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, an outer surface of the section having a first area and a second area which are disjoint areas located on different sides of the leaky transmission-line structure, wherein the outer con ductor comprises one or more openings arranged along a longitudinal direction of the section enabling leakage of the electromagnetic waves from the leaky transmis- sion-line structure; and a dielectric strip of a second dielectric material arranged on the outer surface of the section so as to cover only said first area at least at one or more locations of said one or more openings.

According to another aspect, there is provided a method comprising providing a leaky transmission-line structure for guiding electromagnetic waves, wherein the leaky transmission-line structure comprises one or more openings ar ranged along a longitudinal direction of the leaky transmission-line structure so as to enable leaking of the electromagnetic waves from the leaky transmission-line structure; providing a dielectric strip; and attaching the dielectric strip to an outer surface of the leaky transmission-line structure so as to cover at least one or more locations of 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 DESCR1PT10N 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;

Figure 2 illustrates an alternative leaky-wave structure according to an exemplary embodiment;

Figure 3 illustrates two guided modes of a leaky coaxial cable: a bifilar mode (on the left) and a monofilar mode (on the right);

Figure 4A and 4B illustrate a leaky-wave structure according to an ex emplary embodiment;

Figure 5 illustrates coupling loss of a coaxial cable loaded with a sinus oidally modulated reactance surface (thin line) and a corresponding conventional leaky coaxial cable (thick line);

Figures 6A and 6B illustrate a leaky-wave structure according to an ex emplary embodiment

Figure 7 illustrates coupling loss of a coaxial cable loaded with three se ries fed conformal square patch antennas (dashed line) and a corresponding con- ventional leaky coaxial cable (solid line); and

Figure 8 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 (i.e., a view of the yz- plane) and Figure 1C shows a view from "above" (i.e., a view of the x -plane) of a section of the leaky-wave structure. Figure IB may correspond to the cut-plane vis ible in Figure 1A on the right-hand side. Moreover, Figure 1A may be considered to illustrate a semi-infinite leaky- wave structure with the section 120 providing leaky-wave properties extending to the positive x-direction indefinitely ln prac tice, said section 120 providing leaky-wave properties is terminated from both ends as shown in Figure 1C.

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 dielectric strip 110 arranged on said leaky transmission-line structure 101. Specifically, the leaky transmission-line structure 101 in the illustrated embodiment is a leaky coaxial ca ble (sometimes also called a leaky feeder or a radiating 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 embodiments different cross-sec tional 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 coax ial cables such as foamed polyethylene, solid polyethylene, polyethylene foam, pol- ytetrafluoroethylene or air space polyethylene. The dimensions of the coaxial cable may be according to a standard type of coaxial cable, 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 one continuous opening 105 arranged parallel to the longitudinal direction of the coax ial 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 opening 105 is rectangular in shape, other shapes may be employed in other em bodiments. For example, the opening 105 may be shaped like an ellipse extending along the longitudinal direction ln some embodiments, the opening 105 may be arranged at an angle relative to the x-axis. ln other embodiments, two or more openings may be employed as will be discussed in detail in relation to Figure 2.

Before discussing the dielectric strip 110 according to embodiments in detail, the operation and properties of a conventional leaky coaxial cable, that is, the leaky coaxial cable 101 without the dielectric strip 110, is discussed briefly. The main operating principle of the leaky coaxial cable is that said one or more open ings 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) simulta neously acts as a waveguiding and radiating structure. Due to this leakage of en ergy, line amplifiers inserted at regular intervals are often used in practical scenar ios 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 a dielectric strip (or a slab) 110 of second dielectric material to cover at least one of said one or more openings 105 in the outer conductor of the leaky coaxial cable 101. ln some embodiments, the die- lectric strip 110 may cover all of said one or more openings 105, as depicted in Figure 1C. Depending on the permittivity of the second dielectric material and di mensions of said strip 110, different improved radiation properties (e.g., reflection coefficient, radiation pattern) may be achieved. Specifically, a more directive radi ation pattern may be achieved compared to the conventional leaky coaxial cable with right choice of permittivity of the second dielectric material as will be de scribed in the following.

ln some embodiments, a leaky transmission-line structure 101 (or spe cifically a section 120 thereof) comprises an outer surface having a first area 130 and a second area 140 which are disjoint (i.e., not overlapping) areas located on different sides of the leaky transmission-line structure 101. The first and second area 130, 140 may be aligned with each other in the longitudinal direction. Said outer surface may correspond to the outer surface of the outer conductor 104, as in the embodiment illustrated in Figure 1A, or to another surface such as an outer surface of a jacket enclosing the outer conductor 104. The dielectric strip 110 of the second dielectric material may be arranged (on the outer surface of the section 120) to cover only said first area 130 at least at one or more locations of said one or more openings 105. Consequently, the dielectric strip 110 of the second dielec tric material cannot fully enclose the leaky transmission-line structure 101. More over, the first area 130 may comprise (or be located over) all or only some of said one or more openings 105. ln Figure 1A, the first area 130 may be the area of the outer surface of the outer conductor 104 in contact with the dielectric strip 110 and the second area may be defined, for example, as any area of the outer surface of the outer conductor 104 aligned with the first area in the longitudinal direction and not overlapping with the first area.

The dielectric strip 110 of the second dielectric material may be ar- ranged along the longitudinal direction, as illustrated in Figures 1A and 1C, or at least predominantly along the longitudinal direction (i.e., at an angle significantly smaller than 90° relative to the longitudinal direction). Moreover, the dielectric strip 110 may be adapted to curve along (or conform to) the outer conductor 104 (or specifically to its outer surface), as illustrated in Figures 1A and IB. The dielec- trie strip 110 (or the first area 130 as discussed in the previous section) may cover only a relatively small part (e.g, less than one half or even less than one fourth) of the circumference of the outer conductor 104. ln some embodiments, the dielectric strip 110 may cover over one half of the circumference of the outer conductor 104, but does not fully enclose it. ln other embodiments, the strip 110 may be a planar strip. A planar strip may be employed, for example, with leaky coaxial cables where the width of the opening 105 is small enough that that the curvature over the width of the opening 105 is negligible. The dielectric strip 110 may extend over the whole opening 105 so that a small air gap is formed between the dielectric strip 110 and the dielectric layer 103 of the first dielectric material. The air gap may be small enough so that it has no significant effect on the waveguiding or radiation proper ties of the leaky-wave structure. The dielectric strip 110 may be only slightly larger in both longitudinal direction (i.e., direction along x-direction) and in azimuthal di rection (i.e., angular direction along the curvature of the outer conductor and or thogonal to the x-direction) than the opening 105, as illustrated in Figure 1C. ln other embodiments, the dielectric strip 110 may be adapted to fit into the opening 105 and thus the dielectric strip 110 of the second dielectric material and the die lectric layer 103 of first dielectric material may be in contact with each other.

ln some embodiments, the thickness of the dielectric strip may be smaller or significantly smaller than the width of the dielectric strip ln some em- bodiments, the thickness of the dielectric strip may be 0.1 mm - 1 cm, depending on the operational frequencies and the permittivity of the second dielectric mate rial. ln other embodiments, the thickness of the dielectric strip may be 1 mm - 4 mm.

ln some embodiments, two or more dielectric strips 110 may be ar- ranged over or within said one or more openings 105 of the outer conductor 104, instead of a single continuous strip 110. Each of said two or more dielectric strips 102 may or may not be adapted to curve along the surface of the outer conductor 104.

ln order to provide improved radiation performance compared to a conventional leaky coaxial cable, the permittivity of the second dielectric material should be at least larger than the permittivity of air (or of any other medium in which the leaky- wave structure is immersed, e.g., water). Preferably, the permit tivity of the second dielectric material should be considerably larger than the per mittivity of air (e.g., relative permittivity being 11). According to an embodiment, the relative permittivity of the second dielectric material is larger than two. Ac cording to another embodiment, the relative permittivity of the second dielectric material is larger than four, preferably larger than six, even more preferably larger than eight. According to yet another embodiment, the relative permittivity of the second dielectric material is larger than nine, preferably larger than ten, even more preferably larger than eleven.

According to some embodiments, the permittivity of the second dielec tric material is larger than the permittivity of the first dielectric material ln some embodiments, the ratio of the permittivity of the second dielectric material to the permittivity of the first dielectric material may be larger than two, preferably larger than four and even more preferably larger than six (or even eight). The above em bodiments relating to permittivities of both the first and the second dielectric ma terials may be equally defined using corresponding relative permittivities.

The second dielectric material may have low dielectric losses. For ex ample, the loss tangent of the second dielectric material may be smaller than 0.002 (e.g., approximately 0.001) at the operational frequencies of the leaky- wave struc ture. The relative permittivity of the first dielectric material may preferably be rel atively close to one (e.g., 1.25).

When said conditions of high permittivity and low losses for the second dielectric material are fulfilled, the dielectric strip 110 is capable of acting as a cou- pling element which couples (or facilitates the coupling of) the propagating elec tromagnetic wave from the coaxial cable to the outside space (i.e., free space) via said one or more openings 105. As the wavelength of an electromagnetic wave in side a dielectric material is defined to be inversely proportional to the square root of the relative permittivity (i.e., the dielectric constant) of the dielectric material, the electromagnetic wave is more "tightly packed" into a high-permittivity dielec- trie material compared to a low-permittivity dielectric material or vacuum ln other words, more wavelengths of the electromagnetic wave may be comprised in a given length of the dielectric material. The inclusion of the low-loss, high-permittivity di electric strip 110 results in the leaking electromagnetic field being concentrated farther from the outer conductor which, in turn, prevents the leaking electromag- netic energy from coupling to the outer surface of the outer conductor 104 and spe cifically to a propagating monofilar wave mode of the leaky coaxial cable 101 (to be discussed in detail in relation to Figure 3). The leaky- wave structure comprising a high-permittivity dielectric strip produces a radiation pattern which is directed substantially in a direction orthogonal to the longitudinal direction (i.e., x-axis) and opposite to the opening 105.

The dielectric strip 110 may be attached to the opening 105 and/or the outer conductor 104 during the manufacturing of the leaky coaxial cable 101 or it may be attached at a later stage using, e.g., glue or adhesive tape. The dielectric strip may be manufactured using an extrusion method.

lt should be noted that the placement of the dielectric strip 110 on top of the outer conductor 104 (i.e., external to the outer conductor 104) is beneficial compared to the placement of the dielectric strip 110 inside the leaky-wave trans- mission-line structure 101 (i.e., between the inner and outer conductors) in that it enables adjusting radiation properties of the leaky transmission-line structure 101 without modifying or breaking the leaky transmission-line structure 101 itself ln other words, the manufacturing of a dielectric-based leaky-wave structure where a dielectric strip is placed on the leaky- wave transmission structure 101 is much simpler compared to the manufacturing of a dielectric-based leaky-wave structure where the dielectric strip is placed inside the leaky-wave transmission structure 101.

ln some embodiments, the leaky-wave structure of Figures 1A, IB and 1C may further comprise an outer plastic sheath or a jacket (not shown in Figures 1A, IB and 1C) arranged either around the whole leaky- wave structure or around the outer conductor 104 of the transmission-line structure 101, as described also above ln the latter case, the dielectric strip 110 may be arranged on top of the outer plastic sheath or jacket. The outer plastic sheath or jacket may be thin and have a relatively low permittivity (e.g., relative permittivity being between 1 and 3) so that it has minimal effect on the electrical performance of the cable (both in terms of waveguiding and radiation leakage).

While in the exemplary leaky-wave structure (i.e., a leaky coaxial cable) of Figures 1A, IB and 1C, the outer conductor 104 of the leaky coaxial cable 101 comprised one continuous opening extending in the longitudinal direction, in other embodiments two or more openings may be arranged in the outer control 104. One such an alternative exemplary embodiment is illustrated in Figure 2 showing a view and an orientation of the leaky-wave structure similar to Figure 1C, i.e., a view of the x -plane with the leaky-wave structure oriented along x-axis. Apart from the differing configuration of the openings 207 on the outer conductor, the leaky-wave structure of Figure 2 may be similar to the leaky-wave structure of Figures 1A, IB and 1C.

Figure 2 illustrates an exemplary embodiment where two or more openings 207 (specifically, 15 openings denoted by dashed lines in the illustrated case) are arranged periodically along the longitudinal direction (i.e., x-axis) in a section 220 of a leaky transmission-line structure 201. The spacing between adja cent openings 207 of said two or more openings may be smaller free-space wave lengths of the leaky-wave structure. Preferably, said spacing is (very) small com- pared to operational free-space wavelengths of the leaky-wave structure (i.e., said spacing is "electrically small"). The (operational) free-space wavelengths of the leaky-wave structure correspond to the operational frequencies of the leaky-wave structures, that is, the frequencies at which the leaky-wave structure is adapted to operate as a radiating structure (i.e., an antenna) and as a transmission line. When said spacing is small or very small (e.g., at least smaller than the smallest opera tional wavelength divided by twenty), the electromagnetic behavior of said two or more opening 207 is approximately the same as the electromagnetic behavior of a corresponding single continuous opening (as depicted in Figures 1A, IB and 1C) and thus the leaky coaxial cable is able to operate in coupled mode.

The width of said two or more openings may also be smaller than the smallest operational wavelength though larger than the spacing between adjacent openings. While Figure 2 illustrates an exemplary leaky-wave structure with rec tangular openings 207, said openings may have another shape (e.g., an elliptical shape, a circular shape, a rounded rectangular shape or a diamond shape) in other embodiments. lt should be appreciated that while Figures 1A, IB and 1C and Figure 2 and some subsequent Figures illustrate coaxial cable -based embodiments, in other embodiments a different type of leaky transmission-line structure may be used ln general, the leaky transmission-line structure may be based on any closed trans- mission-line structure, that is, on any transmission-line structure where the elec tromagnetic waves propagate only within a limited space defined by an outer con ductor of the transmission-line structure ln other words, no electromagnetic en ergy 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 el lipsoidal waveguide or a multi-conductor coaxial cable ln some embodiments, a partially open transmission-line structure may be used to realize the leaky trans- mission-line structure. For example, a microstrip line, a coplanar line or a stripline with opening(s) in the ground plane may be employed.

ln addition to the coupled radiation mode, two distinct guided modes are supported by conventional coupled mode leaky coaxial cables (like the ones illustrated in Figures 1A, IB and 1C and Figure 2 with the additional dielectric strip). These two guided modes are illustrated in Figure 3. The bifilar or coaxial mode (illustrated on the left in Figure 3) is mostly confined between the inner and outer conductor of the coaxial cable though some of the electromagnetic energy leaks to the outside space via the opening(s). This leakage is utilized commonly for communication purposes. On the contrary, the monofilar mode (illustrated on the right in Figure 3) is spread over the outer surface of the outer conductor, similar to a surface wave lf one these guided modes could be converted into propagating modes, the radiation efficiency of the leaky-wave structure and/or directivity may be improved and in general further degrees of freedom for the design of the radia tion performance are provided.

One way to achieve conversion of a guided mode (specifically, the bifilar mode) to a radiating mode is using a sinusoidally modulated reactance surface (SMRS). SMRS refers to any surface whose modal surface impedance (i.e., surface impedance pertaining to a particular mode in question) is sinusoidally modulated along the surface. By periodically modulating the surface reactance above a thresh old period, single or multiple discrete modes of the surface wave may become propagating ln this way, electromagnetic energy is radiated away from the surface wave. The surface impedance, h, of a SMRS with the direction of wave propagation assumed to be in x-direction, is given by

where h ₀ is the free-space wave impedance (« 120p W), X is the average surface reactance normalized by the free-space wave impedance, M is the modulation fac tor governing the leakage rate and a is the periodicity of modulation. The parame- ters ( X , M, a] define, to a large extent, the characteristics of a SMRS.

Figures 4A and 4B illustrate a leaky-wave structure comprising a die lectric strip which implements a SMRS according to an embodiment. Specifically, Figure 4A shows a view from "above" (i.e., a view of the x -plane) and Figure 4B shows a view from "the side" (i.e., a view of the xz-plane) with the leaky- wave struc- ture oriented along the x-direction. ln Figures 4A and 4B, the dielectric strip 410 is illustrated using a dotted pattern for improved clarity.

Referring to Figures 4A and 4B, the illustrated leaky-wave structure may be similar to the one illustrated in Figure 1A, IB and 1C with one key differ ence: the dielectric strip 410 is loaded with a SMRS 406 providing a sinusoidal var- iation in the impedance (or specifically reactance) of a surface wave mode. Specif ically, two or more metallic elements 406 are deposited on a surface of a dielectric strip 410 of the second dielectric material facing away from the outer conductor of the leaky coaxial cable 401 in a section 420 of the leaky coaxial cable 401. Said two or more metallic elements are adapted to be excitable by the electromagnetic waves leaking from the transmission-line structure via one or more openings 405 (exemplary embodiment with only one opening shown in Figure 4A). The dielectric strip 410 of the second dielectric material may be a dielectric layer of a printed circuit board (PCB) and said two or more metallic elements 406 may be metallized elements of said PCB shaped using a copper patterning method. The copper pat- terning method may comprise, for example, one of silk screen printing, photoen graving, milling, laser resist ablation or laser etching or any other established cop per patterning method ln other embodiments, said one or more metallic elements may be separate metallic elements curving along the dielectric strip 410 and being fixed to it.

Said two or more metallic elements 406 may be two or more conformal or curved patches (e.g., 16 conformal patches as illustrated in Figures 4A and 4B) arranged along the longitudinal direction (i.e., x-axis). The width of each conformal patch in the longitudinal direction and/or a spacing of adjacent conformal patches may be modulated to modulate the reactance for a surface wave mode in a sinusoi- dal manner. Specifically, the reactance may follow the reactance given by equation (1). ln some embodiments such as the one illustrated in Figures 4A and 4B, said two or more conformal patches may be rectangular patches and a center of each rectangular patch may be arranged along a common line parallel to the longitudinal direction ln other embodiments, different shape of metallic (or metallized) ele ments may be employed (e.g., circular or elliptical). The length of each conformal patch (i.e., length along the azimuthal direction as described above) may be equal ln some embodiments, at least some of said two or more metallic elements 406 may be arranged off-center relative to said common line parallel to the longitudinal di rection. The length of each conformal rectangular patch 406 along the azimuthal direction may be equal (or substantially equal) to the width of the dielectric strip 410 along the azimuthal direction as illustrated in Figures 4A and 4B. ln other em bodiments, the length of each rectangular patch 406 may be smaller than the width of the dielectric strip 410.

Figures 4A and 4B illustrates, for simplicity and clarity, only a single pe riod of the sinusoidal pattern ln other embodiments, said two or more metal- lic/metallized elements 406 may comprise multiple periods of the illustrated si nusoidally modulated pattern.

As mentioned above, the leaky-wave structure loaded with a SMRS as illustrated in Figures 4A and 4B provides an increased radiation efficiency com pared to a conventional leaky-wave structure (e.g., a corresponding conventional leaky coaxial cable). As the impedance matching of the leaky- wave structure is not significantly affected by the introduction of the SMRS, the signal strength of the sig nal transmitted by the leaky-wave structure is also increased. Another way to quantify the improvement in the performance of the leaky-wave structure is to evaluate the coupling loss defined as

L dB] =— 10 log(P _r /P _t ), (2)

where P _r is the power received by a standard half-wavelength dipole antenna and P _t is the input power of the leaky-wave structure. The coupling loss is a parameter which characterizes the mutual coupling intensity between leaky-wave structure and outside environment. Thus, a higher coupling loss corresponds to a larger por- tion of the electromagnetic waves (or electromagnetic energy) fed to the leaky- wave structure being radiated to free space.

Figure 5 illustrates coupling loss of a leaky coaxial cable loaded with a SMRS and a corresponding conventional leaky coaxial cable. The result is based on full-wave electromagnetic simulations of two exemplary structures having the fol- lowing dimensions. The total length of the simulated coaxial cable -based structure was 830 mm. There was a 44 mm long section of a conventional coaxial cable at both of the ends of the simulated structure. The inner and outer conductor diame ters of the coaxial line were 9.3 mm and 25.2 mm, respectively. The relative per mittivity of the first and the second dielectric material was chosen to be 1.4 and 11, respectively. Five periods of the modulated reactance dielectric strip, conformed over the leaky coaxial cable were simulated. The width, length and thickness of the dielectric strip were 12 mm, 800 mm and 3 mm respectively, sufficient to cover the openings of the coaxial cable (being similar to as illustrated in Figure 2).

ln the simulations, the electric field was calculated at a distance of one meter from the center of the cable in both cases with a simulated electric field probe and consequently the coupling loss was calculated utilizing equation (2). The coupling as a function of frequency is illustrated in Figure 5 for a conventional leaky coaxial cable with a thick line and for a SMRS-loaded leaky coaxial cable with a thin line. Apart from the lowest frequencies, there was a significant improvement (roughly 5-10 dB) in the coupling loss. As the wavelength decreases, it becomes more comparable to the chosen period length of the design which, in turn, causes an improved coupling loss performance especially at higher frequencies.

Arranging one or more metallic elements so as to form a SMRS as dis cussed in relation Figures 4A, 4B and 5 is not the only way to improve the perfor mance (e.g., radiation efficiency and field strength) of the leaky-wave structure us ing one or more metallic (loading) elements deposited on the outer surface of the dielectric strip. Figures 6A and 6B illustrate a leaky-wave structure with a metal- backed dielectric strip according to an alternative embodiment. Specifically, Figure 6A shows a view from "above" (i.e., a view of the x -plane) and Figure 6B shows a view from "the side" (i.e., a view of the xz-plane) with said alternative leaky-wave structure oriented along the x-direction. Apart from the inclusion of said one or more metallic elements, the leaky-wave structure illustrated in Figures 6A and 6B may be similar to the one discussed in relation to Figure 1A, IB and 1C. ln Figures 6A and 6B, the dielectric strip 610 is illustrated using a dotted pattern for improved clarity.

ln the embodiment illustrated in Figures 6A and 6B, one or more metal lic elements are deposited on a surface of a dielectric strip 610 of the second die lectric material facing away from the outer conductor of the leaky transmission line structure 601 (e.g., a leaky coaxial cable as illustrated in Figures 6A and 6B) in a section 620 of a leaky transmission-line structure 601 and said one or more me tallic elements are adapted to be excitable by the electromagnetic waves leaking from the transmission-line structure via one or more openings 605, similar to the embodiments discussed in relation to Figures 4A and 4B. Moreover, said metallic (or metallized) elements may also be manufactured as discussed above in relation to Figures 4A and 4B.

However, instead of forming a SMRS structure, said one or more metal- lie elements consist of a single continuous metallic element forming two or more series fed conformal patch antennas 606, 607, 608 arranged along the longitudinal direction (i.e., x-axis) with the outer conductor of the leaky coaxial cable 601 acting as a ground plane. Each pair of adjacent conformal patch antennas may be con nected to each other via at least one narrow conformal metal strip 616, 617 forming a microstrip feed line to achieve the series feeding. Each narrow conformal metal strip 616, 617 may be of equal length and/or width. Thus, each series fed patch antenna may be adapted to be excited by the electromagnetic waves leaking from the leaky coaxial cable via said one or more openings and/or upon excitation of an adjacent series fed patch antenna via at least one narrow conformal metal strip at one or more radio frequencies ln other words, the dielectric strip 610 may act as a primary coupling element for coupling the electromagnetic waves from the leaky coaxial cable to free space (similar to previous embodiments) and each conformal patch antenna 606, 607, 608 may act as a secondary coupling element further fa cilitating said coupling at a specific frequency band defined by the dimensions of the given conformal patch antenna. As conformal patch antennas have directive ra diation patterns with the radiation being directed predominantly in directions or thogonal (or near orthogonal) to the patch surface (maximum being in the direc tion orthogonal to the surface of the patch at a center point of the patch), the intro duction of said two or more series fed conformal patch antennas 606, 607, 608 also has a directive effect on the radiation pattern of the leaky-wave structure at the operational frequencies (i.e., resonance frequencies) of the conformal patch anten nas 606, 607, 608. Consequently, the (maximum) gain of the leaky-wave structure at the operational frequencies of the conformal patch antennas 606, 607, 608 is also increased.

To provide said coupling in multiple independent frequency bands sim ultaneously, said two or more series fed conformal patch antennas 606, 607, 608 may have different dimensions providing different resonance frequencies when ex cited by the electromagnetic waves leaking from the transmission line structure via said one or more openings 605 or by the adjacent patch antennas via the microstrip feed lines 616, 617. ln particular in the case of simple patch antenna shapes such as a rectangular or circular patch antenna, the length of the patch (that is, length from the feed point to the opposite edge) determines the resonance frequency along with the permittivity of the second dielectric material (which is the same for all the series fed conformal patch antennas) ln the exemplary embodiment illus trated in Figures 6A and 6B, three series fed conformal patch antennas 606, 607, 608 having a square shape all have different dimensions and thus different reso nance frequencies. Each conformal patch antenna 606, 607, 608 may be resonant at its fundamental (l/2) resonance frequency as well as at one or more higher res onance frequencies corresponding higher resonance modes supported by the leaky coaxial cable 601. Therefore, an additional improvement in the radiation effi- ciency/coupling loss/gain is expected in three sets of frequency bands of the three conformal patch antenna 606, 607, 608.

Said two or more series fed patch antennas may be rectangular or square conformal patch antennas arranged so that two edges of each series fed conformal patch antenna are aligned with the longitudinal direction, as illustrated in Figures 6A and 6B. Further, the center of each rectangular conformal patch an tenna and each narrow metal strip 616, 617 may be arranged along a common line parallel to the longitudinal direction ln other embodiments, a different type of patch antenna or even multiple different types patch antennas may be used. The types of patch antennas which may be used in different embodiments comprise, for example, a rectangular patch antenna, a circular patch antenna, an elliptical patch antenna, a triangular patch antenna, a circular ring patch antenna and a disc sector -shaped patch antenna. Also in such embodiments, conformal patch antennas may be arranged substantially along a line parallel to the longitudinal direction.

ln some embodiments, all the series fed patch antennas may have the same shape and equal dimensions ln other embodiments, the longitudinal center points of adjacent patch antennas may have equal spacing relative to each other.

ln some embodiments, at least some of the narrow conformal metal strips 616, 617 implementing microstrip feed lines are connected to said two or more series fed patch antennas 606, 607, 608 by using inset feeding ln some em- bodiments, coupling circuitry comprising one or more distributed and/or discrete circuit elements in series or parallel may be arranged between at least some of said two or more series fed patch antennas 606, 607, 608.

While the leaky-wave structures illustrated in Figures 4A, 4B, 6A and 6B comprise a single opening 405, 605 in the outer conductor of the leaky coaxial cable 401, 601, two or more openings may be arranged in other embodiments, sim ilar to as described in relation to Figure 2. All or only some of said two or more openings may be covered by the dielectric strip and thus are able to excite said one or more metallic elements deposited on the dielectric strip.

Figure 7 illustrates the increase in coupling loss resulting from loading a leaky coaxial cable with series fed conformal patch antennas. The results are based on full-wave electromagnetic simulations of a leaky coaxial cable loaded with said three series fed conformal patch antennas having different dimensions (as il lustrated in Figures 6A and 6B) and a corresponding conventional leaky coaxial cable. The coupling loss was calculated similar to as was discussed in relation to Figure 5. The coupling as a function of frequency is illustrated for a conventional leaky coaxial cable with a solid line and for a patch antenna -loaded leaky coaxial cable with a dashed line. A significant improvement in the coupling loss (i.e., higher coupling loss) may be observed in Figure 7 at most of the studied frequencies.

ln some embodiments, said one or more metallic elements deposited on a surface of the dielectric strip of the second dielectric material facing away from the outer conductor of the leaky coaxial cable may be individual resonant elements

(that is, not forming a part of a SMRS or being series fed). For example, each metal lic element may form an individual patch antenna with the outer conductor acting as the ground plane.

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 8.

Referring to Figure 8, there is initially provided, in block 801, a leaky transmission-line structure for guiding electromagnetic waves. The leaky trans- mission-line structure (e.g., a leaky coaxial cable) comprises one or more openings arranged along a longitudinal 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 discussed in relation to any of the previous embodiments. Further, there is provided, in block 802, a dielectric strip. Said dielectric strip may be any dielectric strip (including any dielectric strip with metallic elements deposited on its surface) discussed in relation to any of the previous embodiments. Finally, said dielectric strip is attached, in block 803, to the leaky transmission-line structure (or specifically to the outer surface of the leaky transmission line structure) so as to cover at least at one or more locations of said one or more openings. The attach- ing may be carried out, for example, using an adhesive (i.e., a glue) or an adhesive tape. 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.