Patent Description:
Designing antennas for use in difficult propagation environments, for example in a human body-mounted device or a vehicle-mounted device, is challenging since the environment may have adverse effects on the antenna, including a reduction in radiation efficiency, input impedance variation, radiation pattern fragmentation and polarization distortion. Simultaneously, there is a demand for low profile, minimum volume antenna structures for use in such environments. Currently available antennas tend to be either too large or have insufficient performance to meet all of the demands of modern day applications. In particular, known antennas that perform well in difficult propagating environments are not of a physical form that suit commercial needs. Accordingly, PCB integrated or chip antennas currently used in industry tend to exhibit poor performance. For applications involving challenging environments, these antennas are failing, meaning a communication link cannot be achieved.

It would be desirable to mitigate the problems outlined above.

Document <CIT> (<NUM>-<NUM>-<NUM>) discloses a patch antenna in which a closed curve slot is formed in the conductive patch and centred around a feed probe in order to create a vertically polarized radiation pattern.

The invention provides an antenna as claimed in claim <NUM>. Preferred features of the invention are recited in the dependent claims appended hereto.

The slots are preferably arranged to form a plurality of concentric rings. The, or each, ring is preferably circular.

Advantageously, the slots are arranged such that the, or each, ring is symmetrical about said at least one axis. Preferably, the slots are arranged such that the, or each, ring is symmetrical about both of said perpendicular axes.

In preferred embodiments, the, or each, ring comprises one or more slots, preferably two slots. Each slot is preferably shaped to form a respective half of the respective ring. The, or each, ring is preferably circular and each slot is arc-shaped, e.g. substantially semi-circular.

In preferred embodiments, the, or each, ring comprises two or more slots, arranged end-to-end and being spaced apart to leave an intra-ring gap between adjacent ends of adjacent slots. The, or each, slot of any one of said rings are arranged with respect to the, or each, slot of the, or each, adjacent ring such that the respective intra-ring gaps of adjacent rings are not aligned along any axis in the plane of the radiating structure. The preferred arrangement is such that the intra-ring gaps of any two adjacent rings are evenly spaced apart around the centre of the rings.

In a preferred embodiment, the slots of any one ring are angularly displaced about the ring centre by <NUM>° with respect to the slots of the, or each, adjacent ring such that the respective intra-ring gaps are angularly spaced apart by <NUM>° about the ring centre.

Optionally, the slots are arranged to form four concentric rings. Alternatively, the slots are arranged to form three concentric rings.

Advantageously, said slots are arranged to create a meandering current path on said radiating structure from said inner portion of said radiating structure to an outer portion of said radiating structure.

Advantageously, the slots are arranged symmetrically about two perpendicular axes that lie in the plane of the radiating structure.

In preferred embodiments, the feed structure comprises a feed line and a feed connector connected between the feed line and the inner portion of the radiating structure. The feed connector typically connects with said radiating structure at a feed point, wherein, preferably, at least one axis of symmetry extends through said feed point.

Typically, said radiating structure is rectangular, and wherein said at least one axis is parallel with a respective edge of the radiating structure.

Typically, said at least one axis extends through a centre of said inner portion.

In preferred embodiments, at least one shorting connector connected between the radiating structure and the ground plane, preferably between said inner portion and said ground plane. Said at least one shorting connector is preferably arranged symmetrically with respect to said at least one axis.

In preferred embodiments, the antenna comprises a planar radiating structure, a ground plane and a feed structure, the radiating structure comprising a plurality of slots arranged symmetrically in concentric rings around an inner portion of the radiating structure. The slots are advantageously arranged to create a meandering current path on the radiating structure. The preferred antenna produces an omnidirectional, monopole-like radiation field, and is relatively small with relatively high performance making it suitable for use in a wide variety of applications including those with challenging environments.

Further advantageous aspects of the invention will be apparent to those ordinarily skilled in the art upon review of the following description of a specific embodiment and with reference to the accompanying drawings.

An embodiment of the invention is now described by way of example and with reference to the accompanying drawings in which:.

Referring now to the drawings there is shown, generally indicated as <NUM>, an antenna embodying the invention. The antenna <NUM> comprises a radiating structure <NUM> and a ground plane <NUM>. The radiating structure <NUM> and ground plane <NUM> are spaced apart from each other in a first direction, which may be referred to as the Z-axis direction, and are preferably parallel with each other. In preferred embodiments the radiating structure <NUM> and ground plane <NUM> are aligned, or substantially aligned, with each other in the Z-axis direction, but in any event preferably at least partially overlap with each other in the Z-axis direction. In preferred embodiments, the antenna <NUM> is cuboid in shape, although may take other shapes in alternative embodiments.

In use, the antenna <NUM> is typically mounted on a substrate (not shown), for example a printed circuit board (PCB) or integrated circuit (IC) substrate, such that the radiating structure <NUM> faces away from the substrate, while the ground plane faces towards the substrate. Accordingly, the radiating structure <NUM> may be said to be located at the top of the antenna <NUM>, and the ground plane <NUM> located at the bottom, and as such the Z-axis may be referred to as the top-to-bottom direction.

The radiating structure <NUM> may be formed from any electrically conductive material suitable for antenna radiating structures, typically metal, e.g. copper.

In preferred embodiments, the radiating structure <NUM> comprises a planar, or patch, radiating element. The patch <NUM> may be rectangular or square in shape, or may take other shapes, e.g. circular or elliptical. The patch <NUM> may have straight edges, or may have non-straight edges, for example meandered or fractal edges. In any event, the radiating structure <NUM> is preferably planar in form and preferably lies in an X-Y plane, where X and Y represent an X-axis and Y-axis respectively, and wherein the X, Y and Z axes are mutually orthogonal.

The radiating structure <NUM> is typically provided on an electrically insulating, or non-conductive, support structure <NUM>, which may be referred to as a substrate, and which may comprise a block of electrically insulating material, preferably dielectric material. In alternative embodiments, the support structure <NUM> may comprise a stack of layers of electrically insulating, or dielectric, material. Any conventional electrically insulating, or dielectric material, may be used to form the support structure <NUM>, for example laminate material for use in circuit boards or microwave or RF applications. The radiating structure <NUM> may be provided as a layer or patch of conductive material on the top surface of the substrate <NUM>.

The ground plane <NUM> may be formed from any electrically conductive material suitable for forming antenna ground planes, typically metal, e.g. copper. The ground plane <NUM> may be connected to electrical ground in any convenient manner. The ground plane <NUM> may be rectangular or square in shape, or may take other shapes, usually to match the shape of the radiating structure <NUM>. The ground plane <NUM> preferably lies in an X-Y plane.

The ground plane <NUM> is optionally provided on an electrically insulating support structure <NUM>, which in the illustrated embodiment is provided at the bottom of the support structure <NUM>. The support structure <NUM> typically comprises an electrically insulating substrate, e.g. formed from a dielectric material, and may be provided on or integrated with the support structure <NUM> in any conventional manner. Alternatively, the ground plane <NUM> may be provided on the support structure <NUM>. The ground plane <NUM> may be provided as a conductive layer on a surface, preferably a bottom surface, of the support structure <NUM> or other surface, e.g. the bottom of the structure <NUM>. The support structure, or substrate, <NUM> may be part of the support structure <NUM>, e.g. they may be provided by a single block of electrically insulating, or dielectric, material, or it may be formed separately from the structure <NUM> and fixed thereto by any conventional means. The support structures <NUM>, <NUM> may be formed from the same material (especially when they are formed as a single block) or may be formed from different material. Any conventional electrically insulating, or non-conductive material, may be used to form the substrates <NUM>, <NUM>, especially dielectric material. For example dielectric composite material, or laminate material, for use in circuit boards or microwave or RF applications may be used. By way of example, either one or both of the substrates <NUM>, <NUM>, as applicable, may be formed from a ceramic-filled hydrocarbon thermoset material (which may be glass-reinforced), or any conventional epoxy/glass composite material, plastics/glass composite material, or paper/epoxy composite material.

The antenna <NUM> comprises a feed structure <NUM> that is typically located between the radiating structure <NUM> and the ground plane <NUM>. The feed structure <NUM> is coupled to an external feed connector <NUM>, which may be part of the antenna <NUM> or may be an external structure. In use, the antenna <NUM> is connected to external circuitry (not shown), typically comprising an RF transmitter, RF receiver or RF transceiver, via the connector <NUM>. In a transmitting mode of the antenna <NUM>, the feed structure <NUM> receives excitation signals from the external circuitry via the connector <NUM>, and feeds the excitation signals to the radiating structure <NUM> for transmission thereby. In a receiving mode of the antenna <NUM>, the feed structure <NUM> feeds received signals from the radiating structure <NUM> to the external circuitry via connector <NUM>. The connector <NUM> may take any suitable conventional form, for example comprise an SMA connector or other device suitable for sending signals to and receiving signals from the antenna <NUM>.

In preferred embodiments, the feed structure <NUM> comprises a feed line <NUM>, typically in the form of a microstrip feed line. The feed line <NUM> may be formed from any electrically conductive material, typically metal, e.g. copper. The feed line <NUM> is located between, and is preferably parallel with, the radiating structure <NUM> and ground plane <NUM>. The feed line <NUM> is spaced apart from the radiating structure <NUM> and the ground plane <NUM> in the Z-axis direction. The feed line <NUM> has a first, or free, end <NUM> located between the radiating structure <NUM> and the ground plane <NUM>, and a second end <NUM> (which may be referred to as the feed end) coupled to the connector <NUM> (at least in use). The end <NUM> of the feed line <NUM> is aligned with an inner portion <NUM> of the radiating structure <NUM>, the inner portion <NUM> typically being located centrally of the structure <NUM>. The second end <NUM> is typically located at, or adjacent, a peripheral portion, e.g. side or edge, of the antenna <NUM>. In preferred embodiments, the feed line <NUM>. The preferred arrangement is such that the feed line <NUM> extends in the X or Y direction.

Typically, the feed line <NUM> is provided on a substrate of electrically insulating material, preferably a dielectric material. Typically, the feed line <NUM> is provided as a conductive, e.g. metallic, strip on a surface of the substrate. Conveniently, the feed line <NUM> is provided on the same substrate <NUM> as the ground plane <NUM>, on the opposite surface to the ground plane <NUM>. In the illustrated embodiment, the feed line <NUM> is formed in the top surface of substrate <NUM> and the ground plane <NUM> is on the bottom surface. In the illustrated embodiment, the feed connector <NUM> passes through the substrate <NUM>. The ground plane <NUM> is shaped to define a region <NUM> of electrically insulating material around to the connector <NUM>.

In preferred embodiments, the feed structure <NUM> also comprises a second feed connector <NUM> which connects the feed line <NUM> to the radiating structure <NUM> in order to convey excitation signals between the feed line <NUM> and the radiating structure <NUM>. The second feed connector <NUM>, which may conveniently take the form of a conductive post or pin, may be formed from any suitable conductive material, e.g. copper or other metallic material. The feed connector <NUM> extends from the free end <NUM> of the feed line <NUM> to a feed point <NUM> located in the inner portion <NUM> of the radiating structure <NUM>. The feed connector <NUM> is preferably perpendicularly disposed with respect to the radiating structure <NUM>.

In alternative embodiments (not illustrated) the feed structure <NUM> may take other forms, not necessarily comprising the feed line <NUM> and/or the feed connector <NUM>. More generally, the feed structure <NUM> may be coupled with, or connected to, the radiating structure <NUM> by any conventional means. For example, the feed structure <NUM> may be a proximity-coupled feed structure, or an aperture-coupled feed structure, or other arrangement comprising a feed line that is indirectly coupled to the radiating structure <NUM> (e.g. electromagnetically coupled but not necessarily mechanically coupled).

In preferred embodiments, the antenna <NUM> includes at least one, and typically a plurality of, electrically conductive shorting connectors <NUM> connecting the radiating structure <NUM>, in particular the inner portion <NUM> of the radiating structure <NUM>, to the ground plane <NUM>. The connectors <NUM> create an electrical connection between the radiating structure <NUM> and ground plane <NUM> to short the radiating structure <NUM> to the ground plane <NUM>. The shorting connectors <NUM> typically take the form of a pin or a post. The shorting connectors <NUM> are preferably perpendicularly disposed with respect to the radiating structure <NUM>.

The shorting connectors <NUM> are preferably arranged symmetrically with respect to at least one axis in the X-Y. In particular, the shorting connectors <NUM> are arranged symmetrically about at least one X-Y axis through the feed point <NUM>. In the illustrated embodiment, first and second shorting connectors 32A, 32B are arranged symmetrically about an axis through the feed point <NUM> in the Y direction only. The shorting connectors <NUM> are preferably located adjacent the feed point <NUM>. Placing the connectors <NUM> close to the centre of region <NUM> improves impedance match performance and positional symmetry across one axis and will reduce radiation pattern impurity.

The shorting connectors <NUM> are preferably arranged symmetrically with respect to the feed line <NUM>, typically about the longitudinal axis of the feed line <NUM>. In preferred embodiments, only two shorting connectors 32A, 32B are provided, although in other embodiments a single shorting connector <NUM> may be provided, or more than two shorting connectors <NUM> may be provided. The shorting conductors <NUM> may have any cross-section shape, e.g. circular or rectangular, and their size (width and/or length) may be adjusted to suit the application and/or the optimization of the antenna <NUM>. The, or each connector <NUM> does not have to be in the form of a post (or pin), and may for example take any other convenient form, e.g. an elongate strip or wall of conductive material, which may run parallel with the ground plane <NUM>.

In use, the shorting connectors 32A, 32B cause nulls in the radiation field, or electric field (E-field), of the antenna <NUM> between the radiating structure <NUM> and the ground plane <NUM>. The nulls provided by the shorting connectors <NUM> facilitate production of the desired omnidirectional radiation pattern, and also facilitate miniaturization of the antenna <NUM>. In alternative embodiments (not illustrated), especially where the requirement for miniaturisation is lower, the shorting connectors <NUM> may be omitted.

In preferred embodiments, the radiation field of the antenna <NUM>, at least in one resonant mode, typically at least one higher order resonant mode of operation, has a monopole-like, or monopolar, radiation pattern or shape. In particular, the radiation field is omnidirectional in the azimuth plane.

A plurality of slots <NUM> are formed in the radiating structure <NUM>. The slots <NUM> are arranged symmetrically with respect to at least one axis in the X-Y plane, i.e. the plane in which the radiating structure <NUM> lies, and preferably with respect to two perpendicular axes in the X-Y plane. In preferred embodiments, the axis, or one of the axes, about which the slots <NUM> are symmetrical is parallel with the longitudinal axis of the feed line <NUM>. In preferred embodiments in which the radiating structure <NUM> is rectangular, or square, in shape, the axis, or each of the axes, about which the slots <NUM> are symmetrical is parallel with a respective edge of the radiating structure <NUM>. In preferred embodiments, the shorting connectors <NUM> are symmetrically arranged with respect to the same axis/axes as the slots <NUM>. The, or each, axis of symmetry passes through the inner portion <NUM>, preferably through the centre of the inner portion <NUM>.

The slots <NUM> are arranged around the inner portion <NUM> of the radiating structure <NUM> such that the inner portion <NUM> is located at the centre of the slot arrangement (and preferably also at the centre of the radiating structure <NUM>). In the illustrated embodiment, the feed point <NUM> is located centrally on the X axis but is offset from the centre of the Y axis, and so is not located exactly at the centre of the inner portion <NUM>. In alternative embodiments, the feed point <NUM> may be located elsewhere in the inner portion, preferably centrally located on at least one of the X and Y axes, and preferably close to the centre. In the illustrated embodiment, the shorting pins <NUM> are located centrally on the Y axis. In alternative embodiments, the shorting pins <NUM> may be located elsewhere in the inner portion, preferably close to the centre.

The slots <NUM> are arranged to form at least one but preferably a plurality of rings <NUM> around the inner portion <NUM>. Preferably, each ring <NUM> comprises two or more slots <NUM> arranged in a ring-like manner. Within each ring <NUM>, the respective slots <NUM> are arranged end-to-end with an intra-ring gap <NUM> between adjacent ends of adjacent slots <NUM>. The intra-ring gaps <NUM> comprise conductive material since they are part of the radiating structure <NUM>. Alternatively, the or each ring <NUM> may be formed by a single C-shaped slot with an intra-ring gap between its ends. The size of the intra-ring gaps <NUM>, in particular the slot-to-slot length, may vary depending on the application, for example in order to tune the antenna <NUM>, e.g. with respect to resonant frequency(ies) and/or bandwidth. Within any given ring <NUM>, the size of each intra-ring gap <NUM> is preferably the same since this facilitates provision of a symmetrical ring arrangement.

In preferred embodiments the rings <NUM> are circular, but they may alternatively take other shapes, e.g. square, rectangular or other regular or symmetrical curved or polygonal shape. In preferred embodiments, each slot <NUM> is arc-shaped but other shapes may be used, e.g. C-shaped, U-shaped, curved or polygonal depending on the shape of the ring.

In preferred embodiments, there is a plurality of rings <NUM> of slots <NUM>, the rings <NUM> being arranged concentrically around the inner portion <NUM>. Adjacent rings <NUM> are spaced apart by an annular inter-ring gap <NUM>. The inter-ring gaps <NUM> comprise conductive material since they are part of the radiating structure <NUM>. The size of the inter-ring gaps <NUM>, in particular the slot-to-slot width, may vary depending on the application, for example in order to tune the antenna <NUM>, e.g. with respect to resonant frequency(ies) and/or bandwidth. For any given inter-ring gap <NUM>, its width is preferably constant since this facilitates provision of a symmetrical ring arrangement.

The slots <NUM> may be formed in any conventional manner, e.g. by cutting, masking or etching. In any event, each slot <NUM> defines a non-conductive region of the radiating structure <NUM>, and the edges <NUM> of the slots <NUM> are interfaces between the non-conductive slot area and the surrounding conductive material of the radiating structure <NUM>, including the intra-ring gaps <NUM> and the inter-ring gaps <NUM>.

In preferred embodiments, the slots <NUM> are arranged such that the rings <NUM> are symmetrical about the, or each, axis of symmetry in the X-Y plane. Each ring <NUM> preferably has the same number of slots <NUM>. Preferably, the slots <NUM> all have the same width.

In preferred embodiments, each ring <NUM> comprises (only) two slots 40A, 40B. Each slot 40A, 40B is preferably the same size (preferably in length and width). Each slot 40A, 40B is shaped to form a respective half of the respective ring <NUM>. For example, in preferred embodiments in which the rings <NUM> are circular, each slot 40A, 40B is arc-shaped, preferably substantially semi-circular. In alternative embodiments, there may be more than two slots in each ring <NUM>. It is preferred however that there is an even number of slots <NUM> in each ring <NUM> since this facilitates creating symmetry about two perpendicular axes, which helps create the desired radiation field shape. It is found that resonant frequency reduction is adversely impacted by using any more than two slots per ring.

The slot(s) <NUM> of any one ring <NUM> are arranged with respect to the slot(s) <NUM> of the, or each, adjacent ring <NUM> such that the respective gap(s) <NUM> of adjacent rings <NUM> are not aligned along any axis in the X-Y plane. Advantageously, this non-aligned arrangement of slots <NUM>, creates a maze-like or meandering current path from the feed point <NUM> to the outer edges of the radiating structure <NUM>. As a result, the current path is relatively long (in comparison with cases where the gaps <NUM> are aligned), and this improves the minimisation achieved.

Preferably, the arrangement is such that the intra-ring gaps <NUM> of any two adjacent rings <NUM> are, collectively, evenly spaced apart around the centre of the rings <NUM>. For example, in the preferred embodiment (as illustrated) in which each ring <NUM> has two slots 40A, 40B, the slots 40A, 40B of any one ring <NUM> are angularly displaced about the ring centre by <NUM>° with respect to the slots 40A, 40B of the, or each, adjacent ring <NUM> such that the respective four gaps <NUM> (two of each ring) are angularly spaced apart by <NUM>° about the ring centre.

In a preferred embodiment (as illustrated), there are (only) four rings <NUM>. In another preferred embodiment (not illustrated), there are (only) three rings. In other embodiments there may be more than four or fewer than three rings. With each additional ring of slots, the resonant frequency of the antenna is reduced, which facilitates the desired miniaturisation. However, with each additional ring, there are diminishing returns with regard to the reduction of resonant frequency vs increased area, and the complexity required to add the additional rings.

In preferred embodiments, the antenna <NUM> generates a higher order resonant mode that is achieved by driving the feed structure <NUM> with an alternating excitation signal within the resonant frequency impedance bandwidth of the antenna <NUM>. By way of example, the antenna <NUM> may be configured to operate in the <NUM>, <NUM> and <NUM> Industrial and Scientific Medical (ISM) bands. The shorting posts 32A, 32B force 'nulls' in the E-field between the radiating element <NUM> and ground plane <NUM>. Accordingly, a higher order mode is generated which causes the antenna <NUM> to generate a monopole-like radiation pattern. The symmetrical maze-like pattern of slots <NUM> in the radiating structure <NUM> causes a corresponding pattern in surface current on the radiating structure <NUM>, which allows significant miniaturisation of the antenna <NUM> without disrupting the monopole-like radiation pattern, which is an important requirement for many commercial applications. For example, the dimensions (X x Y x Z) of a conventional higher mode antenna configured to operate in the <NUM> band is approximately <NUM> x <NUM> x <NUM>, whereas the dimensions of the antenna <NUM> are approximately <NUM> x <NUM> x <NUM> for the same operating band.

When the antenna <NUM> is mounted on a PCB or other substrate, the electric field (E-Field) is normal to the PCB/substrate and the antenna is sufficiently small that it is suitable for use in a broad range of applications. Having the E-field oriented in this way means that dominant propagating modes in dynamic and difficult environments are supported. By way of example, the antenna <NUM> may exhibit a performance improvement of up to <NUM> dB in comparison with conventional antennas, which can mean the difference between the relevant device of which the antenna is part working or not.

For any given application, the dimensions of the slots <NUM> may be determined through iterative design in simulation. Changing slot dimensions impacts a number of factors, mainly resonant frequency and bandwidth and so may be tuned according to the specific requirements of the application. For example, creating narrower slots <NUM> in the rings <NUM> reduces resonant frequency but also reduces bandwidth.

More generally, the following design considerations are noted. The overall X-Y dimensions of the radiating structure <NUM> are related to the desired wavelength, and increasing the X-Y dimensions decreases the resonant frequency of the antenna <NUM>. Adding a ring <NUM> reduces the resonant frequency and bandwidth. Reducing the inter-ring gap width reduces resonant frequency and bandwidth. Reducing slot width reduces resonant frequency and bandwidth. Increasing the height of the radiating structure <NUM> above the ground plane <NUM> increases bandwidth. Decreasing the diameter of the shorting connectors <NUM> reduces resonant frequency and bandwidth. Decreasing the feed connector <NUM> to shorting connector <NUM> spacing reduces resonant frequency and bandwidth. Any feature that reduces resonant frequency tends to reduce radiation efficiency to varying extents.

Claim 1:
An antenna (<NUM>) comprising:
a radiating structure (<NUM>);
a ground plane (<NUM>); and
a feed structure (<NUM>) coupled to the radiating structure,
wherein the radiating structure comprises a plurality of slots (<NUM>) located around an inner portion (<NUM>) of the radiating structure, the slots (<NUM>) being arranged symmetrically about at least one axis that extends along a surface of the radiating structure, the slots (<NUM>) being arranged to form a plurality of rings (<NUM>) around said inner portion, each ring (<NUM>) comprising one or more of said slots (<NUM>), characterized in that each ring (<NUM>) includes at least one intra-ring gap (<NUM>) between adjacent slot ends, and wherein the, or each, slot (<NUM>) of any one of said rings (<NUM>) is arranged with respect to the, or each, slot (<NUM>) of the, or each, adjacent ring (<NUM>) such that the respective intra-ring gaps (<NUM>) of adjacent rings (<NUM>) are not aligned along any axis that extends along the surface of the radiating structure (<NUM>).