F1ELD OF THE INVENTION

The present invention relates to leaky transmission lines. BACKGROUND

The following background description art may include insights, discov eries, understandings or disclosures, or associations together with disclosures not known to the relevant art prior to the present invention but provided by the pre sent disclosure. Some such contributions disclosed herein may be specifically pointed out below, whereas other such contributions encompassed by the present disclosure the invention will be apparent from their context.

Leaky-wave structures such as leaky-wave antennas, leaky waveguides and leaky cables are modified transmission-line structures which enable a part of the electromagnetic energy propagating inside a transmission line as electromag netic waves to leak from the transmission line to the outside space in a controlled manner. Conventionally, this leakage is achieved by providing one or more open ings (or slots) in the outer conductor of the otherwise closed transmission-line structure. Leaky-wave structures have found applications especially in closed en vironments where radio communication needs to be provided for moving vehicles, for example, in tunnels, underground roads and subways ln such scenarios, con ventional antenna solutions (i.e., point source antenna solutions) provide insuffi cient coverage unless a large number of periodically distributed antennas is em ployed.

The radiation pattern provided by most leaky-wave structures is roughly omnidirectional, that is, the radiation is spread almost equally to all direc tions orthogonal to the longitudinal direction of the leaky-wave structure (e.g., a direction along a length of a leaky cable/waveguide). While the omnidirectionality of the provided radiation is not a problem in many of the aforementioned multi- path-heavy closed environments, this property limits the use of leaky-wave struc tures in many application where there is a need for higher gain in a particular di rection. For example, in a scenario where a leaky-wave structure is arranged along a corridor of an office building leading to multiple offices located in either side of the corridor, a large part of the radiated electromagnetic energy is wasted if an om nidirectional leaky-wave structure is used. Thus, there is need for a leaky-wave so lution providing more adjustable radiation performance compared to the conven tional omnidirectional solutions. SUMMARY

The following presents a simplified summary of features disclosed herein to provide a basic understanding of some exemplary aspects of the inven tion. This summary is not an extensive overview of the invention lt is not intended to identify key/critical elements of the invention or to delineate the scope of the invention lts sole purpose is to present some concepts disclosed herein in a sim plified form as a prelude to a more detailed description.

Various embodiments of the invention comprise an apparatus and a method as defined in the independent claims. Further embodiments of the inven- tion are disclosed in the dependent claims.

According to an aspect, there is provided a leaky transmission-line structure for guiding electromagnetic waves, the leaky transmission-line structure comprising a section with at least one inner conductor, an outer conductor enclos ing said at least one inner conductor and a layer of a first dielectric material sepa- rating the outer conductor from said at least one inner conductor, wherein the outer conductor comprises one or more openings arranged along a longitudinal di rection of the section enabling leaking of the electromagnetic waves from the leaky transmission-line structure; and one or more resonator antennas mounted over said one or more openings so as to be excitable by the electromagnetic waves leak- ing from the leaky transmission-line structure and adapted to be resonant at one or more radio frequencies supported by the leaky trans-mission-line structure.

According to another aspect, there is provided a method comprising providing a leaky transmission-line structure for guiding electromagnetic waves, the leaky transmission-line structure comprising one or more openings arranged along a longitudinal direction of the leaky transmission-line structure so as to ena ble leaking of the electromagnetic waves from the leaky transmission-line struc ture; providing one or more resonator antennas; and attaching said one or more resonator antennas over said one or more openings.

One or more examples of implementations are set forth in more detail in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims.

BR1EF DESCRIPTION OF THE DRAW1NGS

ln the following the invention will be described in greater detail by means of preferred embodiments with reference to the attached drawings, in which Figures 1A, IB and 1C illustrate a leaky-wave structure according to an exemplary embodiment;

Figures 2A, 2B, 3, 4A, 4B and 4C illustrate alternative leaky-wave struc tures according to exemplary embodiments; and

Figure 5 illustrates a method according to an embodiment.

DETA1LED DESCRIPTION OF THE INVENTION

The following embodiments are exemplary. Although the specification may refer to "an", "one", or "some" embodiment(s) in several locations, this does not necessarily mean that each such reference is to the same embodiment(s), or that the feature only applies to a single embodiment. Single features of different embodiments may also be combined to provide other embodiments. Furthermore, words "comprising", "containing" and "including" should be understood as not lim iting the described embodiments to consist of only those features that have been mentioned and such embodiments may contain also features/structures that have not been specifically mentioned.

Figures 1A, IB and 1C illustrate a leaky-wave structure according to an exemplary embodiment. Specifically, Figures 1A, IB and 1C illustrate a leaky-wave structure oriented along an x-axis (as defined in said Figures) from three perspec tives. Namely, Figure 1A provides a perspective view of the leaky-wave structure showing a cut-plane, Figure IB shows a cross-sectional view of a cross section A (i.e., a view of the z-plane at said cross section A) and Figure 1C shows a view from "above" (i.e., a view of the xy-plane) of a section 120 of the leaky- wave structure. The cross section A is illustrated in Figure 1C with a dashed line. Figure 1A may be considered to illustrate a semi-infinite leaky-wave structure with the section providing leaky-wave properties extending to the positive x-direction indefinitely ln Figures 1A, IB and 1C as well as in the following Figures, a dotted line is used to denote objects behind other objects.

The leaky-wave structure of Figures 1A, IB and 1C (or specifically the section 120 of the leaky transmission-line structure in Figure 1C) comprises two main elements: a leaky transmission-line structure 101 and a radio frequency res onator 110 mounted over said leaky transmission-line structure 101 at a first dis tance 120. Specifically, the leaky transmission-line structure 101 in the illustrated embodiment is a leaky coaxial cable (sometimes also called a leaky feeder or a ra diating cable). The leaky coaxial cable comprises an inner conductor 102 and an outer conductor 104 (or shield) which are separated from each other by a dielectric layer 103 of a first dielectric material. The inner and outer conductor 102, 104 are arranged along the same axis (hence they are "coaxial") ln the illustrated example, the inner conductor 102 has a circular cross section and the outer conductor has a cross section of a circular ring with a relatively thin width though in other embod- iments different cross-sectional shapes (e.g., elliptical) may be employed ln some other embodiments, two or more inner conductors may be used. The first dielectric material of the dielectric layer 103 may be any conventional dielectric material conventionally used in coaxial cables such as foamed polyethylene, solid polyeth ylene, polyethylene foam, polytetrafluoroethylene or air space polyethylene. The dimensions of the coaxial cable may be according to a standard type of coaxial ca ble, for example, according to M1L-C-17 standard or according to NF-C-93550 standard.

To provide the leaky- wave property of the coaxial cable, the outer con ductor 102 comprises one or more openings 105 (i.e., slots, holes or apertures) ar- ranged along the longitudinal direction (x-direction) of the section 120 enabling leakage of the electromagnetic waves from inside the coaxial cable to outside space (i.e., to free space) ln the illustrated example, the outer conductor comprises two openings 105 having a shape of an elongated rectangle arranged orthogonal to the longitudinal direction of the coaxial cable section 120 as illustrated in Figure 1C using a dashed line. The dimensions of the opening(s) may be defined for operation at a specific frequency range with a certain longitudinal loss (i.e., signal loss along the cable). While the illustrated openings 105 are rectangular in shape, other shapes may be employed in other embodiments. For example, the openings 105 may be shaped like ellipses ln some embodiments, the openings 105 may be ar- ranged at an angle relative to the -axis. ln other embodiments, a single opening, as will be discussed in detail in relation to Figure 3, or three or more openings may be employed.

Before discussing the resonator antenna(s) 110 according to embodi ments in detail, the operation and properties of a conventional leaky coaxial cable, that is, the leaky coaxial cable 101 without the resonator antenna(s) 110, is dis cussed briefly. The main operating principle of the leaky coaxial cable is that said one or more openings 105 in the outer conductor leak electromagnetic energy of the propagating guided wave inside the coaxial cable 101 over the entire length of the coaxial cable 101. Thus, a leaky coaxial cable (or any leaky transmission-line structure) simultaneously acts as a waveguiding and radiating structure. Due to this leakage of energy, line amplifiers inserted at regular intervals are often used in practical scenarios to be to boost the signal back up to acceptable levels.

Based on the radiation mechanism, a leaky coaxial cable may be one of two types: coupled mode or radiating mode leaky coaxial cable. The type of the leaky coaxial cable depends on the geometry, dimensions and spacing of said one or more openings. Typically, the radiating mode leaky coaxial cable has an outer conductor with two or more openings arranged periodically along the longitudinal direction (i.e., along x-axis) while the outer conductor of the coupled mode leaky coaxial cable has a single continuous opening extending in the longitudinal direc tion as illustrated in Figures 1A, IB and 1C. The coupled mode operation may also be achieved by using two or more openings arranged so as to approximate a single larger opening, for example, by using a loosely woven outer braid as the outer con ductor or by using a set of very closely spaced transverse slots. While the radiating mode operation is based on arranging the openings so as to have resonances be tween the apertures (i.e., the openings) similar to a resonant antenna, the coupled mode operation is based on the generation of surface waves, similar to surface wave antennas. The following embodiments are predominantly operating using the coupled mode.

The performance of a leaky coaxial cable is generally characterized by its longitudinal attenuation per unit length and its coupling loss compared to a standard dipole antenna at a specific distance. The longitudinal attenuation is mainly due to conductor and dielectric losses in the cable while coupling loss is a characteristic of the slot aperture (i.e., size and dimensions of the one or more openings).

Far-field radiation pattern of a finite length ordinary leaky coaxial cable is roughly omnidirectional in the radial direction and roughly end-fire in the axial (i.e., longitudinal) direction ln other words, the radiation is spread equally to all directions orthogonal to the longitudinal direction of the leaky-wave structure. The electromagnetic field generated by the leaky coaxial cable is predominantly polar ized along the longitudinal direction (assuming the opening(s) in the outer conduc tor are narrow and symmetric).

As mentioned above, a conventional leaky coaxial cable has an almost omnidirectional pattern. While such a radiation pattern may be preferable in, for example, mines and railway and subway tunnels where the leaky coaxial cables are currently widely in use, indoor communication scenarios (e.g., office scenarios) of- ten require more robust and flexible solutions providing adjustable radiation per formance. One option according to embodiments for addressing this deficiency in view of indoor (office) scenarios is arranging or mounting one or more resonator antennas 110 (or equally resonant antennas or open resonators) over said one or more openings 105 in the outer conductor of the leaky coaxial cable 101. Specifi cally, said one or more resonator antennas 110 are arranged so that they are excit able by the electromagnetic waves leaking from the leaky coaxial cable 101. Fur ther, said one or more resonator antennas 110 are adapted to be resonant at one or more radio frequencies supported by the leaky coaxial cable 101. ln other words, each radio frequency resonators 110 is arranged and tuned such that when a signal having one of said one or more radio frequencies is transmitted via the leaky coax ial cable 101, the electromagnetic waves leaking from said one or more openings 105 to outside space are capable of resonantly exciting the radio frequency reso nator. Consequently, the resonator antenna 110 radiates electromagnetic wave ac- cording to its radiation pattern (for that particular resonant mode) and thus effec tively modifying the radiation pattern of leaky coaxial cable ln most case, the result is a total radiation pattern for the leaky-wave structure having larger (maximum) gain compared to the corresponding "unloaded" leaky transmission-line structure.

Said one or more resonator antennas 110 may be any resonant anten- nas or electromagnetic resonator structures which enable (re)radiation of the elec tromagnetic energy gathered by the resonator structure to directions other than the direction of the opening(s). For example, a rectangular cavity resonator having a single opening facing said one or more openings of the outer conductor 105 would not provide the desired effect, but a cavity resonator having two openings (preferably at opposite side) would. Each resonator antenna 110 may be arranged over a single opening 105 or multiple openings, as depicted in Figures 1A, IB and 1C. Each resonator antenna 110 may be arranged at a first distance 130 from the outer conductor 105 of the leaky coaxial cable 101.

ln some embodiments, each of the one or more resonator antennas 110 may comprise an elongated coupling element 114 (e.g., a metal strip or an aper ture) capable of coupling to the electromagnetic waves leaking from the leaky co axial cable 101 at said one or more radio frequencies ln such embodiments, an orientation of each elongated coupling element 114 relative to the longitudinal di rection determines strength of the coupling. Depending on the structure of the res- onator antenna 110, the elongated coupling element may be a resonant element by itself and/or it may be used for feeding to a primary resonant structure (e.g., a par tially open metal cavity). The elongated coupling element 114 may have, for exam ple, a rectangular shape, an elliptical shape or it may have a more complex shape such as a shape corresponding to an end-loaded dipole antenna (e.g., a primary metal strip segment along a first direction joined at each end with secondary, shorter metal strip segments orthogonal to the first direction forming an elongated Ή' shape).

ln some embodiments including the one illustrated in Figures 1A, IB and 1C, said one or more resonator antennas comprise at least one (partially) met- allized dielectric resonator antenna 110. The metallized dielectric resonator an tenna 110 comprises a conventional dielectric resonator antenna (DRA) or dielec tric resonator, that is, a block of dielectric material, and a metallization applied on certain surfaces of the dielectric material. Specifically, the metallized dielectric res onator antenna 110 may have a first metallized surface 111 facing said one or more openings 114, a first nonmetallized surface 112 facing away from said one or more openings 114 and one or more second metallized surfaces 113 connecting the first metallized surface to the first nonmetallized surface ln such embodiments, the elongated coupling element 114 may be an elongated opening (e.g., a rectangular opening) in a metallization of the first metallized surface 111 enabling feeding of the leaking electromagnetic waves to the metallized dielectric resonator antenna 110. Due to the first nonmetallized surface, the electromagnetic energy fed to the metallized dielectric resonator antenna 110 is only temporarily stored inside the metallized dielectric resonator antenna 110. Eventually (that is, after multiple re flections from the metallized surfaces) the electromagnetic waves are able to "es- cape" the metallized dielectric resonator antenna 110 and radiate to the outside space. The shape of the radiation pattern depends on the geometry of the metal lized dielectric resonator antenna 110 though increased directivity/gain to the di rection of the first nonmetallized surface (i.e., to the +z-direction in Figures 1A, IB and 1C) is to be expected. According to the above definition, the metallized dielec- trie resonator antenna 110 may have a variety of different shapes such as a cylinder (e.g., a right/oblique circular/elliptical cylinder), a prism (a polyhedron comprising an n-sided polygonal base), a cube or a rectangular cuboid.

ln Figures 1A, IB and 1C, the illustrated metallized dielectric resonator antenna 110 has a cylindrical shape (specifically the shape of a right circular cylin- der) and is arranged so that bases 111, 112 of said at least one metallized cylindri cal dielectric resonator antenna 110 are parallel to the longitudinal direction (i.e., x-axis) and symmetrically relative to the outer conductor 104. The symmetry rela tive to the outer conductor 104 may be defined so that opposing points on the edge (or rim) of the first base 111 are equally distant from the outer conductor 104 (or, to be precise, from the outer conductor 104 if it would have no openings) ln view of the above more general definition for the metallized dielectric resonator antenna

110, the first metallized surface corresponds to a first base 111 of the cylindrical dielectric resonator antenna 110, the first nonmetallized surface corresponds to a second base 112 of the cylindrical dielectric resonator antenna (denoted by a dashed line in Figure IB) and said one or more second metallized surfaces corre- sponding to a side 113 of the cylindrical dielectric resonator antenna 110. The first metallized surface 111 has two elongated openings 105 which have a rectangular shape and are aligned along the -direction (i.e., parallel to the two openings 105 of the outer conductor 104).

ln a conventional (nonmetallized) DRA or dielectric resonator, the per- mittivity (i.e., the dielectric constant) of the dielectric material needs to be high as the reflections inside the resonant structure causing the resonances occur due to the contrast in permittivities of the dielectric material and air. However, no such requirement exists for the metallized dielectric resonator antenna 110 as practi cally perfect reflection already occurs at the metallized surfaces due to the metalli- zation. However, as the wavelength inside a dielectric material decreases as a func tion of permittivity and the resonance frequencies of the dielectric body of the res onator antenna shift up accordingly, using a high permittivity dielectric has the benefit (similar to a conventional DRA) of enabling the minimization of the metal lized dielectric resonator antenna 110. The dielectric losses of the dielectric mate- rial should, similar to conventional DRAs or dielectric resonators, preferably be low for good performance. For example, the loss tangent of the dielectric material used may be at least lower than 0.005 and preferably lower than 0.001.

Each dielectric resonator antenna 110 may be adapted to be resonant at one or more frequencies (called resonance frequencies) supported by the leaky coaxial cable 101 as described above. Each of said one or more frequencies may relate to a radiating resonance mode of a metallized dielectric volume 115 of the metallized dielectric resonator antenna 110 or a resonance mode of the elongated opening 114. The resonance modes of the metallized dielectric volume 115 may comprise wave modes of one or more of the following types: TE (transverse elec- trie), TM (transverse magnetic), quasi-TE and quasi-TM, HE, EH, quasi-HE and quasi-EH. For example, the commonly used TEoia mode (a quasi-TEon mode) may be employed. The resonance frequencies of said modes depend on the geometry and dimensions of the dielectric resonator antenna 110 (or specifically the metal lized dielectric volume 115). The choice of the used wave mode also affects the total radiation pattern of the leaky-wave structure, as different wave modes produce different radiation patterns. Additionally or instead, the elongated opening 114 may have resonant length. For example, the elongated opening 114 may have a length of l/4 or l/2, where l is the free-space wavelength ln this case, the metal lized dielectric 115 of the dielectric resonator antenna 110 may act simply as a type of coupling element between the resonant elongated opening 114 and the free space affecting also the radiation pattern of the dielectric resonator antenna 110.

Each of said one or more resonator antennas 110 may be mounted over said one or more openings 105 using mounting means (not shown in Figures 1A, IB and 1C for clarity). Said mounting means may comprise one or more molded pieces of non-conductive material (e.g., foam core) adapted to be placed between the leaky coaxial cable 101 and said one or more resonator antennas 110 for fixing said one or more resonator antennas 110 at the first distance 130 and attaching means, such as glue or screws, for attaching each resonator antenna 110, corre sponding one or more molded pieces of non-conductive material and the leaky co axial cable 101 to each other. Said mounting means may comprise only or mostly materials which have minimal effect on the electromagnetic behavior of the leaky- wave structure. Alternatively, said mounting means may comprise a rig or a stand having a first end section attachable at one or more sides of the leaky coaxial cable 101 having no openings and a second end section attached via a middle section to the first section attachable to said one or more resonator antennas 110 (e.g., to the side 113 of the cylinder in the illustrated exemplary embodiment).

ln some embodiments, said mounting means may comprise a clamping element clamped around the leaky coaxial cable 101 and connected to said one or more resonator antennas 110. The clamping element may be a plastic snap-on clamp 200 as illustrated in Figures 2A and 2B. Similar to Figures IB and 1C, Figures 2A shows a cross-sectional view of a cross section A (i.e., a view of the z-plane at said cross section A) and Figure 2B shows a view from "above" (i.e., a view of the xy-plane). The cross section A illustrated in Figure 2B with a dashed line corre spond to the corresponding cross section in Figures IB and 1C. ln Figures 2A and 2B, a dotted line is used to denote objects behind other objects only for the snap- on clamp for clarity. The leaky coaxial cable 101 and the resonator antenna 110 (i.e., a metallized dielectric resonator antenna) may correspond to the correspond ing structures as discussed in relation to Figures 1A, IB and 1C.

Referring to Figures 2A and 2B, the plastic snap-on clamp 200 may com prise two clamping arms 201, 202 arranged around the leaky coaxial cable 101 and a center section 203 between said two clamping arms 201, 202 attached to said one or more resonator antennas 110 using, e.g., glue. The center section 203 may fur ther comprise an opening 203 to allow for the leaking electromagnetic waves to couple to the resonator antenna 110 substantially without obstruction. The plastic snap-on clamp 200 may be made of any suitably stiff plastic (e.g., PVC). ln the lon- gitudinal direction (i.e., x-direction), the snap-on clamp 200 may extend over the whole length of said one or more resonator antennas 110 or only a part of said length as shown in Figure 2B. While in Figures 2A and 2B a separate clamp 200 is arranged for each resonator antenna 110, in other embodiments a single clamp may be attached to two or more resonator antennas ln some embodiments, the arms 201, 202 of the plastic snap-on 200 clamp may fully enclose the leaky coaxial cable 101.

ln Figures 1A, IB and 1C, said one or more elongated openings 114 (or specifically two elongated rectangular openings) are aligned with one or more openings 105 (or specifically also two elongated rectangular openings) of the outer conductor 104. ln this configuration, while electromagnetic waves are able to cou ple from the leaky coaxial 101 cable to the dielectric resonator antenna 110, the amount of electromagnetic energy coupling to the dielectric resonator antenna is not fully maximized due to the narrowness of the elongated opening 114 in the longitudinal direction. By changing the orientation of the dielectric resonator an- tenna 110 by rotating the dielectric resonator antenna 110 around a rotating axis 140 orthogonal to the longitudinal direction, the orientation of the elongated open ing 114 (relative to the longitudinal direction) may be changed. This, in turn, affects the strength of the coupling. The strength of the coupling has a direct effect on the radiation pattern and the maximum directivity and gain. Depending on the use case, a highly directive leaky-wave structure may be more advantageous over a less directive leaky-wave structure or vice versa. Thus, the orientation in which the di electric resonator antenna 110 is to be installed may be chosen to best suit the needs of a particular use case/application.

Figure 3 illustrates a leaky-wave structure according to an alternative exemplary embodiment where the orientation of the dielectric resonator antenna 310 is different from the orientation of the dielectric resonator antenna 110 of Fig ures 1A, IB and 1C. The alternative exemplary embodiment illustrated in Figure 3 shows a view and an orientation of the leaky- wave structure and a section 320 thereof similar to Figure 1C and Figure 2B, i.e., a view of the xy-plane with the leaky- wave structure oriented along x-axis. Apart from the differing orientation of the dielectric resonator antenna 310 and the differing opening(s) of the outer conduc tor, the leaky- wave structure of Figure 3 maybe similar to the leaky- wave structure of Figures 1A, IB and 1C and/or Figures 2A and 2B.

Two differences exist between the leaky-wave structure of Figure 3 and the leaky-wave structure of Figure 1C both of which contribute in increasing the directivity and gain of the leaky-wave structure. Firstly, while the dielectric reso nator antenna 310 may be similar to the dielectric resonator antenna 110 of Figure 1C, its orientation is different ln Figure 3, the dielectric resonator antenna 310 and consequently also the elongated opening 314 is oriented at an angle with the lon gitudinal direction (i.e., x-axis). ln other words, the dielectric resonator antenna 310 corresponds to the dielectric resonator antenna 110 rotated around the rota tional axis 140. As the size of the elongated opening 314 in the longitudinal direc tion is increased, the leaking electromagnetic waves are able to couple more strongly to the dielectric resonator antenna 310 increasing the directivity and gain of the leaky-wave structure. Another factor contributing to an increase in the di rectivity and gain is the differing number and shape of the one or more opening 305 of the outer conductor of the leaky coaxial cable 301. lnstead of having two narrow rectangular openings oriented along -axis as in Figure 1C, the outer con ductor of the leaky coaxial cable 301 comprises a single, considerably wider but equally opening 305 over which the dielectric resonator antenna 310 is arranged. Obviously, as the opening 305 covers the previous two openings, a larger portion of the electromagnetic energy guided by the leaky coaxial cable is able to leak from the leaky coaxial cable 301. lt should be noted that while said two modifications were implemented together in the embodiment of Figure 3, they are in no way de pendent on each other and thus only one of said may be implemented in another embodiment.

ln some embodiments, instead of providing resonator antennas which may be installed in different orientations as discussed above, said one or more res onator antennas may be rotatably mounted to allow for rotation of each resonator antenna (and its elongated coupling element) around the rotating axis (axis 140 in Figure 1A) orthogonal to the longitudinal direction without detaching the resona tor antenna. Additionally or instead, said one or more resonator antennas may be adjustably mounted to allow for adjusting the first distance (distance 130 in Figure IB) between a corresponding resonator antenna and the leaky coaxial cable. This adjustment may also be used to increase or decrease the strength of the coupling ln contrast to the rotational adjustment, the adjustment of the first direction may be used to change the strength of the coupling even in the case of resonator antenna lacking an elongated coupling element or other directional element. For example, the resonator antenna may be a conventional, nonmetallized dielectric resonator antenna having a cylindrical shape or other shape having rotational symmetry. For example, Figures 1A, IB, 1C, 2A, 2B and 3 may correspond to such a scenario as suming that none of the surfaces of the cylinder 110, 310 are metallized and that the permittivity of the dielectric material is sufficiently high. Rotating such a die lectric resonator antenna, obviously, has no effect on the coupling of the electro magnetic energy from the leaky transmission-line structure to the dielectric reso nator antenna, but adjusting the first distance has.

The adjustability of the orientation and/or the first distance may be re alized also with a plastic snap-on clamp as illustrated in Figure 2A and 2B with some modification. To achieve the adjustability in the orientation a rotation plat form arranged between the center section of the plastic snap-on clamp and the res onator antenna(s) may be used, for example. The rotation platform may comprise an opening, preferably matching the opening in plastic snap-clamp (i.e., element 204 of Figures 2A and 2B). The adjustment in the first distance may be arranged with a similar platform providing adjustment in height, instead of rotation angle.

ln addition to the applications outlined above, potential applications for the leaky-wave structure according to embodiments include different RF1D (Radio Frequency ldentification) application environments (e.g., smart environments and smart desks) . Each RF1D chip or tag require a certain signal level in order to operate properly ln many RF1D application environments, multiple RF1D tags are located in close proximity with each other. The electromagnetic signal fed to an RF1D tag should have a signal level high enough to meet the required signal level but low enough so that adjacent RF1D tags are not affected negatively by the electromag netic signal. Thus, by using a leaky-wave structure where the gain pattern is di rective and easily adjustable for feeding the RF1D tags is beneficial. The embodi ments may also be used for powering (ultra-) low-power sensors in various loT (In ternet of Things) applications in a similar manner. Figures 4A, 4B and 4C illustrate a leaky-wave structure according to an alternative exemplary embodiment. Similar to Figures 1A, IB and 1C, respectively, Figure 4A shows a perspective view of the leaky-wave structure with a cut-plane, Figure 4B shows a cross-sectional view of a cross section B and Figure 4C shows shows a view from "above" (i.e., a view of the xy-plane) of a section 420 of the leaky- wave structure. The cross section B is illustrated in Figure 4C with a dashed line. The illustrated leaky coaxial cable 401 and its orientation are similar to the leaky coaxial cable 101 of Figures 1A, IB and 1C.

Referring to Figures 4A, 4B and 4C, the illustrated leaky-wave structure comprises a resonator antenna 410 with an elongated coupling element 414, simi lar to the embodiments illustrated in previous Figures. However, instead of corre sponding to an opening in a metallic surface, the elongated coupling element 412 is itself a metallic element. Specifically, the elongated coupling element 412 in Fig ures 4A, 4B and 4C may be a metallized element 414 printed on a substrate 415. ln the illustrated example, the metallized element 414 has the shape of a rectangle though other shapes may be used in other embodiments ln contrast to earlier em bodiments, the metallized element 412 does not serve to feed the electromagnetic energy to another resonator structure but acts itself as a resonant radiating ele ment. The metallized element may be, for example, a l/2 printed dipole antenna or a printed end-loaded dipole antenna. The metallic resonant element 414 may be arranged on a plane parallel to a tangential plane of the outer conductor 404 where the tangential (or tangent) plane is defined for a point on an outer surface of the outer conductor without said one or more openings 105 closest to the metallic res onant element 414. ln other words, the tangential plane is defined as a plane tan- gential to an outer surface of the outer conductor without said one or more open ings 105 at a point closest to the metallic resonant element 314. According to an alternative (but equivalent) definition, the metallic resonant element 414 may be arranged to be orthogonal to a surface normal (or a surface normal vector) defined for a point on the outer surface of the outer conductor (without said one or more openings 105) closest to the metallic resonant element 414. ln view of Figures 4A, 4B and 4C, this definition means that the metallized element is arranged along a z- plane. The metallized element 414 may be arranged on the side of the substrate facing said one or more openings 105 (i.e., facing -z-direction as defined in Figures 4A, 4B and 4C). The strength of the coupling may be adjusted also in this embodi- ment by rotating the resonator antenna 414 around a rotational axis 440 and/or by adjusting the first distance 430. The adjustment may be achieved by installing the resonator antenna 410 at a different orientation and/or a different distance or rotatably/adjustably mounting the resonator antenna 410. Similar mounting means may be employed as discussed with earlier embodiments.

ln some embodiments, the substrate 415 may have little effect on the electromagnetic behaviour of the leaky- wave structure. To this end, the substrate 415 may be thin (e.g., less than 1 mm) and made of a dielectric material having low permittivity (e.g., relative permittivity less than 2) and/or low dielectric losses ln other embodiments, the substrate 415 may be thicker and/or have larger permit tivity in order to guide the electromagnetic waves away from the outer conductor 104 and thus mitigate any unwanted coupling to the outer conductor 104. For ex ample, the relative permittivity of the dielectric material of the substrate 415 may be larger than 10 (e.g., 11) and/or have a thickness of 2-4 mm (e.g., 3 mm).

ln some embodiments, the resonator antenna 410 may, alternatively, be oriented upside-down compared to Figures 4A, 4B and 4C. ln other words, the res- onator antenna 410 may be arranged so that the metallized element is facing away from said one or more openings 105 (i.e., facing the +z-direction as defined in Fig ures 4A, 4B and 4C). ln such embodiments, the substrate 415 may even be mounted directly on the leaky coaxial cable 110. To achieve this, the substrate may be moulded to conform to the outer conductor 104 of the leaky coaxial cable 101.

ln some embodiments, the metallic element 412 may be a metallic (res onant) element other than a metallized element printed on a substrate. For exam ple, the metallic element 414 may be a metallic foil element (i.e., a piece of thin metallic foil) or a metal wire element. Said metallic foil/metal wire element may be attached (using, e.g., glue or tape) to a piece of support material such as foam (e.g., polyurethane foam). The embodiment illustrated in Figures 4A, 4B and 4C may equally correspond to a foil resonator -based embodiment with the element 414 being the metallic foil and the element 415 being the support material ln some embodiment, said piece of support material may be (moulded and) mounted di rectly on the leaky coaxial cable 110, similar to as discussed for a substrate with a printed metallized element in the previous paragraph.

lt should be appreciated that while Figures 1A, IB and 1C, Figures 2A and 2B, Figure 3 and Figures 4A, 4B and 4C illustrate leaky coaxial cable -based embodiments, in other embodiments a different type of leaky transmission-line structure may be used such as a leaky multi-conductor coaxial cable ln general, the leaky transmission-line structure may be based on any closed transmission-line structure, that is, on any transmission-line structure where the electromagnetic waves propagate only within a limited space defined by an outer conductor of the transmission-line structure ln other words, no electromagnetic energy leaks to the space outside a closed transmission-line structure unless one or more openings are introduced to the outer conductor according to embodiments. For example, the transmission-line structure may be a rectangular, spherical or ellipsoidal wave guide. ln some embodiments, a partially open transmission-line structure may be used to realize the leaky transmission-line structure. For example, a microstrip line, a coplanar line or a stripline with opening(s) in the ground plane may be em ployed.

According to an embodiment, there is provided a method for providing a leaky- wave structure according to any of the previous embodiments. Said method is illustrated in Figure 5.

Referring to Figure 5, there is initially provided, in block 501, a leaky transmission-line structure for guiding electromagnetic waves. The leaky trans- mission-line structure comprises one or more openings arranged along a longitu dinal direction of the leaky transmission-line structure so as to enable leaking of the electromagnetic waves from the leaky transmission-line structure. The leaky transmission-line structure may be any leaky transmission-line structure dis cussed in relation to any of the previous embodiments. Further, there is provided, in block 502, one or more resonator antennas. Said one or more resonator anten nas may also be any resonator antennas discussed in relation to any of the previous embodiments. Finally, said one or more resonator antennas are attached, in block 503, over said one or more openings. The attaching may be carried out, for exam ple, using a plastic snap-on clamp as discussed in relation to Figures 2A and 2B. Alternatively, the attaching may be carried out using mounting means comprising one or more molded pieces of non-conductive material (e.g., foam core) adapted to be placed between the leaky coaxial cable and said one or more resonator antennas for fixing said one or more resonator antennas at a first distance and attaching means, such as glue or screws, for attaching each resonator antenna corresponding one or more molded pieces of non-conductive material and the leaky coaxial cable to each other, as described above.

Even though the invention has been described above with reference to examples according to the accompanying drawings, it is clear that the invention is not restricted thereto but can be modified in several ways within the scope of the appended claims. Therefore, all words and expressions should be interpreted broadly and they are intended to illustrate, not to restrict, the embodiment lt will be obvious to a person skilled in the art that, as technology advances, the inventive concept can be implemented in various ways. Further, it is clear to a person skilled in the art that the described embodiments may, but are not required to, be com bined with other embodiments in various ways.