SE544295C2 - Contactless millimetre-wave array antenna element - Google Patents

Abstract

An antenna element (100) for an array antenna (200) comprising a ridge gap waveguide (110), RGW, formed by a first conductive layer (120) and a second conductive layer. A ridge (140) is arranged along a waveguide path on any of the conductive layers (120). The ridge comprises a ridge matching section (141) arranged in connection to an open end of the RGW (111), where a cross sectional shape of the ridge (140) varies along the ridge matching section. The first conductive layer (120) comprises a metamaterial structure (150) arranged on both sides of the waveguide path, where the metamaterial structure is arranged to present a high-impedance surface facing the second conductive layer. The metamaterial structure comprises a metamaterial matching section (151) arranged in connection to the open end (111) of the RGW and in connection to the ridge matching section (141), where a distance between the metamaterial structure (150) and the ridge (140) varies along metamaterial matching section.

Description

TITLECONTACTLESS MILLIMETRE-WAVE ARRAY ANTEN NA ELEMENT TECHNICAL FIELD The present disclosure relates to radiating antenna elements, particularly antennaelements for array antennas, and to methods for producing antenna elements. Theantennas elements are suited for use in, e.g., telecommunication and radar transceivers.

BACKGROUND Wireless communication networks comprise radio frequency transceivers, such asradio base stations used in cellular access networks, microwave radio linktransceivers used for, e.g., backhaul into a core network, and satellite transceiverswhich communicate with satellites in orbit. A radar transceiver is also a radio frequency transceiver since it transmits and receives radio frequency signals.

Radio transceivers, in general, comprise antenna arrangements. An antennaarrangement may comprise an array antenna, which in turn comprises a plurality ofradiation elements, i.e., antenna elements. Existing wide-angle beam-steeringantenna arrangements at high frequencies, e.g., the millimeter-wave spectrum, mayutilize planar radiating elements based on printed circuit board (PCB), lowtemperature co-fried ceramics (LTCC), and monolithic microwave integrated circuit(MMIC) technology. Such elements, however, typically demonstrate low-efficiencyand narrow bandwidth at high frequencies. Hollow metal waveguide-basedconfigurations, such as array antennas of open-ended rectangular and ridgewaveguides, often present better efficiency, and bandwidth. Unfortunately, suchconfigurations have high design complexity and large manufacturing costs. lnaddition, waveguides often require a good electrical contact between conductingelements forming the waveguide structure and active electronics integration inside metal-enclosed waveguide channels is very challenging.

There is a need for high-performing antenna elements that are easy and cost-effective to manufacture.

SUMMARYlt is an object of the present disclosure to provide improved antenna elements, which, i.a., offer high performance and a simple and const-effective manufacturing process.

This object is at least in part obtained an antenna element for an array antenna. Theantenna element comprises a ridge gap waveguide, RGW, formed by a firstconductive layer and a second conductive layer, where each conductive layer isassociated with a respective thickness. The RGW comprises an open end arrangedto radiate electromagnetic waves. A ridge is arranged along a waveguide path on anyofthe conductive layers, where the ridge has a height in a normal direction to a surfaceon one of the conductive layers and a width in a direction substantially perpendicularto the height. The ridge comprises a ridge matching section arranged in connectionto the open end of the RGW, where wherein a cross sectional shape of the ridgevaries along the ridge matching section. The first conductive layer comprises ametamaterial structure arranged on both sides of the waveguide path, where themetamaterial structure is arranged to present a high-impedance surface facing thesecond conductive layer. The metamaterial structure is arranged on both sides of theridge at respective distances from the ridge, where the distances are measuredsubstantially perpendicular to a normal direction of a surface of one of the conductivelayers and substantially perpendicular to a tangent of the ridge. The metamaterialstructure comprises a metamaterial matching section arranged in connection to theopen end of the RGW and in connection to the ridge matching section, where at leastone of the distances between the metamaterial structure and the ridge varies along metamaterial matching section.

The combination of the ridge matching section and the metamaterial matching sectionadvantageously lowers the RGW cut-off frequency. With only a ridge matchingsection, the transformation of the ridge introduces reflections at lower frequencies,which increases RGW cut-off frequency. The lowered cut-off frequency leads to awideband impedance performance. According to aspects, the 10-dB impedancebandwidth is at least 30% for the broadside radiation regime. That is noticeably wider as compared to rectangular and ridge waveguide array elements.

The combination of the ridge matching section and the metamaterial matching sectionfurther reduces H-plane coupling between adjacent antenna elements in an array.The E-plane of the electromagnetic waves is in a plane along the extension direction of the ridge and the height direction of the ridge, and the H-plane is in a plane along the extension direction of the ridge and the width direction of the ridge. The reducedcoupling advantageously increases the H-plane beam-steering range. Thus, the H-plane beam-steering range of an array antenna comprising the disclosed antennaelement is noticeably wider compared to rectangular and ridge waveguide array elements.

The contactless design due to the RGW configuration allows for relaxedmanufacturing and assembling tolerances and provides an additional volume which simplifies active electronics integration to the antenna element.

The disclosed antenna element presents all of the above mentioned advantageswithout comprising radiation efficiency, which remains the same as for previously known metal-enclosed hollow waveguide array elements.

According to aspects, the metamaterial structure comprises a repetitive structure ofconductive pins arranged periodically protruding from the first conductive layer. This provides a high-performing metamaterial structure that is easy to manufacture.

According to aspects, wherein the metamaterial matching section comprises a firstpart arranged at a first distance from the ridge and a second part arranged at a seconddistance from the ridge. This stepped type of matching effectively lowers the RGW cut-off frequency while being easy to manufacture.

According to aspects, any of the conductive layers comprise a conductive layerdecoupling section arranged in connection to the open end of the RGW and inconnection to the ridge matching section, where the thickness of the conductive layervaries along the conductive layer decoupling section. The conductive layer decouplingsection reduces E-plane coupling between adjacent antenna elements in an array.The reduced coupling advantageously increases the E-plane beam-steering range.Thus, the E-plane beam-steering range of an array antenna comprising the disclosedantenna element is noticeably wider compared to rectangular and ridge waveguide array elements.

According to aspects, the conductive layer decoupling section comprises a first partwith a first thickness and a second part with a second thickness. According to furtheraspects, the conductive layer decoupling section comprises a flat surface facing the waveguide path. This enables a simple and effective manufacturing process.

According to aspects, wherein the ridge matching section comprises a stepped height impedance transformer, a tapered height impedance transformer, a stepped width impedance transformer, and/or a tapered width impedance transformer. Any of suchtransformers provides a wideband impedance matching that enables widebandimpedance performance of the antenna element. Any of these transformers are also easy to manufacture.

There is also disclosed herein a telecommunication or radar transceiver comprising the antenna element according to the discussion above.

There is also disclosed herein an array antenna comprising a plurality of the antenna element according to the discussion above.

According to aspects, the metamaterial structure of the array antenna comprises arepetitive structure of conductive pins arranged periodically protruding from the firstconductive layer, wherein metamaterial matching section comprises a first part with asingle row of pins and a second part with two rows of pins, where the rows extendalong the ridge. This way adjacent antenna elements in the H-plane direction canshare protruding pins, which reduces the size of the array antenna without sacrificing performance.

There is also disclosed herein a method for producing an antenna element, the method comprising:providing a first conductive layer having a metamaterial structure; arranging a second conductive layer to face the first conductive layer, wherein each conductive layer is associated with a respective thickness; arranging a ridge along a waveguide path on any of the conductive layers, the ridgehaving a height in a normal direction to a surface on one of the conductive layers anda width in a direction substantially perpendicular to the height, thereby forming a ridge gap waveguide, RGW, comprising an open end to radiate electromagnetic waves, wherein the metamaterial structure is arranged on both sides of the waveguide path,and wherein the metamaterial structure is arranged to present a high-impedancesurface facing the second conductive layer, and wherein the metamaterial structureis arranged on both sides of the ridge at respective distances from the ridge, wherethe distances are measured substantially perpendicular to a normal direction of asurface of one of the conductive layers and substantially perpendicular to a tangent of the ridge, arranging a ridge matching section on the ridge in connection to the open end of theRGW, wherein a cross sectional shape of the ridge varies along the ridge matching section; and arranging a metamaterial matching section on the metamaterial structure inconnection to the open end of the RGW and in connection to the ridge matchingsection, wherein at least one of the distances between the metamaterial structure and the ridge varies along metamaterial matching section.

The methods disclosed herein are associated with the same advantages as discussedabove in connection to the different apparatuses. There are furthermore disclosed herein control units adapted to control some of the operations described herein.

Generally, all terms used in the claims are to be interpreted according to their ordinarymeaning in the technical field, unless explicitly defined othenNise herein. Allreferences to "a/an/the element, apparatus, component, means, step, etc." are to beinterpreted openly as referring to at least one instance of the element, apparatus,component, means, step, etc., unless explicitly stated othenNise. The steps of anymethod disclosed herein do not have to be performed in the exact order disclosed,unless explicitly stated. Further features of, and advantages with, the presentinvention will become apparent when studying the appended claims and the followingdescription. The skilled person realizes that different features of the present inventionmay be combined to create embodiments other than those described in the following, without departing from the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS The present disclosure will now be described in more detail with reference to the appended drawings, where Figures 1A-1C schematically illustrate different perspectives of an example antenna element, Figures 2A-2C schematically illustrate different perspectives of an example array antenna, and Figure 3 is a flow chart illustrating methods.

DETAILED DESCRIPTION Aspects of the present disclosure will now be described more fully with reference tothe accompanying drawings. The different devices and methods disclosed herein can,however, be realized in many different forms and should not be construed as beinglimited to the aspects set forth herein. Like numbers in the drawings refer to like elements throughout.

The terminology used herein is for describing aspects of the disclosure only and isnot intended to limit the invention. As used herein, the singular forms "a", "an" and"the" are intended to include the plural forms as well, unless the context clearly indicates othenNise.

As mentioned, there is a need for high-performing antenna elements that are easyand cost-effective to manufacture, especially at millimeter-waves. ln a preferredembodiment, the present disclosure proposes an antenna element 100 for an arrayantenna 200 design based on an open-ended ridge gap waveguide (RGW) - astructure that employs a tvvo-dimensional electromagnetic bandgap structure allowinga contactless design with an air gap between forming metal parts. The disclosure isespecially beneficial for use in millimeter-wave (e.g., W-band and above) wide-anglebeam-steering antenna systems due to the element's high radiation efficiency, wideimpedance bandwidth, and contactless configuration, which significantly alleviatesmanufacturing and assembling tolerances and provides an additional physical volume for active beamforming circuitry integration inside the waveguide.

Figures 1A-1C show different perspectives of an example of the disclosed antennaelement 100. The antenna element comprises an RGW 110 formed by a firstconductive layer 120 and a second conductive layer 130, where each conductive layeris associated with a respective thickness. The RGW comprises an open end 111arranged to radiate electromagnetic waves. Furthermore, a ridge 140 is arrangedalong a waveguide path on any of the conductive layers 120, 130. The ridge has aheight in a normal direction to a surface on one of the conductive layers and a widthin a direction substantially perpendicular to the height. The ridge further comprises aridge matching section 141 arranged in connection to the open end of the RGW 111,where a cross sectional shape of the ridge 140 varies along the ridge matchingsection. The first conductive layer 120 comprises a metamaterial structure 150arranged on both sides of the waveguide path, where the metamaterial structure isarranged to present a high-impedance surface facing the second conductive layer 130. The metamaterial structure is arranged on both sides of the ridge 140 at respective distances from the ridge, where the distances are measured substantiallyperpendicular to a normal direction of a surface of one of the conductive layers 120,130 and substantially perpendicular to a tangent of the ridge. Furthermore, themetamaterial structure comprises a metamaterial matching section 151 arranged inconnection to the open end 111 of the RGW and in connection to the ridge matchingsection 141, where at least one of the distances between the metamaterial structure 150 and the ridge 140 varies along metamaterial matching section. ln Figures 1A-1C, the E-plane of the electromagnetic waves is the yz-plane, and theH-plane is the xz-plane. ln the figures, the first conductive layer 120 with themetamaterial structure 150 and the second conductive layer 130 are arranged in acontactless manner and form an air-filled gap between the second conductive layer130 and the metamaterial structure 150. Electromagnetic energy is concentrated inthe gap between the ridge 140 and second conductive layer 130. ln the exampleelement, the metamaterial structure 150 comprises a repetitive structure of conductivepins arranged periodically protruding from the first conductive layer 120. The pins maybe integrally and preferably monolithically formed on the conductive layer. Othermetamaterial structures are also possible, which are discussed in more detail below.The pin structure and the second conductive layer 130 form a two-dimensionalelectromagnetic bandgap structure, thus blocking electromagnetic leakage from thewaveguide path, to, e.g., adjacent antenna elements in an array. Preferably, at leasttwo rows of pins are arranged at the element's input for sufficient isolation. ln Figures 1A and 1B, the input is arranged on the left side of the element.

Preferably, the distances between the ridge and the metamaterial structure onrespective sides of ridge are equal, i.e., at a particular point along the ridge, thedistance from the ridge to the metamaterial structure on one side is equal to thedistance from the ridge to the metamaterial structure on the other side. This providesa symmetry to the RGW. Preferably, the distances along the metamaterial matchingsections are also equal to provide symmetry. As mentioned, the distances betweenthe metamaterial structure 150 and the ridge measured substantially perpendicular toa normal direction of a surface of one of the conductive layers 120, 130 andsubstantially perpendicular to a tangent of the ridge. ln Figure 1B, these distances arein the x-dimension. The tangent of the ridge can extend from a center point along thewidth of the ridge. Substantially perpendicular to a normal direction of a surface of one of the conductive layers means along the surface of the conductive layer.

A layer is planar element with t\No sides, or faces, and is associated with a thickness.The thickness is much smaller than the dimension of the faces, i.e., the layer is a flator approximately planar element, i.e., as in an arcuate shape. According to someaspects, a layer is rectangular or square. However, more general shapes are alsoapplicable, including circular or elliptical disc shapes. The conductive layers may bemade from a metal such as copper or aluminum, or from a non-conductive materiallike PTFE or FR-4 coated with a thin layer of an electrically conductive material likegold or copper. The conductive layers may also be made from a material with anelectric conductivity comparable to that of a metal, such as a carbon nanostructure orelectrically conductive polymer. As an example, the electric conductivity of theconductive layers can be above 103 Siemens per meter (S/m). Preferably, the electric conductivity is above 105 S/m. ln Figures 1A-1C, the waveguide path is a straight path, and the ridge comprises arectangular cross section and extends in a straight direction. More general shapes ofthe waveguide path and the ridge are possible. For example, both the waveguide pathand the ridge may extend in an S-shape, i.e., when viewed from the perspective inFigure 1B, the ridge may form an S-shape. The cross section of the ridge may bemore general as well. For example, the cross section may have a half-circle shape.

Such shape still has a width and height according to the discussions above. ln an array antenna 200 comprising a plurality of the antenna element 100, theelement input can be connected to a beamforming network that comprises a powerdistribution system and phase-shifting circuitry. A distribution system can comprise: acorporate feed network based on RGW power combiner/dividers, where all H-planefeed networks are interconnected through a common E-plane corporate feed network;or a quasi-optical feed network providing a spatial excitation of array antenna RGWinputs inside a common electrically large waveguide environment. The ridge can bearranged on any of the first 120 and the second 130 conductive layers, but is preferably arranged on the first conductive layer with the metamaterial structure 150.

On the other side of the antenna element, i.e., at the open end 111, the RGWcomprises the ridge matching section 141. The ridge matching section 141 providesan impedance matching between the RGW and free space, thereby enabling efficientradiation through the RGW element open end 111. ln the example antenna elementin the figures, the ridge matching section 141 comprises a stepped height impedance transformer. ln particular, the ridge 140 comprises a first, second, and third height at the ridge matching section, where the ridge comprises the third height elsewhere. lnother words, the matching section comprises two steps, where the height of the ridgeis reduced at each step. Any numbers of steps are also possible, but at least tvvo arepreferred to achieve a wideband impedance matching. The ridge matching section141 may alternatively, or in combination, comprise any of a tapered height impedancetransformer, a stepped width impedance transformer, and a tapered width impedancetransformer. Other shapes of the impedance transformation are also possible, i.e., any change in cross sectional shape of the ridge. ln Figure 1B, it can be seen that the metamaterial matching section 151 is arrangedin connection to the open end 111 of the RGW and in connection to the ridge matchingsection 141, where the distances between the metamaterial structure 150 and theridge 140 varies along metamaterial matching section. ln particular, on both sides ofthe ridge, the metamaterial matching section 151 comprises a first part arranged at afirst distance from the ridge 140 and a second part arranged at a second distancefrom the ridge. ln the figure, the first part is to the left and the second part is to theright. lt can further be seen that the metamaterial structure is arranged at the firstdistance from the ridge for the remainder of the RGW to the left of the metamaterialmatching section. ln the example of the figure, the metamaterial matching sectioncomprises a transition from an arrangement with two rows of pins (in the x-dimension)to a single row of pins. ln general, the metamaterial matching section can be arrangedat three or more discrete distances, arranged in a tapered fashion, or more general transitions.

According to aspects, the start of the distance variation of the metamaterial matchingsection is arranged directly adjacent to the start of the cross sectional variation of theridge matching section. For example, if the metamaterial matching section comprisest\No discrete distances on respective sides of the ridge and the ridge matching sectioncomprises t\No discrete heights of the ridge, the step from the first distance to thesecond distance can be directly adjacent to the transition from the first ridge height tothe second ridge height. Directly adjacent can mean in a direction perpendicular to a tangent of ridge at the height transition of the ridge.

According to other aspects, the ridge matching section is arranged on a section of theridge that has a length in the extension direction of the ridge equivalent to 0.5-10wavelengths ofthe operational frequency of the antenna element. According to further aspects the metamaterial matching section is arranged on a section of the metamaterial structure that has a length in the extension direction of the ridgeequivalent to 0.5-10 wavelengths ofthe operational frequency of the antenna element.According to additional aspects, the sections of the of the ridge matching section andthe metamaterial matching section have the same lengths. The lengths of thesesections may be measured from the start of the variation (of cross sectional shape orof distances) to the open end 111 of the RGW.

The combination of the ridge matching section and the metamaterial matching sectionadvantageously lowers the RGW cut-off frequency. As mentioned, the ridge matchingsection provides an impedance matching between the RGW and free space, therebyenabling efficient radiation through the RGW element open end. With only a ridgematching section, however, i.e., without the metamaterial matching section, thetransformation of the ridge introduces reflections at lower frequencies, whichincreases the RGW cut-off frequency. Such reflections arise from a change in wavepropagation conditions introduced by the varying cross sectional shape of the ridge.The combination of the ridge matching section and the metamaterial matching sectionallows the RGW to have an effective impedance matching of the ridge while notintroducing any unwanted reflections. Thereby, the combination results in a lower cut-off frequency compared to prior art. The lowered cut-off frequency, in turn, leads to amore wideband impedance performance. According to aspects, the 10-dB impedancebandwidth is at least 30% for the broadside radiation regime. That is noticeably wider compared to rectangular and ridge waveguide array elements.

The combination of the ridge matching section and the metamaterial matching sectionfurther reduces H-plane coupling (i.e., in the x-dimension in the figures) betweenadjacent antenna elements 100 in an array. An array antenna comprising open-endedwaveguides typically suffers from unwanted coupling between the elements. Themetamaterial matching section and the combination of the ridge matching section andthe metamaterial matching section also introduce coupling between elements.However, the matching sections can be arranged such that the resulting couplingdestructively interferes with the coupling that is originally present. Thereby, the overallThe advantageously increases the H-plane beam-steering range. Thus, the H-plane coupling between antenna elements is lowered. reduced coupling beam-steering range of an array antenna comprising the disclosed antenna element 100 is noticeably wider compared to rectangular and ridge waveguide array elements. 11 The contactless design due to the RGW configuration allows for relaxedmanufacturing and assembling tolerances and provides an additional physical volume which simplifies active electronics integration to the antenna element.

The disclosed antenna element presents all of the above-mentioned advantageswithout compromising radiation efficiency, which remains the same as for previously known metal-enclosed hollow waveguide array elements.

The antenna element 100 may comprise E-plane grooves, i.e., any of the conductivelayers 120, 130 may comprise a conductive layer decoupling section 121, 131arranged in connection to the open end 111 of the RGW and in connection to theridge matching section 141, where the thickness of the conductive layer varies alongthe conductive layer decoupling section 121, 131. ln particular, the conductive layerdecoupling section 121, 131 may comprise a first part with a first thickness and asecond part with a second thickness. ln the example of Figure 1A, both conductivelayers 120, 130 comprise respective conductive layer decoupling sections with firstand second parts. The first parts are arranged to the left in the figure and the secondparts are arranged to the right in the figure at the open end 111 of the RGW. Theconductive layers of the rest of the RGW have thickness equal to the first thickness.ln the example of Figure 1A, the conductive layer decoupling section 121, 131comprises a flat surface facing the waveguide path. ln other words, a grove isarranged on the surface facing away from the waveguide path. ln general, theconductive layer decoupling section 121, 131 can be arranged at three or morediscrete thickness, or the thickness can be arranged in a tapered fashion or in moregeneral transitions. The placement of the conductive layer decoupling section may bearranged in connection to the open end 111 of the RGW and the ridge matchingsection 141 in a similar way as the placement of the metamaterial matching section 151, as discussed above.

The conductive layer decoupling section 121, 131 reduces E-plane coupling (i.e., inthe y-dimension in the figures) between adjacent antenna elements 100 in an array.The decoupling section forms a groove between adjacent elements. This groove isarranged to form an electromagnetic soft surface. Such surface is arranged to prohibitelectromagnetic waves to propagate past it. According to aspects, the groove formedby the conductive layer decoupling section 121, 131 has a length in the extensiondirection of the RGW corresponding to a quarter of a wavelength of the operational frequency of the antenna element. 12 The reduced coupling advantageously increases the E-plane beam-steering range.Thus, the E-plane beam-steering range of an array antenna comprising the disclosedantenna element 100 is noticeably wider as compared to rectangular and ridge waveguide array elements.

According to aspects, the conductive layer decoupling section 121, 131 comprisesone or more grooves in the extension direction of the RGW arranged at the open end111 of the RGW. ln other words, in addition to the varying thickness of the conductivelayer in Figure 1A, a groove can be arranged at the open end of the of the RGW. Thisgroove may extend a similar length along the RGW as the thickness variation of theconductive layer decoupling section. Such groove may have a rectangular shape.Such groove may also extend along the whole conductive layer. ln the array antennain Figure 2A, the conductive layer decoupling sections form a single groove betweenthe elements in the y-dimension. Additional grooves in the conductive layer can reduce E-plane coupling further.

Some example dimensions of the example antenna element 100 in Figures 1A-1C,arranged to operate at a center frequency of 95 GHz, are: a third ridge height of 0.71mm (to the left in the figures); a second ridge height of 0.62 mm; a first ridge height of0.38 mm; a ridge width of0.3 mm; a conductive layerthickness of0.5 mm; an E-planegroove with a depth of 0.3 mm and a length of 0.75 mm in the extension direction ofthe ridge; a first and a second distance between the pins and the ridge (i.e., thedistance between the sidewall of the ridge to the nearest pin sidewall along x-axis) of0.3 mm and 0.45 mm, respectively; a square cross section of the pins of 0.3 mm by0.3 mm; a height of the pins of 0.76 mm; and a distance between centers of adjacentpins of 0.6 mm. The ridge matching section comprises three different heights wherethe first height is arranged on a section of the ridge with a length of 0.74 mm and thesecond height is arranged on a section of the ridge with a length of 0.67 mm. The firstdistance between the pins from the ridge extends on part of the metamaterial section with a dimension of 1.8 mm in a direction along the extension direction of the ridge.

There is also disclosed herein an array antenna 200 comprising a plurality of theantenna element 100 according to the discussions above. The disclosed elementscan be arranged in a rectangular array grid as shown in Figures 2A-2C to form arequired aperture configuration of an electronically scanned array antenna. ln thefigure, a row of elements is arranged in the x-dimension. The elements can, alternatively, be arranged in a triangular grid, i.e., every other row is offset in the x- 13 dimension compared to adjacent rows above and below. More general configurationsof the elements are also possible, where the elements preferably are arranged periodically in one or two planes (x- and y-dimensions). ln an example of the array antenna 200, the first conductive layer comprises a ridgeand pins formed in a single manufacturing process (e.g., CNC milling, siliconmicromachining, 3D printing, etc.). ln the H-plane, i.e., x-dimension, multiple antennaelements can be fabricated on a common bottom metal plate as shown in Figures 2Band 2C. ln Figure 2A, the first conductive layer of one element is separate from thesecond conductive layer of an element below. However, these two layers may beformed from a single conductive layer. ln other words, the bottom metal plate of eachrow of elements can be used to form the top metal plate for the lower row of array elements. Here bottom means downward in Figures 2A and 2C. ln an example array antenna 200, the metamaterial structure 150 comprises arepetitive structure of conductive pins arranged periodically protruding from the firstconductive layer 120. Furthermore, the metamaterial matching section 151 comprisesa first part with a single row of pins and a second part with two rows of pins, wherethe rows extend along the ridge 140. More specifically, the first part is arranged at afirst distance from the ridge 140 and the second part is arranged at a second distancefrom the ridge. ln Figures 2A-2C, the rest of the RWG elements comprise two rows of pins adjacent to both sides of the respective ridges.

The antenna elements in the example array antenna in Figures 2A-2C are comprisedof a unit cell 160 of the antenna element in Figures 1B and 1C. ln the array, adjacentantenna elements 100 in the x-dimension share protruding pins between the adjacent elements.

There is also disclosed herein a telecommunication or radar transceiver comprisingthe antenna element 100 and/or an array antenna 200 according to the discussions above.

The metamaterial structure is arranged to form a high impedance surface, such as anartificial magnetic conductor (AMC). lf the high impedance faces an electricallyconductive surface (i.e., a low impedance surface such as a perfect electric conductor,PEC, in the ideal case), and if the two surfaces are arranged at a distance apart lessthan a quarter of a wavelength at a center frequency, no electromagnetic waves in afrequency band of operation can, in the ideal case, propagate along or between the intermediate surfaces since all parallel plate modes are cut-off in that frequency band. 14 The two surfaces may also be arranged directly adjacent to each other, i.e.,electrically connected to each other. The center frequency is often in the middle of thefrequency band of operation. ln a realistic scenario, the electromagnetic waves in thefrequency band of operation are attenuated per length along the intermediatesurfaces. Herein, to attenuate is interpreted as to significantly reduce an amplitude orpower of electromagnetic radiation, such as a radio frequency signal. The attenuationis preferably complete, in which case attenuate and block are equivalent, but it is appreciated that such complete attenuation is not always possible to achieve The metamaterial structure may replace the walls of a waveguide to form a gapwaveguide. Such waveguide may comprise a ridge to form a ridge gap waveguide(RGW).

The metamaterial structure 150 of the first conductive layer 120 in the antennaelement 100 is arranged to form a high-impedance surface, e.g., an AMC surface,that faces the second conductive layer 130. This high-impedance surface, togetherwith the second conductive layer 130, which is a low impedance surface and in theideal case a PEC surface, forms an electromagnetic bandgap between the twosurfaces. Thus, the metamaterial structure 150 and the second conductive layer 130are arranged to prevent electromagnetic waves in a frequency band of operation frompropagating intermediate the metamaterial structure and the second conductive layer130, except along intended waveguide paths, i.e., along the RGW. Advantageously,the metamaterial structure enables compact designs, low loss, low leakage, and forgiving manufacturing and assembling tolerances.

There exists a multitude of metamaterial structures. Such structures often compriseelements arranged in a periodic or quasi-periodic pattern in one, two or threedimensions. Herein, a quasi-periodic pattern is interpreted to mean a pattern that islocally periodic but displays no long-range order. A quasi-periodic pattern may berealized in one, two or three dimensions. As an example, a quasi-periodic pattern canbe periodic at length scales below ten times an element spacing, but not at length scales over 100 times the element spacing.

A metamaterial structure may comprise at least two element types, the first type ofelement comprising an electrically conductive material and the second type ofelement comprising an electrically insulating material. Elements of the first type maybe made from a metal such as copper or aluminum, or from a non-conductive material like PTFE or FR-4 coated with a thin layer of an electrically conductive material like gold or copper. Elements of the first type may also be made from a material with anelectric conductivity comparable to that of a metal, such as a carbon nanostructure orelectrically conductive polymer. As an example, the electric conductivity of elementsof the first type can be above 103 Siemens per meter (S/m). Preferably, the electricconductivity of elements of the first type is above 105 S/m. ln other words, the electricconductivity of elements of the first type is high enough that the electromagneticradiation can induce currents in the elements of the first type, and the electricconductivity of elements of the second type is low enough that no currents can beinduced in elements of the second type. Elements of the second type may optionallybe non-conductive polymers, vacuum, or air. Examples of such non-conductive element types also comprise FR-4 PCB material, PTFE, plastic, rubber, and silicon.

Elements of the first and second type may be arranged in a pattern characterized byany of translational, rotational, or glide symmetry, or a periodic, quasi-periodic or irregular pattern.

The physical properties of the elements of the second type also determines thedimensions required to obtain attenuation of electromagnetic propagation past themetamaterial structure. Thus, if the second type of material is chosen to be differently from air, the required dimensions of the first type of element changes.

The elements of the first type may be arranged in a periodic pattern with somespacing. The spaces between the elements of the first type constitute the elements ofthe second type. ln other words, the elements of the first type are interleaved withelements of the second type. lnterleaving of the elements of the first and second type can be achieved in one, two or three dimensions.

A size of an element of either the first or the second type, or both, is smaller than thewavelength in air of electromagnetic radiation in the frequency band. As an example,defining the center frequency as the frequency in the middle of the frequency band,the element size is between 1/5th and 1/50th of the wavelength in air ofelectromagnetic radiation at the center frequency. Here, the element size isinterpreted as the size of an element in a direction where the electromagnetic wavesare attenuated, e.g., along a surface that acts as a magnetic conductor. As anexample, for an element comprising a vertical rod with a circular cross-section andwith electromagnetic radiation propagating in the horizontal plane, the size of the element corresponds to a length or diameter of the cross-section of the rod. 16 A type of metamaterial structure comprises electrically conductive protrusions on anelectrically conductive substrate. The protrusions may optionally be encased in adielectric material. lt is appreciated that the protrusions may be formed in manydifferent shapes, like a square, circular, elliptical, rectangular, or more generally shaped cross-sections. lt is also possible that the protrusions are mushroom shaped, as in, e.g., a cylindricalrod on an electrically conductive substrate with a flat electrically conductive circle ontop of the rod, wherein the circle has a cross section larger than the cross section ofthe rod, but small enough to leave space for the second element type between thecircles in the metamaterial structures. Such mushroom-shaped protrusion may beformed in a printed circuit board, wherein the rod comprises a via hole, which may or may not be filled with electrically conductive material.

The protrusions have a length in a direction facing away from the electricallyconductive substrate. ln general, if the element of the second type is air, the protrusionlength corresponds to a quarter of the wavelength in air at the center frequency. Thesurface along the tops of the protrusions is then close to a perfect magnetic conductorat the center frequency. Even though the protrusions are only a quarter wavelengthlong at a single frequency, it presents a high-impedance surface at a frequency bandaround that single frequency. This type of metamaterial structure thus presents a bandof frequencies where electromagnetic waves may be attenuated, when themetamaterial structure faces a low impedance surface. ln a non-limiting example, thecenter frequency is 15 GHz and electromagnetic waves in the frequency band 10 to20 GHz propagating intermediate the metamaterial structure and an electrically conductive surface are attenuated.

As another example, a type of metamaterial structure comprises a single slab ofelectrically conductive material into which cavities have been introduced. The cavitiesmay be air-filled or filled with a non-conductive material. lt is appreciated that thecavities may be formed in different shapes such as elliptical, circular, rectangular, ormore general cross-section shapes. ln general, the length (in a direction facing awayfrom the electrically conductive substrate) corresponds to a quarter of the wavelength at the center frequency.

There is also disclosed herein a method for producing an antenna element 100, as is shown in Figure 3. The method comprises: providing S1 a first conductive layer 120 having a metamaterial structure 150; 17 arranging S2 a second conductive layer 130 to face the first conductive layer 120, where each conductive layer is associated with a respective thickness; arranging S3 a ridge 140 along a waveguide path on any of the conductive |ayers 120,130, the ridge having a height in a normal direction to a surface on one of theconductive |ayers and a width in a direction substantially perpendicular to the height,thereby forming a ridge gap waveguide, RGW, comprising an open end 111 to radiate electromagnetic waves, wherein the metamaterial structure 150 is arranged on both sides of the waveguidepath, and wherein the metamaterial structure is arranged to present a high-impedancesurface facing the second conductive layer 130, and wherein the metamaterialstructure is arranged on both sides of the ridge 140 at respective distances from theridge, where the distances are measured substantially perpendicular to a normaldirection of a surface of one of the conductive |ayers 120, 130 and substantially perpendicular to a tangent of the ridge, arranging S4 a ridge matching section 141 on the ridge 140 in connection to the openend of the RGW 111, wherein a cross sectional shape of the ridge 140 varies along the ridge matching section; and arranging S5 a metamaterial matching section 151 on the metamaterial structure 150in connection to the open end 111 of the RGW and in connection to the ridge matchingsection 141, wherein at least one of the distances between the metamaterial structure 150 and the ridge 140 varies along metamaterial matching section.

Claims (14)

1. An antenna element (100) for an array antenna (200), the antenna elementcomprising a ridge gap waveguide (110), RGW, formed by a first conductive layer(120) and a second conductive layer (130), wherein each conductive layer isassociated with a respective thickness, the RGW comprising an open end (111) arranged to radiate electromagnetic waves, wherein a ridge (140) is arranged along a waveguide path on any of the conductivelayers (120, 130), the ridge having a height in a normal direction to a surface on oneof the conductive layers and a width in a direction substantially perpendicular to theheight, wherein the ridge comprises a ridge matching section (141) arranged inconnection to the open end of the RGW (111), wherein a cross sectional shape of the ridge (140) varies along the ridge matching section, wherein the first conductive layer (120) comprises a metamaterial structure (150)arranged on both sides of the waveguide path, wherein the metamaterial structure isarranged to present a high-impedance surface facing the second conductive layer(130), and wherein the metamaterial structure is arranged on both sides of the ridge(140) at respective distances from the ridge, where the distances are measuredsubstantially perpendicular to a normal direction of a surface of one of the conductive layers (120, 130) and substantially perpendicular to a tangent of the ridge, wherein the metamaterial structure comprises a metamaterial matching section (151)arranged in connection to the open end (111) of the RGW and in connection to theridge matching section (141), wherein at least one of the distances between themetamaterial structure (150) and the ridge (140) varies along metamaterial matching section.

2. The antenna element (100) according to claim 1, wherein the metamaterialstructure (150) comprises a repetitive structure of conductive pins arranged periodically protruding from the first conductive layer (120).

3. The antenna element (100) according to any previous claim, wherein themetamaterial matching section (151) comprises a first part arranged at a first distance from the ridge (140) and a second part arranged at a second distance from the ridge.

4. The antenna element (100) according to any previous claim, wherein any of theconductive layers (120, 130) comprise a conductive layer decoupling section (121, 131) arranged in connection to the open end (1 1 1) of the RGW and in connection to the ridge matching section (141 ), wherein the thickness of the conductive layer varies along the conductive layer decoupling section (121, 131).

5. The antenna element (100) according to claim 4, wherein the conductive layerdecoupling section (121, 131) comprises a first part with a first thickness and a second part with a second thickness.

6. The antenna element (100) according to any of claims 4-5, wherein theconductive layer decoupling section (121, 131) comprises a flat surface facing the waveguide path.

7. The antenna element (100) according to any previous claim, wherein the ridge matching section (141) comprises a stepped height impedance transformer.

8. The antenna element (100) according to any previous claim, wherein the ridge matching section (141) comprises a tapered height impedance transformer.

9. The antenna element (100) according to any previous claim, wherein the ridge matching section (141) comprises a stepped width impedance transformer.

10. The antenna element (100) according to any previous claim, wherein the ridge matching section (141) comprises a tapered width impedance transformer.

11. A telecommunication or radar transceiver comprising the antenna element (100) according to any of claims 1-10.

12. An array antenna (200) comprising a plurality of the antenna element (100) according to any of claims 1-10.

13. The array antenna (200) according to claim 12, wherein the metamaterialstructure (150) comprises a repetitive structure of conductive pins arrangedperiodically protruding from the first conductive layer (120), wherein metamaterialmatching section (151) comprises a first part with a single row of pins and a second part with two rows of pins, wherein the rows extend along the ridge (140).

14. A method for producing an antenna element (100), the method comprising:providing (S1) a first conductive layer (120) having a metamaterial structure (150); arranging (S2) a second conductive layer (130) to face the first conductive layer (120), wherein each conductive layer is associated with a respective thickness; arranging (S3) a ridge (140) along a waveguide path on any of the conductive layers (120, 130), the ridge having a height in a normal direction to a surface on one of the conductive layers and a width in a direction substantially perpendicular to the height,thereby forming a ridge gap waveguide, RGW, comprising an open end (111) to radiate electromagnetic waves, wherein the metamaterial structure (150) is arranged on both sides of the waveguidepath, and wherein the metamaterial structure is arranged to present a high-impedancesurface facing the second conductive layer (130), and wherein the metamaterialstructure is arranged on both sides of the ridge (140) at respective distances from theridge, where the distances are measured substantially perpendicular to a normaldirection of a surface of one of the conductive layers (120, 130) and substantially perpendicular to a tangent of the ridge, arranging (S4) a ridge matching section (141) on the ridge (140) in connection to theopen end of the RGW (111), wherein a cross sectional shape of the ridge (140) varies along the ridge matching section; and arranging (S5) a metamaterial matching section (151) on the metamaterial structure(150) in connection to the open end (1 11) of the RGW and in connection to the ridgematching section (141), wherein at least one of the distances between themetamaterial structure (150) and the ridge (140) varies along metamaterial matching section.