essentially, the isolation element is a short-circuit resonator. The isolation element is an inverted U-shaped structure having an overall length (L) of one wavelength of the wave radiated from the antenna elements.

The spacing between the isolation element and the antenna elements is set as a quarter wavelength of the wave radiated from the antenna elements. The mutual coupling between the antenna elements can only be adjusted by regulating the length, L. This does not provide sufficient design freedom to optimise the performance of the isolation element.

Statement of the Invention

In a first aspect of the present invention, there is provided a method of reducing mutual coupling between a plurality of antenna elements formed on a first surface of a substantially laminar substrate, the method comprising providing at least one conductive element on said first surface, said conductive element being positioned between said antenna elements such that each of said antenna elements is substantially isolated from radiation emitted by at least one adjacent antenna element.

In a second aspect of the present invention, there is provided a method of making an array antenna structure comprising providing a laminar substrate defining first and second opposing planar surfaces, forming a plurality of antenna elements on said first planar surface, said antenna elements being spaced apart from each other, providing at least one conductive element on said first surface, said conductive element being positioned between said antenna elements such that each of said antenna elements is substantially isolated from radiation emitted by at least one adjacent antenna element.

Provision of a conductive element between the antenna elements can allow mutual coupling between the antenna elements to be reduced without increasing the overall size (in particular the thickness) of the antenna array. In addition, the conductive element is not connected to a ground plane of the antenna array. This can provide a number of advantages. For example, the antenna array can be fabricated easily by using a printing or an etching process known in the art, without creating any via holes to connect the conductive element to the ground plane of the antenna array.

The conductive element may be an open-circuit transmission line. In contrast with a short-circuit resonator, an open-circuit transmission line can provide sufficient design freedom to reduce mutual coupling between the antenna elements. For example, the length or the width of the open-circuit transmission line, or the distance between the conductive element and the antenna elements, can be varied accordingly to achieve optimum performance.

The open-circuit transmission line may be a band-stop filter.

In a preferred embodiment of the above aspects, said providing said conductive element includes forming at least one strip line on said first surface.

In another preferred embodiment of the above aspects, said providing of said conductive element includes mounting at least one conductive filament on said first surface. This can allow isolation between the antenna elements to be provided without forming the conductive element onto the first surface.

In a third aspect of the present invention, there is provided a method of reducing mutual coupling between a plurality of antenna elements, each of the antenna elements being formed on a first surface of a substantially laminar substrate, the method comprising positioning an isolation element between said antenna elements such that each of said antenna elements is substantially isolated from radiation emitted by at least one adjacent antenna element, and wherein said isolation element is formed by providing at least one conductive element on a surface of a further substantially laminar substrate defining first and second opposing surfaces.

The conductive element may be an open-circuit transmission line.

The open-circuit transmission line may be a band-stop filter.

In a preferred embodiment of the above aspects, said providing of said conductive element includes forming at least one strip line on said surface.

In another preferred embodiment of the above aspects, said providing said conductive element includes mounting at least one conductive filament on said surface.

Said at least one conductive element may be provided on said first surface of said further laminar dielectric substrate.

Alternatively, said at least one conductive element may be provided on said second surface of said further laminar dielectric substrate.

In another preferred embodiment, each of said antenna elements comprises a transmission element formed on said first surface, the transmission element comprising a radiating element and a transmission line providing electrical connection between the radiating element and a connection point for establishing electrical connection with the transmission element, the radiating element being substantially tapered towards a narrow end thereof connected with the transmission line, the distal, wider end thereof having formed therein a substantially v-shaped notch thereby defming two lobes which diverge with increasing distance from said transmission line, wherein outer edges of said lobes have formed therein a plurality of serrations to inhibit propagation of signal waves at said outer edges.

In a fourth aspect of the present invention, there is provided an antenna array comprising a laminar dielectric substrate defining first and second opposing planar surfaces, a plurality of antenna elements formed on said first planar surface, said antenna elements being spaced apart from each other, and at least one conductive element being positioned between said antenna elements such that each of said antenna elements is substantially isolated from radiation emitted by at least one adjacent antenna element.

Brief description of the drawings

Embodiments of the present invention will now be described with reference to the accompanying drawings, wherein: Figure 1 illustrates a typical planar antenna array exemplary of the prior art; Figure 2 illustrates a planar antenna array in accordance with a first specific embodiment of the invention including an isolating element interposed between two antenna elements of said array; Figure 3 illustrates a planar antenna array in accordance with a second specific embodiment of the invention including an isolating element interposed between two antenna elements of said array; Figure 4 illustrates a plan view of a planar antenna array including two isolating elements positioned in parallel with each other between two antenna elements of said array; Figure 5 illustrates a planar antenna array in accordance with a third specific embodiment of the invention including an arrangement of an isolating element interposed between two antenna elements of said array; Figure 6 illustrates a planar antenna array in accordance with a third specific embodiment of the invention including an alternative arrangement of the isolating element of figure 5 interposed between two antenna elements of said array; Figure 7 illustrates a plan view of a front surface of an antenna according to a fourth embodiment of the present invention; Figure 8 illustrates a plan view of a back surface of the antenna of figure 7.

Figure 9 illustrates a side view of the antenna of figure 7; Figure 10 illustrates an antenna array according to a fifth embodiment of the present invention; and Figure II illustrates the frequency responses of the mutual coupling between two antenna elements of the antenna array of figure 10.

Detailed Description

The present invention will be 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 be apparent, however, to a person skilled in the art that these specific details need not be employed to practice the present invention.

Figure 2 illustrates a planar antenna array 30 according to a first embodiment of the present invention. As shown in Figure 2, the planar antenna array 30 comprises a substantially laminar substrate 32 defining a first surface 34 and a second opposing surface (not shown), two antenna elements 36, 38 (in this example, rectangular patch antennas) printed on the first surface 34 of the dielectric substrate 32, connection points 40, 42 for each of the respective antenna elements, and a ground element 44 printed on the second surface of the dielectric substrate 32. It will be readily appreciated that any other forms of antenna elements, such as circular patch antennas, may also be used.

Further, the antenna elements need not be identical in shape; that is, one of the antenna elements could be a rectangular patch antenna and the other antenna element could be a circular patch antenna. In addition, the antenna elements may operate at different frequencies, for example, one of the antenna elements may operate in the GSM (Global System for Mobile Communications) band and the other antenna element may operate in the UMTS (Universal Mobile Telecommunications System) band. The planar antenna array may be capable of being utilised in transmission and reception.

It will also be appreciated by the person skilled in the art that any suitable form of connection points 40, 42 may be employed. For this reason, details of the connection points 40, 42 of the antenna array 30 are not described in detail.

An isolating element 46 is provided on the first surface 34 of the dielectric substrate 32.

As illustrated in Figure 2, the isolating element 46 is interposed between the two antenna elements 36, 38.

The antenna elements 36, 38 and the isolating element 46 may be etched or printed on the dielectric substrate 32 using techniques that are well known in the art. Similarly, the ground element 44 may be etched or printed on the dielectric substrate 32 to form a common ground plane of the planar antenna array 30.

In this example, the isolating element is a strip line having a length of approximately half-wavelength at the centre frequency of one of the antenna elements. The isolating element in Figure 2 is interposed between the antenna elements. The person skilled in the art would appreciate that the isolating element 46 may be of any form. In addition, it may not be equidistant from each of the antenna elements.

Essentially, the isolating element may be a contiguous bandwidth band-stop filter of a multiple discrete frequency band-stop filter that has an open-circuit transmission line characteristics, capable of suppressing current at the emission of the antenna elements.

According to an alternative embodiment of the present invention, as shown in Figure 3, the isolating element is replaced by a copper wire 66.

Furthermore, a plurality of isolating elements 78 may be positioned parallel to one another. Figure 4 illustrates a plan view of this arrangement.

As illustrated in Figure 5, in another embodiment of the present invention, each of the antenna elements 182, 184 and the isolating element 186 are fabricated on different laminar substrates 188,190, and 192.

In addition, as shown in Figure 6, the isolating element (not shown) is printed on a second surface of the laminar substrate 208; that is, on the surface which is opposite to the surface of the antenna elements 202, 204. As for earlier described embodiments, the isolating element in this case is a strip line having a length of approximately half-wavelength at the centre frequency of one of the antenna elements. Similarly, the isolating element is interposed between each of the antenna elements.

A small planar antenna structure is described in UK patent application no. GB2439110A. Different configurations of the small planar antenna are also described in the same document. Generally, the small planar antennas provide omnidirectional characteristics in their azimuthal direction of radiation and a shallow radiation null along its geometrical symmetry axis of the radiating element. An example of this small planar antenna is illustrated in Figures 7 to 9.

Figure 7 shows a front surface of the planar antenna element 100 comprising a radiating element 102, a transmission line 104 and a ground plane element 106 printed on the dielectric substrate 108. The transmission line 104 has a signal feed point 110 to provide (and to receive) signal to and from the radiating element 102.

The opposing end of the signal feed point of the transmission line 104 is connected to the radiating element 102. The radiating element 102 is shaped as a segment having two opposed slant edges 112, which diverge outwardly from an apex 114 of the segment.

The two opposed slant edges 112 diverge with increasing distance from the transmission line 104 such that the radiating element 102 tapers outwardly from the transmission line 104. The radiating element 102 possesses two distal peripheral edges (116 and 118) which are arcuate and which respectively bridge the terminal outermost ends of the two opposed slant edges 112 and form the curved outermost peripheries of the radiating element 102.

The radiating element 102 has two corresponding series of serrations 120 each formed within a respective one of the two opposed slant edges 112. Each serration of a given series of serrations is formed by a pair of successive angular (tapering) notches which extend into the radiating element 102 from the respective slant edge 112. Each tapering notch has notch edges which converge to terminate within the radiating element 102 at a right-angled apex 122.

Each such serration, and the series of serrations 120 collectively, present a slow-wave structure to a signal propagating along the slant edge 112. Essentially, the slow-wave structure formed along the slant edge 112 of the radiating element 102 is provided with a meander which slows down the progress of a signal wave travelling along the slant edge 112. This is achieved by constraining the signal wave to progress along the longer meandering slant edge rather than to progress directly along a shorter linear slant edge.

As a result, the radiating element is operable over a wide bandwidth of signal frequencies without increasing the physical size of the radiating element 102.

The meanders of the slant edge 112 are shaped such that the Q-factor of the antenna is minimised thereby reducing aperture clutter by reducing the relative magnitude of a signal reaching the terminal (open circuit) end of the slant edge 112 where signal reflection tends to occur, this being the source aperture clutter. The Q-factor of the radiating element 102 is given as: stored energy Q factorcx rate of energy loss Thus, the relative magnitude of a sigfal reaching the terminal outer edge of the slant edge 112 (i.e. relative to the magnitude of that signal at the beginning of the slant edge 112) is sensitively dependent upon the rate of loss of energy from the signal during propagation along the slant edge. By suitably shaping the meanders of the slow-wave structure, the described specific embodiment of the present invention may enhance the rate of radiative energy loss of the propagating signal as it progresses along the slant edge thereby reducing aperture clutter.

Successive serrations of each series of serrations 120 are shaped to increase in size relative to the preceding serrations in a log-period manner. Thus, the serrations in a given series have a common shape. In this example the common shape is a straight-edged serration with two tapering edges extending from the body of the radiating element 102 at predetermined angles and converging at increasing distance from the body of the radiating element 102 to a terminal right-angular serration tip or apex 122.

Each serration in a given series of serrations 120 possesses two tapering edges which each extend from the body of the radiating element 102 at the same predetermined angles as occurs in respect of the edges of an adjacent serration of the series, and also converge at a right angular serration apex 122. The ratio of the lengths of the two tapering edges of any given serration is shared by all serrations in the same series since all serrations in a given series share the same general shape. However, due to log-periodic scaling, the lengths themselves increase by a predetermined scaling value such that the ratio of a serration edge length of a given serration and the corresponding edge length of the succeeding serration has a constant predetermining ratio value shared by all such neighbouring serrations.

Furthermore, each series of serrations 120 is arranged such that the distance between the location of the apex 114 of the segment of the radiating element 102 and the location of the serration increase log-periodically as one encounters successive serrations of a given series. The result is that the ratio of the aforesaid distance, as between two neighbouring (successive) serrations, is equal to a constant predetermined ratio value shared by all such neighbouring serrations. The location of the serration may be considered to be the location of the apex 122 of the tip of the serration in question, for

example.

The planar antenna 100 also includes a ground plane 126 printed on a back surface of the planar antenna 100 as shown in Figure 8.

In order to design an antenna which is capable of operating over a wide bandwidth, biasing arid impedance effects of the associated DC networks must be considered from an RF or microwave perspective. DC biasing achieved from the use of RF chokes and resistors is effective only if the chokes are effectively an open circuit with no resonances, and if the combinations of inductance, resistance and capacitance do not limit the ability of the circuit to respond broad band.

The ground plane 126 comprises a plurality of slots 128 along its two longitudinal edges 130. The slots 128 along the longitudinal edges 130 have different lengths 132 and are spaced from each other at irregular intervals. In this configuration, the slots are essentially series inductance that function as RF choke to attenuate any unwanted signals.

As shown in Figure 10, the small planar antenna 100 of Figure 7 is implemented in an antenna array 140 having a plurality of the small planar antennas 100 arranged in two rows in a staggered arrangement. An isolating element 142, as described above, is positioned between each of the planar antenna to reduce mutual coupling between one another.

Figure 11 illustrates the measured frequency responses 150 of the mutual coupling between two of the planar antennas of Figure 10. As clearly shown in Figure 11, the mutual coupling between the antennas is reduced approximately 10 dB when an isolating element 142 is positioned between the antennas 100.

Figure 11 also illustrates that the mutual coupling of the antennas 100 can be further reduced by positioning two isolating elements in parallel.

The embodiments of the above-described method can provide sigmficant reduction in mutual coupling between antenna elements in a planar antenna array without increasing the overall size of the antenna array. Furthermore, the described technique can provide sufficient design freedom such that optimum performance of the isolating element can be achieved without compromising the performance of the antenna array.

It will be appreciated that the foregoing provides description of specific embodiments of the invention and that no limitation on the scope of protection sought herein is to be implied therefrom. The scope of protection sought is to be determined from the claims, read with reference to, but not bound by, the description and drawings.

Claims

CLAIMS: A method of reducing mutual coupling between a plurality of antenna elements formed on a first surface of a substantially laminar substrate, the method comprising: providing at least one conductive element on said first surface, said conductive element being positioned between said antenna elements such that each of said antenna elements is substantially isolated from radiation emitted by at least one adjacent antenna element.
2. A method of making an array antenna structure comprising: providing a laminar substrate defining first and second opposing planar surfaces; forming a plurality of antenna elements on said first planar surface, said antenna elements being spaced apart from each other; providing at least one conductive element on said first surface, said conductive element being positioned between said antenna elements such that each of said antenna elements is substantially isolated from radiation emitted by at least one adjacent antenna element.
3. A method according to claim 1 or claim 2 wherein said conductive element is an open-circuit transmission line.
4. A method according to claim 3 wherein said open-circuit transmission line is a band-stop filter.
5. A method according to any one of the preceding claims wherein said providing said conductive element includes forming at least one strip line on said first surface.
6. A method according to any one of claims 1 to 5 wherein said providing said conductive element includes mounting at least one conductive filament on said first surface.
7. A method according to any one of the preceding claims wherein each of said antenna elements comprises a transmission element formed on said first surface, the transmission element comprising a radiating element and a transmission line providing electrical connection between the radiating element and a connection point for establishing electrical connection with the transmission element, the radiating element being substantially tapered towards a narrow end thereof connected with the transmission line, the distal, wider end thereof having formed therein a substantially v-shaped notch thereby defining two lobes which diverge with increasing distance from said transmission line, wherein outer edges of said lobes have formed therein a plurality of serrations to inhibit propagation of signal waves at said outer edges.
8. A method of reducing mutual coupling between a plurality of antenna elements, each of the antenna elements being formed on a first surface of a substantially laminar substrate, the method comprising: positioning an isolation element between said antenna elements such that each of said antenna elements is substantially isolated from radiation emitted by at least one adjacent antenna element; and wherein said isolation element is formed by providing at least one conductive element on a surface of a further substantially laminar substrate defining first and second opposing surfaces.
9. A method according to claim 8 wherein said conductive element is an open-circuit transmission line.
10. A method according to claim 9 wherein said open-circuit transmission line is a band-stop filter.
11. A method according to claims 8 to 10 wherein said providing said conductive element includes forming at least one strip line on said surface.
12. A method according to any one of claims 8 to 11 wherein said providing said conductive element includes mounting at least one conductive filament on said surface.
13. A method according to any one of claims 8 to 12 wherein said at least one conductive element is provided on said first surface of said further laminar dielectric substrate.
14. A method according to any one of claims 8 to 13 wherein said at least one conductive element is provided on said second surface of said further laminar dielectric substrate.
15. An antenna array comprising: a laminar dielectric substrate defining first and second opposing planar surfaces; a plurality of antenna elements formed on said first planar surface, said antenna elements being spaced apart from each other; at least one conductive element interposed between said antenna elements such that each of said antenna elements is substantially isolated from radiation emitted by at least one adjacent antenna element.
16. An antenna array according to claim 15 wherein said conductive element is an open-circuit transmission line.
17. An antenna array according to any one of claims 16 wherein said open-circuit transmission line is a band-stop filter.
18. An antenna array according to any one of claims 15 to 17 wherein said conductive element includes at least one strip line formed on said first surface.
19. -An antenna array according to any one of claims 15 to 18 wherein said conductive element includes at least one conductive element mounted on said first surface.
20. An antenna array according to any one of claims 15 to 19 wherein each of said antenna elements comprises a transmission element formed on said first surface, the transmission element comprising a radiating element and a transmission line providing electrical connection between the radiating element and a connection point for establishing electrical connection with the transmission element, the radiating element being substantially tapered towards a narrow end thereof connected with the transmission line, the distal, wider end thereof having formed therein a substantially v-shaped notch thereby defining two lobes which diverge with increasing distance from said transmission line, wherein outer edges of said lobes have formed therein a plurality of serrations to inhibit propagation of signal waves at said outer edges.
21. An antenna array substantially as herein described with reference to any of Figures 2 to 11 of the accompanying drawings.
22. A method substantially as herein described with reference to any of Figures 2 to 11 of the accompanying drawings.