EP3584887A1 - Structure à ondes de fuite diélectrique - Google Patents

Structure à ondes de fuite diélectrique Download PDF

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Publication number
EP3584887A1
EP3584887A1 EP18178556.9A EP18178556A EP3584887A1 EP 3584887 A1 EP3584887 A1 EP 3584887A1 EP 18178556 A EP18178556 A EP 18178556A EP 3584887 A1 EP3584887 A1 EP 3584887A1
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EP
European Patent Office
Prior art keywords
leaky
wave
openings
longitudinal direction
wave structure
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EP18178556.9A
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German (de)
English (en)
Inventor
Ahmed Handouk
Marko Sonkki
Marko Tuhkala
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Premix Oy
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Premix Oy
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Priority to EP18178556.9A priority Critical patent/EP3584887A1/fr
Priority to PCT/FI2019/050468 priority patent/WO2019243665A1/fr
Publication of EP3584887A1 publication Critical patent/EP3584887A1/fr
Withdrawn legal-status Critical Current

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01Q—ANTENNAS, i.e. RADIO AERIALS
    • H01Q13/00—Waveguide horns or mouths; Slot antennas; Leaky-waveguide antennas; Equivalent structures causing radiation along the transmission path of a guided wave
    • H01Q13/20—Non-resonant leaky-waveguide or transmission-line antennas; Equivalent structures causing radiation along the transmission path of a guided wave
    • H01Q13/203—Leaky coaxial lines

Definitions

  • the present invention relates to leaky transmission lines.
  • 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 electromagnetic 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 openings (or slots) in the outer conductor of the otherwise closed transmission-line structure.
  • Leaky-wave structures have found applications especially in closed environments where radio communication needs to be provided for moving vehicles, for example, in tunnels, underground roads and subways. In such scenarios, conventional antenna solutions (i.e., point source antenna solutions) provide insufficient coverage unless a large number of periodically distributed antennas is employed.
  • the radiation pattern provided by most leaky-wave structures is roughly omnidirectional, that is, the radiation is spread almost equally to all directions 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 multipath-heavy closed environments, this property limits the use of leaky-wave structures in many application where there is a need for higher gain in a particular direction. 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 omnidirectional leaky-wave structure is used. Thus, there is need for a leaky-wave solution providing more adjustable radiation performance compared to the conventional omnidirectional solutions.
  • 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 enclosing said at least one inner conductor, a layer of a first dielectric material separating the outer conductor from said at least one inner conductor and an outer surface having a first area and a second area which are disjoint areas located on different sides of the leaky transmission-line structure, where-in the outer conductor comprises one or more openings arranged along a longitudinal direction of the section enabling leakage of the electromagnetic waves from the leaky transmission-line structure; and a dielectric strip of a second dielectric material arranged to cover only said first area at least at one or more locations of said one or more openings.
  • a method comprising providing a leaky transmission-line structure for guiding electromagnetic waves, wherein the leaky transmission-line structure 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; providing a dielectric strip; and attaching the dielectric strip to the leaky transmission-line structure so as to cover at least at one or more locations of said one or more openings.
  • Figures 1A, 1B and 1C illustrate a leaky-wave structure according to an exemplary embodiment.
  • Figures 1A, 1B and 1C illustrate a leaky-wave structure oriented along an x-axis (as defined in said Figures) from three perspectives.
  • Figure 1A provides a perspective view of the leaky-wave structure showing a cut-plane
  • Figure 1B shows a cross-sectional view (i.e., a view of the yz- plane)
  • Figure 1C shows a view from "above” (i.e., a view of the xy-plane) of a section of the leaky-wave structure.
  • Figure 1B may correspond to the cut-plane visible in Figure 1A on the right-hand side.
  • 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.
  • said section 120 providing leaky-wave properties is terminated from both ends as shown in Figure 1C .
  • the leaky-wave structure of Figures 1A, 1B 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.
  • the leaky transmission-line structure 101 in the illustrated embodiment is a leaky coaxial cable (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").
  • 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-sectional shapes (e.g., elliptical) may be employed. In 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 polyethylene, polyethylene foam, polytetrafluoroethylene or air space polyethylene.
  • the dimensions of the coaxial cable may be according to a standard type of coaxial cable, for example, according to MIL-C-17 standard or according to NF-C-93550 standard.
  • the outer conductor 102 comprises one or more openings 105 (i.e., slots, holes or apertures) arranged 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).
  • the outer conductor comprises one continuous opening 105 arranged parallel 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 opening 105 is rectangular in shape, other shapes may be employed in other embodiments.
  • the opening 105 may be shaped like an ellipse extending along the longitudinal direction. In some embodiments, the opening 105 may be arranged at an angle relative to the x-axis. In other embodiments, two or more openings may be employed as will be discussed in detail in relation to Figure 2 .
  • a leaky coaxial cable that is, the leaky coaxial cable 101 without the dielectric strip 110
  • 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.
  • 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.
  • 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.
  • 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 direction as illustrated in Figures 1A, 1B 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 conductor 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 between 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. In 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 polarized along the longitudinal direction (assuming the opening(s) in the outer conductor are narrow and symmetric).
  • 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) often require more robust and flexible solutions providing adjustable radiation performance.
  • indoor communication scenarios e.g., office scenarios
  • 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.
  • the dielectric strip 110 may cover all of said one or more openings 105, as depicted in Figure 1C .
  • a leaky transmission-line structure 101 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 to cover only said first area 130 at least at one or more locations of said one or more openings 105.
  • the dielectric strip 110 of the second dielectric material cannot fully enclose the leaky transmission-line structure 101.
  • the first area 130 may comprise (or be located over) all or only some of said one or more openings 105.
  • 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 arranged 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 1B . The dielectric 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.
  • the dielectric strip 110 may cover over one half of the circumference of the outer conductor 104, but does not fully enclose it.
  • 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 properties 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 direction (i.e., angular direction along the curvature of the outer conductor and orthogonal to the x-direction) than the opening 105, as illustrated in Figure 1C .
  • 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 dielectric layer 103 of first dielectric material may be in contact with each other.
  • the thickness of the dielectric strip may be smaller or significantly smaller than the width of the dielectric strip. In some embodiments, 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 material. In other embodiments, the thickness of the dielectric strip may be 1 mm - 4 mm.
  • two or more dielectric strips 110 may be arranged 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.
  • 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).
  • the permittivity of the second dielectric material should be considerably larger than the permittivity of air (e.g., relative permittivity being 11).
  • the relative permittivity of the second dielectric material is larger than two.
  • the relative permittivity of the second dielectric material is larger than four, preferably larger than six, even more preferably larger than eight.
  • the relative permittivity of the second dielectric material is larger than nine, preferably larger than ten, even more preferably larger than eleven.
  • the permittivity of the second dielectric material is larger than the permittivity of the first dielectric material.
  • 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 embodiments relating to permittivities of both the first and the second dielectric materials may be equally defined using corresponding relative permittivities.
  • the second dielectric material may have low dielectric losses.
  • 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 structure.
  • the relative permittivity of the first dielectric material may preferably be relatively close to one (e.g., 1.25).
  • the dielectric strip 110 is capable of acting as a coupling element which couples (or facilitates the coupling of) the propagating electromagnetic wave from the coaxial cable to the outside space (i.e., free space) via said one or more openings 105.
  • the wavelength of an electromagnetic wave inside 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 dielectric material compared to a low-permittivity dielectric material or vacuum.
  • 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 dielectric strip 110 results in the leaking electromagnetic field being concentrated farther from the outer conductor which, in turn, prevents the leaking electromagnetic energy from coupling to the outer surface of the outer conductor 104 and specifically 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.
  • the leaky-wave structure of Figures 1A, 1B and 1C may further comprise an outer plastic sheath or a jacket (not shown in Figures 1A, 1B 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.
  • 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).
  • the outer conductor 104 of the leaky coaxial cable 101 comprised one continuous opening extending in the longitudinal direction
  • two or more openings may be arranged in the outer control 104.
  • Figure 2 shows a view and an orientation of the leaky-wave structure similar to Figure 1C , i.e., a view of the xy-plane with the leaky-wave structure oriented along x-axis.
  • the leaky-wave structure of Figure 2 may be similar to the leaky-wave structure of Figures 1A, 1B 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 adjacent openings 207 of said two or more openings may be smaller free-space wavelengths of the leaky-wave structure.
  • said spacing is (very) small compared 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.
  • said spacing is small or very small (e.g., at least smaller than the smallest operational 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, 1B 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.
  • Figure 2 illustrates an exemplary leaky-wave structure with rectangular 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.
  • 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. In 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.
  • the transmission-line structure may be a rectangular, spherical or ellipsoidal waveguide or a multi-conductor coaxial cable.
  • 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 employed.
  • Coupled radiation mode two distinct guided modes are supported by conventional coupled mode leaky coaxial cables (like the ones illustrated in Figures 1A, 1B 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.
  • 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. If 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 radiation performance are provided.
  • SMRS sinusoidally modulated reactance surface
  • ⁇ x j ⁇ 0 X 1 + M cos 2 ⁇ x a
  • ⁇ o the free-space wave impedance ( ⁇ 120 ⁇ ⁇ )
  • X the average surface reactance normalized by the free-space wave impedance
  • M the modulation factor governing the leakage rate
  • a the periodicity of modulation.
  • the parameters ( X, M, a ) define, to a large extent, the characteristics of a SMRS.
  • Figures 4A and 4B illustrate a leaky-wave structure comprising a dielectric strip which implements a SMRS according to an embodiment.
  • Figure 4A shows a view from "above” (i.e., a view of the xy-plane) and
  • Figure 4B shows a view from "the side” (i.e., a view of the xz-plane) with the leaky-wave structure oriented along the x-direction.
  • the dielectric strip 410 is illustrated using a dotted pattern for improved clarity.
  • the illustrated leaky-wave structure may be similar to the one illustrated in Figure 1A, 1B and 1C with one key difference: the dielectric strip 410 is loaded with a SMRS 406 providing a sinusoidal variation in the impedance (or specifically reactance) of a surface wave mode. Specifically, 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.
  • 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 patterning method may comprise, for example, one of silk screen printing, photoengraving, milling, laser resist ablation or laser etching or any other established copper patterning method.
  • 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 sinusoidal manner.
  • the reactance may follow the reactance given by equation (1).
  • 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.
  • each conformal patch i.e., length along the azimuthal direction as described above
  • 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 direction.
  • 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 . In other embodiments, 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 period of the sinusoidal pattern.
  • said two or more metallic/metallized elements 406 may comprise multiple periods of the illustrated sinusoidally modulated pattern.
  • the leaky-wave structure loaded with a SMRS as illustrated in Figures 4A and 4B provides an increased radiation efficiency compared to a conventional leaky-wave structure (e.g., a corresponding conventional leaky coaxial cable).
  • a conventional leaky-wave structure e.g., a corresponding conventional leaky coaxial cable.
  • the signal strength of the signal transmitted by the leaky-wave structure is also increased.
  • the coupling loss is a parameter which characterizes the mutual coupling intensity between leaky-wave structure and outside environment.
  • a higher coupling loss corresponds to a larger portion 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 following dimensions.
  • the total length of the simulated coaxial cable -based structure was 830 mm.
  • the inner and outer conductor diameters of the coaxial line were 9.3 mm and 25.2 mm, respectively.
  • the relative permittivity 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 ).
  • 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 discussed in relation Figures 4A , 4B and 5 is not the only way to improve the performance (e.g., radiation efficiency and field strength) of the leaky-wave structure using 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.
  • Figure 6A shows a view from "above” (i.e., a view of the xy -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.
  • the leaky-wave structure illustrated in Figures 6A and 6B may be similar to the one discussed in relation to Figure 1A, 1B and 1C .
  • the dielectric strip 610 is illustrated using a dotted pattern for improved clarity.
  • one or more metallic elements are deposited on a surface of a dielectric strip 610 of the second dielectric 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 metallic 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 .
  • said metallic (or metallized) elements may also be manufactured as discussed above in relation to Figures 4A and 4B .
  • said one or more metallic 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 connected 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.
  • 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.
  • 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 facilitating said coupling at a specific frequency band defined by the dimensions of the given conformal patch antenna.
  • conformal patch antennas have directive radiation patterns with the radiation being directed predominantly in directions orthogonal (or near orthogonal) to the patch surface (maximum being in the direction orthogonal to the surface of the patch at a center point of the patch)
  • the introduction 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 antennas 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.
  • said two or more series fed conformal patch antennas 606, 607, 608 may have different dimensions providing different resonance frequencies when excited 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.
  • the length of the patch 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).
  • three series fed conformal patch antennas 606, 607, 608 having a square shape all have different dimensions and thus different resonance frequencies.
  • Each conformal patch antenna 606, 607, 608 may be resonant at its fundamental ( ⁇ /2) resonance frequency as well as at one or more higher resonance frequencies corresponding higher resonance modes supported by the leaky coaxial cable 601. Therefore, an additional improvement in the radiation efficiency/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 antenna and each narrow metal strip 616, 617 may be arranged along a common line parallel to the longitudinal direction. In 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.
  • all the series fed patch antennas may have the same shape and equal dimensions.
  • the longitudinal center points of adjacent patch antennas may have equal spacing relative to each other.
  • 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.
  • 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.
  • 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, similar 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 illustrated 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
  • 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).
  • each metallic element may form an individual patch antenna with the outer conductor acting as the ground plane.
  • a leaky transmission-line structure for guiding electromagnetic waves.
  • the leaky transmission-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.
  • a dielectric strip 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.
  • said dielectric strip is attached, in block 803, to the leaky transmission-line structure so as to cover at least at one or more locations of said one or more openings.
  • the attaching may be carried out, for example, using an adhesive (i.e., a glue) or an adhesive tape.

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