Field of the invention
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The present invention relates to an antenna, more specifically to an antenna operable in the GHz range as used for example as monopole antennas in wireless communication.
Description of related art
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A theoretical monopole antenna includes a monopole arranged perpendicular to a nominally infinite or nearly infinite ground plane. There are also approximately planar monopole antennas known where the nominally infinite ground plane is arranged coplanar to a monopole with both mounted onto the surface of a (dielectric) substrate. In other arrangement the ground plane is mounted onto the opposite side of the substrate, typically covering at least a broad uniform strip or all of the opposite side. The actual antenna part of the monopole is linked to other parts of a transmitting and/or receiving device by a monopole feeding line which can be implemented as a central conductor or feed line shielded on both sides by ground feed lines. In many designs the transmitting/receiving part of a monopole antenna has an increased width compare to the width of the feeding line. For example a monopole antenna could flare into a triangular shape or widen into a circular, rectangular or other shapes from the feeding line part of the antenna. This widening is normally created for the purpose of having wider bandwidth.
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The article "Coplanar Waveguide Fed Ultra-Wideband Antenna Over the Planar and Cylindrical Surfaces" from Rafael Lech et al. published in The 8th European Conference on Antennas & Propagation, 2014 (EuCAP 2014), Hague, Netherlands, 6-11 April 2014, pp. 3737-3740 discloses an antenna arrangement similar to the design shown in Fig. 1 to 3. Fig. 1 shows a top view, while Fig. 2 shows the cross-sectional view along line II-II and Fig. 3 shows the cross-sectional view along line III-III. The ground plane in this arrangement is formed by a circular ring-shaped ground conductor 2 surrounding an inner area. A circular monopole conductor 1 is mounted onto the substrate 3 within this inner area or opening of the ground conductor 2. Both are arranged coplanar on the same side 31 of a substrate 3 while the opposite side 32 of the substrate is free of conducting structures.
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The circular monopole conductor 1 is electrically coupled to transmit/receive circuity (not shown) via the monopole feeding line 4. The ground conductor 2 is similarly electrically coupled to ground of the transmit/receive circuitry by the ground feeding lines 5. The ground conductor 2 and the ground feeding lines 5 shield the monopole feeding line 4 coupled to the monopole conductor 1 arranged in the opening of the ring-shaped ground conductor 2. The characteristics of the antenna 11 of FIG. 1 depend mainly on the separation distance between the ground conductor 2 and the monopole 1, particularly on the following geometrical parameters: the radius r1 of the monopole conductor 1, the outer radius r2 and the inner radius r3 of the ring-shaped ground conductor 2, the distance d of the feeding point 101 of the monopole conductor 1 to the inner border 21 of the ring-shaped ground conductor 2 and the distance between the monopole feeding line 4 to the ground feeding line 5 on both sides. The feeding point 101 of the monopole conductor 1 is herein defined as the point at which the monopole conductor 1 begins to widen from the (constant)width of the monopole feeding line. In other words, the feeding point 101 can be understood as the point at which there is the transition from the monopole feeding line into the monopole or monopole conductor 1.
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The antenna as described by Lech et al. has disadvantages. For example, the finite distance d of the feeding point 101 of the monopole 1 from the point where the monopole feeding line 4 crosses the inner circumference of the ground conductor 2 introduces a constraint when determining the correct geometrical parameters of the antenna for a desired performance. On the other hand, reducing the distance d to zero in a device according to Lech et al. would cause a short circuit between the radiation monopole 1 and the ground conductor 2.
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Further, the arrangement of Lech et al. makes it difficult to connect a coaxial connector 6 to the monopole feeding line 4 and the ground feeding line 5. In the state of the art as shown in Figs. 1 to 3, the coaxial connector 6 is mounted on the edge of the substrate 3 so that the ground connector 7 is connected to the ground feeding line 5 and the signal connector 8 is connected to the monopole feeding line 4. Thus, the longitudinal axis of the coaxial connector appears as an exposed stub extending from the edge of the substrate. The known design cannot be easily adapted to change the location of the connector.
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Therefore, it can be seen as an object of the invention to improve the present antenna design to address at least some of the above-mentioned problems associated with the prior art. It is another object of the invention to provide an antenna which allows for a mechanically more stable connection for coupling a feed line from a transmit/receive circuitry to the antenna.
Brief summary of the invention
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According to a first aspect of the invention, there is provided a monopole antenna having on a first side of a plane dielectric substrate a first layer of conducting material being shaped as a of the antenna and defining a boundary around a central area of the substrate free of conducting material, and having a second layer of conducting material being shaped as monopole of the antenna on the opposite side of the substrate, wherein most of the second layer (and hence the monopole of the antenna) is located within the boundary formed by an inner contour of the ground conductor when projected perpendicular onto the second side of the substrate.
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The plane substrate can be flat or curved.
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When referring to most of the second layer being located within the projected perimeter, it is understood that when excluding feeding lines at least 90 percent, or even 95 percent or preferably 100 percent of the area covered by the second structure is within the projected boundary.
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The ground conductor can be a strip or strips of conductive material forming a frame or ring around a central area with all strips connected in operation to a common potential. The inner contour of the ground conductor forms the boundary of the central area or opening. In cases where the ground conductor is structured as ring segments or the like this boundary can be defined by virtually bridging any gaps in the ground conductor structure smoothly to form a continuous line around the central area.
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The second structure or monopole conductor can be a structure which flares or widens after a feeding point to cover a wider area within the projected perimeter. The widened structure can be for example elliptical including circular, rectangular including square, or triangular.
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In terms of design and performance simulation, the described arrangement appears to be more complex than an arrangement as described by Lech et al. as the additional parameters of the thickness of the substrate and its electromagnetic properties must be considered. In addition, it is not immediately evident that similar radiation patterns can be achieved using the new arrangement. However, it can be shown that such radiation patterns can be reproduced and may even be improved with this arrangement.
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It is seen as a further advantage of an antenna in accordance with the above aspect of the inventions that the arrangement of the monopole conductor and of the ground conductor on two opposite sides of the substrate allows for more flexibility when designing a desired antenna shape and the desired properties. In particular the feeding point of the monopole can now be placed very close to or exactly at the projected inner boundary of the ground conductor structure. Hence an equivalent of the distance d between the feeding point and the ground conductor can be made zero or close to zero. Such a value for d effectively eliminates this parameter from the consideration for the purpose of simulating the antenna.
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By arranging the monopole conductor and the structured ground conductor of the described type of antenna on opposite sides or faces of the substrate, it is also possible to implement a mechanically more stable arrangement for connector on the surface of the substrate. It will be appreciated that this and further aspects of the invention as relating to the connector can be implemented also independently of the aspect of the reducing the distance d or other aspects of the invention as described above.
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In one embodiment of the aspect of the invention, the antenna comprises a monopole feeding line on the second side of the substrate connected to the monopole conductor at the feeding point and a ground feeding line on the first side of the substrate connected with the ground conductor. This has the advantage that the arrangement of the monopole feeding line does not disturb the arrangement of the ground feeding line any more. Such an arrangement is not known in the field of antenna design, since usually only feeding lines with infinite ground planes on the other side of the substrate are considered ("strip line"). In order to design the feeding lines in the embodiment, they can be simulated as a macro strip line as known in the microwave engineering field (assuming for example two back-to-back finite width microstrip lines with a virtual infinite ground plane at a certain point in between them).
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In a further embodiment, the antenna comprises a connector on one of the first side and second side of the substrate connected with a ground connector to the ground feeding line and with a signal connector to the monopole feeding line, wherein one of the ground connector and the signal connector is connected by a through hole or via to the other of the first side and second side of the substrate. This allows mounting a connector mechanically stable on the surface of the substrate without changing the resistivity of the feeding lines. In particular, it is possible to include the mating part of a coaxial connector on the substrate such that the longitudinal axis of the coaxial connector is oriented perpendicular to the first and second surface of the substrate.
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Thus, the antenna can include a connector on the first side of the substrate connected by a ground connector to the ground feeding line and by a signal connector and a through hole to the monopole feeding line on the second side of the substrate.
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In one embodiment, the ground conductor structure and/or the ground feeding line is/are symmetrical with respect to a perpendicular projection of the longitudinal axis of the monopole feeding line onto the first side of the substrate.
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In one embodiment, the monopole conductor structure is symmetrically with respect the longitudinal axis of the monopole feeding line.
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In one embodiment, a width of the ground feeding line is larger than a width of the monopole feeding line.
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In one embodiment, the width of the ground feeding line is smaller than the width of the substrate in the same direction.
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In one embodiment, a width of the monopole feeding line, a width of the ground feeding line, a thickness of the substrate and an electromagnetic property of the substrate are configured such that the antenna has a desired input impedance, e.g. 50 Ohm, at the feeding point of the monopole of fifty Ohm. In particular, it can be designed such that feeding line has the desired input impedance at the feeding point and at any point along the monopole feeding line and the ground feeding line on the substrate, at least along the section where these two lines are aligned in parallel on their respective sides of the substrate. In this variant of the invention a connector for the antenna can be mounted at any point along the feeding lines without changing the performance of the antenna.
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In one embodiment, the geometrical center or, in cases where the geometrical center is difficult to define, the center of gravity of the conducting structure forming the ground conductor is offset, preferably in the direction of the feeding point, from the projection of the geometrical center or center of gravity of the monopole conductor onto the first side of the substrate.
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In one embodiment, an outer boundary and/or an inner boundary of the conducting structure forming the ring-shaped ground conductor is essentially similar to the outer boundary line of the structure forming the monopole conductor. In other words the boundaries of both structures of conducting material, whilst not necessarily parallel, closely match each other in shape but usually not in size as the structure forming the monopole is in most cases smaller than the inner boundary of the ground conductor
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In one embodiment, an outer boundary and/or an inner boundary of the ring-shaped ground conductor and/or the monopole conductor have essentially the shape of a circle.
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In one embodiment, the outer boundary and/or an inner boundary of the ring-shaped ground conductor have a common centre, and a circumscribing radius enclosing all of the monopole conductor structure is smaller than the radius of the inner boundary of the ring-shaped ground conductor.
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In one embodiment, geometrical parameters of the ground conductor and the monopole conductor, a thickness of the substrate and an electromagnetic property of the substrate are configured to result in a input impedance of fifty Ohm and a desired frequency behaviour.
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In one embodiment, the antenna is an ultra-wideband (UWB) antenna.
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The above and further aspects of the present invention are described by way of example in the following making reference to the drawings as listed below.
Brief Description of the Drawings
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The invention will be better understood with the aid of the description of an embodiment given by way of example and illustrated by the figures, in which:
- Fig. 1 shows a top view of an antenna of the prior art;
- Fig. 2 shows a cross-section II-II of Fig. 1;
- Fig. 3 shows a cross-section III-III of Fig. 1;
- Fig. 4 shows a top view of an antenna according to an example of the invention;
- Fig. 5 shows a cross-section V-V of Fig. 4;
- Fig. 6 shows a cross-section VI-VI of Fig. 4; and
- Fig. 7 shows the return loss over frequency of an antenna according to an example of the invention
Detailed Description
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Figs. 4 to 6 show one embodiment of an antenna 10 according to an example of the invention. Fig. 4 shows a top view of the embodiment of the antenna, while Fig. 5 and 6 show the cross-sections V-V and VI-VI, respectively.
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The antenna 10 comprises a substrate 13 with a first side 131 and a second side 132, a monopole conductor 11, a ground conductor 12, a monopole feeding line 14, a ground feeding line 15 and a connector 16. The feeding line 14 connects to the monopole at the feeding point 111.
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The substrate 13 is generally made of a dielectric material. The substrate 13 is chosen depending on the desired application. The electromagnetic properties of the substrate 13, especially its permittivity, influence also the characteristics of the antenna 10. Therefore, the properties of the substrate 13 must be considered when choosing the other parameters of the antenna. The substrate 13 of the example is a thin rectangular cuboid or parallelepiped with facing main sides or faces. Preferably, the first side 131 and the second side 132 are parallel to each other and/or flat. However, the substrate 13 can be also bent into a curved shape for specific applications. In the embodiment shown, the substrate 13 is a rigid plate, for example with a constant thickness. However, the substrate 13 can also be a flexible material like a foil and/or could be of varying thickness. The thickness of the substrate 13 refers to the distance separating the first side 131 and the second side 132.
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The ground conductor 12 comprises an electrically conducting material deposited as a layer onto the first side 131 of the substrate 13. The ground conductor 12 is approximately annular. It will be appreciated by a person skilled in the art that any other form of the ground conductor 12 suitable to enclose a central area of the surface of the substrate 13 can be used. Such forms can take an ellipsoidal, a triangular, a rectangular, a multi-angular or any other approximately or nearly closed shape. In the embodiment shown, the ground conductor 12 is defined by two concentric circular borders 121, 122 with an inner radius r12 and an outer radius r13, respectively. In the embodiment shown, the ground conductor 12 has a constant width w=r13 - r12 along its circumference. However, the ground conductor 12 can be of varying width.
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The ground conductor 12 can also be shaped as an open or segmented ring. For example the ground conductor 2 as shown in Fig. 1 illustrates a design of a circular open ground conductor with a single gap, which can be adapted for use in the present invention. And irrespective of the shape of the ground conductor structure 12, it is possible to define a closed boundary of minimal length which forms an inner boundary of the ground conductor structure. For example, any gap between two or more segments of the ground conductor can be bridged by extending the inner boundary across such gaps as indicated by the dashed circle segments of Fig 1.
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Such a boundary, whether physically present or defined, defines in turn a central area on the surface of the substrate 13, substantially enclosed by the ground conductor 12. In the example shown, this inner boundary 121 is simply the inner circle with the radius r12.
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The ground feeding line 15 is made of an electrically conducting material and is connected to the ground conductor 12. The ground feeding line 15 is arranged on the first side of the substrate 13. In the embodiment shown in Figs. 3 to 6, the ground feeding line 15 is a straight strip with constant width up to the point where it merges with the ground conductor 12. In the example shown, the antenna 10 has also a coaxial connector 16 and the ground feeding line 15 connects the ground conductor 12 to a ground connector 17 of the connector 16. However, in variants of this example, the ground feeding line 15 may be omitted when the ground connector 17 can be connected to the ground conductor 12 without a feeding line. Alternatively or in addition, the ground feeding line 15 can comprise two or more ground feeding lines, if, for example, the ground conductor 12 is shaped as an open ring, similar to Fig. 1.
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The monopole conductor 11 in Figs. 3 to 6 is a structure made of an electrically conducting material and deposited onto a second side 132 of the substrate 13. Thus, the monopole conductor 11 can be mounted on a second side 132, different from a first side 131 on which a ground conductor 12 may be deposited. The monopole conductor 11 is arranged such that its projection onto the first side 131 of the substrate 13 is at least partly within the area of the ring-shaped ground conductor 12. In the top view onto the first side of the substrate of Fig. 4, this projection is shown as a dashed circle of radius r11. As shown, the shape and size of the monopole conductor 11 is chosen such that all or at least most of its projection onto the first side 131 of the substrate 13 is within the central area of the ring-shaped ground conductor 12 as defined above.
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In the example, the projection of the monopole conductor 11 onto the second side 132 of the substrate 13 lies within the central area of the ring-shaped ground conductor 12 delimited by the inner boundary 121. The monopole conductor 11 has an exemplary form of a circle 112 with a radius r11. Preferably, the radius r11 is smaller than the inner radius r12 and/or the outer radius r13 of the ground conductor 12. However, in other examples (not shown), the monopole conductor 11 structure may include parts which extend beyond the inner boundary 121 of the central area as defined above or even beyond the outer boundary 122 of the ground conductor structure 12.
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As will be clear to a person skilled in the art neither the shape of the monopole conductor 11 nor the shape of the ground conductor are not limited to a circular shape. They can be ellipsoidal, triangular, rectangular, multi-angular, fractal, or any other shape. Further, the outer boundary 112 of the monopole conductor 11 can be shaped similar to one of the outer boundary and/or the inner boundary of the ground conductor 12 as shown. But the boundaries of the monopole 11 and the ground conductor 12 can also be shaped differently. In the example, the geometric centres of the monopole conductor 11 the ground conductor 12 do not coincide. However, said geometric centres could also coincide.
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In the example shown, the shape of the monopole conductor 11 on the second side 132 of the substrate 13 is chosen such that the outer boundary of its projection onto the first side 131, i.e. the dashed circle with radius r11 is collocated or nearly collocated in at least in one point with the inner boundary, i.e. the circle with the radius r12, of the ground conductor 12. In the example shown, this point is the feeding point 111 of the monopole. In this particular example, the distance between the projection of the feeding point 111 onto the first side 131 and the closest point on the inner boundary 121 of the ground conductor 12 is zero. In contrast to the known prior art that is constrained to a non-zero distance to avoid short-circuiting the monopole conductor 11 and the ground conductor 12, various embodiments of the invention provide more design choices with fewer constraints, enabling a wider variety of antenna designs. However, in accordance with other examples of the invention, it is also possible that the projection of the monopole conductor 11 on the first side 131 of the substrate 13 does not coincide with any point on the inner border 121 of the ring-shaped ground conductor 12.
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The characteristics of the antenna 10 in Figs. 3 to 6, for example the input impedance or the reflection coefficient, depends, among other things, on the thickness of the substrate 13, the electromagnetic property of the substrate 13 and the parameters of the geometrical arrangement of the ground conductor 12 and the monopole conductor 11. In the example shown, the parameters of the geometrical arrangement are, inter alia, r11, r12 and r13. Electromagnetic properties include, for example, the permittivity, permeability, and loss tangent.
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The monopole feeding line 14 is made of an electrically conducting material and is connected to the monopole conductor 11. As mentioned above, the point where the monopole feeding line 14 connects to the monopole conductor 11 is referred to as the feeding point 111. At the feeding point 111 the monopole conductor 11 widens or flares with respect to the width of the feeding line 14. The monopole feeding line 14 and the monopole conductor 11 are arranged on the second side 132 of the substrate 13. In the example shown, the monopole feeding line 14 is a straight strip with a constant width which is preferably (but not necessarily) smaller than the width of the ground feeding line 15. In the shown embodiment, the longitudinal axis of the monopole feeding line 14 is arranged radially to the monopole conductor 11. However, the monopole feeding line 14 can also be curved. The resistivity of the antenna's feeding arrangement depends on the thickness of the substrate 13, the electromagnetic properties of the substrate 13, the width of the ground feeding line 15 and the width of the monopole feeding line 14.
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For example, the input impedance at the feeding point 111 may be designed to match a desired input impedance. The desired input impedance is typically selected to match the transmitting and/or receiving circuitry (not shown). Values used are, for example, 50 Ohm or 75 Ohm.
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If the antenna 10 has also a coaxial connector 16, as shown, the monopole feeding line 14 connects the monopole conductor 11 with a signal connector 18 of the coaxial connector 16. With the impedance being matched to the desired value, e.g. 50 Ohm) at all points along the length of the feeding line 14, the location at which the connector 16 is mounted can be chosen with a greater degree of freedom. In some instances, the monopole feeding line 14 is fully optional and can be omitted and the connector 16 could be connected directly to the monopole conductor 11, for example at the feeding point 111.
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The connector 16 is configured to connect a ground line to the ground conductor 12 and a signal line to the monopole conductor 11. In the example shown, the connector 16 is a coaxial connector comprising a base 19, a ground connector 17 and a signal connector 18. The connector 16 is preferably connected onto one of the first side 131 or second side 132 of the substrate 13, preferably on the first side 131. The connector 16 may be fixed to the substrate 13, possibly with connector terminal(s)(not shown) on the ground feeding line 15 so that an electrical connection is established between the ground connector 17 and the ground feeding line 15. Alternatively, the ground connector 17 of connector 16 could be directly mounted on the ground conductor 12. In this case, the ground feeding line 15 may be superfluous. Since the monopole feeding line 14 is arranged on the second side 132, the signal connector 18 of the connector 16 can be connected by passing from the first side 131 to the second side 132 of the substrate 13 to the monopole feeding line 14, see Fig. 5. In the example shown, the signal connector 18 passes through a hole 133 in the substrate 13 and/or the ground conductor feeding line 15. In this case, the connector 16 is arranged directly on the ground conductor 12, the signal connector 18 can be connected to the monopole conductor 11 or the monopole feeding line 14 via a hole 133.
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Fig. 7 shows the return loss over frequency of an antenna 10 as described above when referring to FIG. 4 with the exemplary parameters r11 = 11.5 mm, r12 = 23.5 mm, r13 = 35.0 mm, the ground feeding width being 10 mm, the monopole feeding width being 3 mm, the thickness of the substrate 13 being 1.6 mm and the permittivity of the substrate 13 being 4.35. This exemplary antenna can be used in all frequencies which show less than -10 dB return loss. Acceptable return loss may be application specific, as will be clear to a person skilled in the art. Here, the present antenna performs as desired from a frequency of 1.8 GHz to at least over 5.5 GHz. Hence its exemplary bandwidth is at least 4.7 GHz.
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While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not of limitation. Likewise, the various diagrams may depict an example structure or other configuration for the invention, which is done to aid in understanding features and functionality that can be included in the invention. Further, it should be understood that the various features, aspects and functionality described in one or more of the individual embodiments are not limited in their applicability to the particular embodiment with which they are described, but instead can be applied, alone or in various combinations, to one or more of the other embodiments of the invention.
