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C. Yepes Julia
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In this letter, we present a design of a linear array of tilted dipoles to achieve radiation patterns with asymmetric angular filtering characteristics. To realize the asymmetric radiation, the dipole elements are spaced by a distance larger than half a wavelength, thus allowing for grating lobes to occur in the visible region. Moreover, the dipoles are loaded with artificial dielectrics to increase the front-to-back ratio and consequently to enable higher gain in certain desired angular regions. Based on the design, a linear array with ten elements is manufactured and tested. The measured results show the ability of such an array to achieve stable gain from broadside up to 90° scanning while implementing a stopband angular filter for negative scanning angles.
...
In this letter, we present a design of a linear array of tilted dipoles to achieve radiation patterns with asymmetric angular filtering characteristics. To realize the asymmetric radiation, the dipole elements are spaced by a distance larger than half a wavelength, thus allowing for grating lobes to occur in the visible region. Moreover, the dipoles are loaded with artificial dielectrics to increase the front-to-back ratio and consequently to enable higher gain in certain desired angular regions. Based on the design, a linear array with ten elements is manufactured and tested. The measured results show the ability of such an array to achieve stable gain from broadside up to 90° scanning while implementing a stopband angular filter for negative scanning angles.
3D Elements for Phased-Array Systems
Analysis and Design
In recent years, radar systems and satellite communications require phased array antennas that are capable of incorporating frequency and angular selectivity while maintaining a low profile. Active phased array antennas must comply with stringent requirements in terms of sensitivity to interference caused by other nearby radiating systems, especially in complex platforms, where a multitude of sensors and radiating systems need to co-exist. In such environments, antennas working at different frequencies can interfere with each other and the implementation of frequency filtering functions with a good out-of-band rejection are needed. Moreover, the interference between different systems can be mitigated by reducing the radiation in the direct path between them. For this purpose, angular filtering functions for pattern shaping can be beneficial. However, standard planar printed circuit board technology puts constraints on the possible antenna elements that can be realized to achieve frequency and angular selectivity. Thus, using new methods such as additive manufacturing technology and 3D printing can provide more degrees of freedom to fabricate complex geometries with a desired operation. For these reasons, this work focuses on studying new solutions for frequency selectivity with rejection of higher order harmonics, developing a spectral method of moment to study antennas to achieve angular shaping and finding the guidelines needed to design such antennas, and testing additive manufacturing technology to find its suitability at high frequencies for phased array antennas. A bandpass miniaturize-element frequency selective surface with harmonic rejection properties has been designed and manufactured. The design is based on an equivalent circuit model, taking a 3-pole Chebyshev bandpass filter as a starting point, where the inter-layer interaction is only described with a single transmission line representing the fundamental Floquet wave. The prototype consists of five metallic layers, interdigitated patches and grids, separated by dielectric slabs and exhibiting good stability over a wide conical incidence range. A practical case to estimate the effects of placing the FSS in the proximity of a wide-scanning wideband connected array of dipoles has been performed. The performance of the array combined with the FSS has been experimentally characterized, defining the optimal distance between FSS and array to avoid the propagation of surface waves between both structures, showing a good response within the FSS bandwidth and a good frequency rejection outside of this bandwidth. A spectral method of moments for tilted elements in free space and in the presence of a backing reflector for infinite and finite arrays has been derived. Such method allows to study dipole and stacked dipole elements and find the guidelines needed to design, in a future, a phased array antenna with an undesired angular range where suppression of gain is intended. The parametric study concluded that the main parameters that shapes the pattern are the inter-element distance between elements and the tilt angle of the elements. The requirements to achieve an asymmetric radiation pattern are a directive element and an inter-elements distance higher than half wavelength, while the tilt of the elements allows to shape the gain levels in the suppressed angular region. To validate this study a linear arrays consisting of tilted dipoles loaded with artificial dielectric layers has been fabricated. The prototype shows a good comparison with simulations and measurements. A simple design of a dipole antenna has been derived and fabricated using Stereolithography process as an additive manufacturing method. The polymer and metal paste used in the process have been characterized and results have been discussed. A good agreement between simulations and measurements has been achieved after including the geometric deviations found in the fabricated antenna. The fabrication process for high frequencies appears to be prone to systematic errors and the challenges related to the use of additive manufacturing technology for high frequency RF antennas and components has been discussed.
...
In recent years, radar systems and satellite communications require phased array antennas that are capable of incorporating frequency and angular selectivity while maintaining a low profile. Active phased array antennas must comply with stringent requirements in terms of sensitivity to interference caused by other nearby radiating systems, especially in complex platforms, where a multitude of sensors and radiating systems need to co-exist. In such environments, antennas working at different frequencies can interfere with each other and the implementation of frequency filtering functions with a good out-of-band rejection are needed. Moreover, the interference between different systems can be mitigated by reducing the radiation in the direct path between them. For this purpose, angular filtering functions for pattern shaping can be beneficial. However, standard planar printed circuit board technology puts constraints on the possible antenna elements that can be realized to achieve frequency and angular selectivity. Thus, using new methods such as additive manufacturing technology and 3D printing can provide more degrees of freedom to fabricate complex geometries with a desired operation. For these reasons, this work focuses on studying new solutions for frequency selectivity with rejection of higher order harmonics, developing a spectral method of moment to study antennas to achieve angular shaping and finding the guidelines needed to design such antennas, and testing additive manufacturing technology to find its suitability at high frequencies for phased array antennas. A bandpass miniaturize-element frequency selective surface with harmonic rejection properties has been designed and manufactured. The design is based on an equivalent circuit model, taking a 3-pole Chebyshev bandpass filter as a starting point, where the inter-layer interaction is only described with a single transmission line representing the fundamental Floquet wave. The prototype consists of five metallic layers, interdigitated patches and grids, separated by dielectric slabs and exhibiting good stability over a wide conical incidence range. A practical case to estimate the effects of placing the FSS in the proximity of a wide-scanning wideband connected array of dipoles has been performed. The performance of the array combined with the FSS has been experimentally characterized, defining the optimal distance between FSS and array to avoid the propagation of surface waves between both structures, showing a good response within the FSS bandwidth and a good frequency rejection outside of this bandwidth. A spectral method of moments for tilted elements in free space and in the presence of a backing reflector for infinite and finite arrays has been derived. Such method allows to study dipole and stacked dipole elements and find the guidelines needed to design, in a future, a phased array antenna with an undesired angular range where suppression of gain is intended. The parametric study concluded that the main parameters that shapes the pattern are the inter-element distance between elements and the tilt angle of the elements. The requirements to achieve an asymmetric radiation pattern are a directive element and an inter-elements distance higher than half wavelength, while the tilt of the elements allows to shape the gain levels in the suppressed angular region. To validate this study a linear arrays consisting of tilted dipoles loaded with artificial dielectric layers has been fabricated. The prototype shows a good comparison with simulations and measurements. A simple design of a dipole antenna has been derived and fabricated using Stereolithography process as an additive manufacturing method. The polymer and metal paste used in the process have been characterized and results have been discussed. A good agreement between simulations and measurements has been achieved after including the geometric deviations found in the fabricated antenna. The fabrication process for high frequencies appears to be prone to systematic errors and the challenges related to the use of additive manufacturing technology for high frequency RF antennas and components has been discussed.
In this work, we present a study on the radiation and impedance properties of antenna arrays with tilted elements. We developed a spectral method of moments (MoM) for the analysis of doubly-periodic arrays with arbitrarily tilted dipole or stacked dipole elements, in free space or in the presence of a backing reflector. By using this analysis method, we study the radiation characteristics of arrays of stacked dipoles over a ground plane, highlighting the variation of the patterns as a function of the inter-element distance and the angle of inclination of the elements. The guidelines derived from the MoM analysis have been applied to the design of a linear array of tilted stacked dipoles.
...
In this work, we present a study on the radiation and impedance properties of antenna arrays with tilted elements. We developed a spectral method of moments (MoM) for the analysis of doubly-periodic arrays with arbitrarily tilted dipole or stacked dipole elements, in free space or in the presence of a backing reflector. By using this analysis method, we study the radiation characteristics of arrays of stacked dipoles over a ground plane, highlighting the variation of the patterns as a function of the inter-element distance and the angle of inclination of the elements. The guidelines derived from the MoM analysis have been applied to the design of a linear array of tilted stacked dipoles.
Conference paper
(2019)
-
Cristina Yepes Julia, Erio Gandini, Stefania Monni, Frank E. Van Vliet, Andrea Neto, Daniele Cavallo
In this work, antenna arrays with tilted dipoles are investigated in terms of radiation and impedance properties. A spectral method of moments (MoM) was developed for the analysis of doubly-periodic arrays (i.e. periodic in both x and ydirections)
with arbitrarily tilted dipole elements, in free space or in the presence of a backing reflector. By the aid of this analysis method, the radiation characteristics of arrays of stacked dipoles over a ground plane are studied, highlighting the variation of the
patterns as a function of the inter-element distance and the angle of inclination of the elements. ...
with arbitrarily tilted dipole elements, in free space or in the presence of a backing reflector. By the aid of this analysis method, the radiation characteristics of arrays of stacked dipoles over a ground plane are studied, highlighting the variation of the
patterns as a function of the inter-element distance and the angle of inclination of the elements. ...
In this work, antenna arrays with tilted dipoles are investigated in terms of radiation and impedance properties. A spectral method of moments (MoM) was developed for the analysis of doubly-periodic arrays (i.e. periodic in both x and ydirections)
with arbitrarily tilted dipole elements, in free space or in the presence of a backing reflector. By the aid of this analysis method, the radiation characteristics of arrays of stacked dipoles over a ground plane are studied, highlighting the variation of the
patterns as a function of the inter-element distance and the angle of inclination of the elements.
with arbitrarily tilted dipole elements, in free space or in the presence of a backing reflector. By the aid of this analysis method, the radiation characteristics of arrays of stacked dipoles over a ground plane are studied, highlighting the variation of the
patterns as a function of the inter-element distance and the angle of inclination of the elements.
Abstract
(2019)
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Cristina Yepes Julia, Ralph van Schelven, Daniele Cavallo, Erio Gandini, Stefania Monni, Frank E. Van Vliet
Antenna arrays located on airplanes or other mobile platforms for satellite communication applications are typically required to support very large scan angles (close to end-_re). How- ever, planar antenna arrays are typically characterized by scan loss. To increase the scan range conformal arrays and multi-panel configuration can be found in literature, but the height of the structures is still too large to be installed on airplanes without significant impact on the aircraft drag. To obtain wide-scan capability while still maintaining a low antenna profile, hybrid scanning mechanism are currently implemented (F. Tiezzi et al., Eu- CAP, 2010). The idea is to scan the beam electronically from broadside to a positive, as high as possible, angle and then to achieve the full coverage by mechanical rotation of the array along the azimuth. In this work we present a study to determine the parameters affecting the power radiated in specific directions by antenna arrays with tilted elements. We developed an efficient Method of Moments (MoM) for the analysis of linear arrays with arbitrarily tilted dipole elements, in free space or in the presence of a backing reflector. By using this analysis method, we study the radiation characteristics of arrays of stacked dipoles over a ground plane, highlighting the variation of the patterns as a function of the inter-element distance and the angle of inclination of the element. For half wavelength inter-element distance the array pattern is rather symmetric even for tilted elements, as expected from the Floquet theory (A.K. Bhattacharyya et al., IEEE TAP, 51, 1572-1581, 2003). However, when the elements are tilted by positive angles and for inter-element distance larger than half wavelength, a null and a reduction in directivity in the radiation pattern is achieved for negative scanning angles. Grating lobes in the visible region are weighted by element pattern. The minimum of the radiation pattern is not necessarily aligned with the dipole axis but, due to mutual coupling, it can move to different angles depending on the combination of inclination angle and distance. The array directivity is almost at for positive angles up to very large scanning directions, and decreases rapidly at specific negative angles that change with the inter-element distance. Moreover, it is shown that, for large arrays (i.e. with more than 10 elements), the shape of the active element pattern does not change significantly as a function of the number of elements. At the conference considerations concerning the effects on the active element pattern of mutual coupling and of the onset of grating lobes for a given grid size/tilt angle will be presented. Moreover, a linear array design with selective pattern characteristics will be presented.
...
Antenna arrays located on airplanes or other mobile platforms for satellite communication applications are typically required to support very large scan angles (close to end-_re). How- ever, planar antenna arrays are typically characterized by scan loss. To increase the scan range conformal arrays and multi-panel configuration can be found in literature, but the height of the structures is still too large to be installed on airplanes without significant impact on the aircraft drag. To obtain wide-scan capability while still maintaining a low antenna profile, hybrid scanning mechanism are currently implemented (F. Tiezzi et al., Eu- CAP, 2010). The idea is to scan the beam electronically from broadside to a positive, as high as possible, angle and then to achieve the full coverage by mechanical rotation of the array along the azimuth. In this work we present a study to determine the parameters affecting the power radiated in specific directions by antenna arrays with tilted elements. We developed an efficient Method of Moments (MoM) for the analysis of linear arrays with arbitrarily tilted dipole elements, in free space or in the presence of a backing reflector. By using this analysis method, we study the radiation characteristics of arrays of stacked dipoles over a ground plane, highlighting the variation of the patterns as a function of the inter-element distance and the angle of inclination of the element. For half wavelength inter-element distance the array pattern is rather symmetric even for tilted elements, as expected from the Floquet theory (A.K. Bhattacharyya et al., IEEE TAP, 51, 1572-1581, 2003). However, when the elements are tilted by positive angles and for inter-element distance larger than half wavelength, a null and a reduction in directivity in the radiation pattern is achieved for negative scanning angles. Grating lobes in the visible region are weighted by element pattern. The minimum of the radiation pattern is not necessarily aligned with the dipole axis but, due to mutual coupling, it can move to different angles depending on the combination of inclination angle and distance. The array directivity is almost at for positive angles up to very large scanning directions, and decreases rapidly at specific negative angles that change with the inter-element distance. Moreover, it is shown that, for large arrays (i.e. with more than 10 elements), the shape of the active element pattern does not change significantly as a function of the number of elements. At the conference considerations concerning the effects on the active element pattern of mutual coupling and of the onset of grating lobes for a given grid size/tilt angle will be presented. Moreover, a linear array design with selective pattern characteristics will be presented.
Abstract
(2019)
-
Cristina Yepes Julia, Erio Gandini, Stefania Monni, Frank E. Van Vliet, Andrea Neto, Daniele Cavallo
We propose a systematic approach to describe planar slot antennas, embedded in generic stratified media. An equivalent transmission line model for the slot is proposed, based on a spectral domain analysis. First, we introduce a method of moments solution to model semi-infinite slots, fed by a deltagap excitation. The solution entails only two basis functions, one located at the feed and the other at the termination. The latter basis function is chosen to properly account for the field diffractive behavior at the antenna end point. An approximate circuit model is then introduced, which describes the main mode propagating along the slot as an equivalent transmission line. Lumped impedances are extracted to accurately describe the source and the end point. This procedure can be used to derive the input impedance of planar antennas with arbitrary length in generic layered media or the interaction between multiple feeds within the same slot.
...
We propose a systematic approach to describe planar slot antennas, embedded in generic stratified media. An equivalent transmission line model for the slot is proposed, based on a spectral domain analysis. First, we introduce a method of moments solution to model semi-infinite slots, fed by a deltagap excitation. The solution entails only two basis functions, one located at the feed and the other at the termination. The latter basis function is chosen to properly account for the field diffractive behavior at the antenna end point. An approximate circuit model is then introduced, which describes the main mode propagating along the slot as an equivalent transmission line. Lumped impedances are extracted to accurately describe the source and the end point. This procedure can be used to derive the input impedance of planar antennas with arbitrary length in generic layered media or the interaction between multiple feeds within the same slot.
Journal article
(2019)
-
Cristina Yepes, Erio Gandini, Stefania Monni, Andrea Neto, Daniele Cavallo, Frank E. van Vliet
In this article, we investigate the radiation and impedance properties of arrays of tilted dipoles. A spectral periodic method of moments (MoM) is developed for the analysis of infinite arrays with arbitrarily tilted dipole elements, in free space or with a backing reflector. With the aid of this analysis method, the radiation characteristics of arrays of stacked dipoles over a ground plane are studied, explaining the variation of the patterns as a function of the interelement distance and the angle of inclination of the elements. Finite linear arrays of tilted dipoles are also investigated, to assess the dependence of the array characteristics on the number of elements. The developed method can be used to design arrays with nonsymmetric radiation patterns for angular filtering or pattern shaping.
...
In this article, we investigate the radiation and impedance properties of arrays of tilted dipoles. A spectral periodic method of moments (MoM) is developed for the analysis of infinite arrays with arbitrarily tilted dipole elements, in free space or with a backing reflector. With the aid of this analysis method, the radiation characteristics of arrays of stacked dipoles over a ground plane are studied, explaining the variation of the patterns as a function of the interelement distance and the angle of inclination of the elements. Finite linear arrays of tilted dipoles are also investigated, to assess the dependence of the array characteristics on the number of elements. The developed method can be used to design arrays with nonsymmetric radiation patterns for angular filtering or pattern shaping.
Conference paper
(2019)
-
Cristina Yepes Julia, Andrea Neto, Daniele Cavallo, E Gandini, Frank E. Van Vliet, Stefania Monni
We present the design of an array of tilted dipoles loaded with artificial dielectrics, to achieve asymmetric radiation patterns. First, a simplified array element consisting of skewed stacked dipoles in the presence of a backing reflector is studied, by means of a dedicated spectral periodic method of moments (MoM). The analysis of the stacked dipoles provides guidelines to design arrays with certain radiation characteristics, by varying the tilt angle and the inter-element spacing. Based on the findings of numerical analysis, we then propose a tilted dipole element design with artificial dielectric loading to implement the pattern asymmetry. The design includes the feeding structure and the balun transition to a 50-Ohm coaxial line.
...
We present the design of an array of tilted dipoles loaded with artificial dielectrics, to achieve asymmetric radiation patterns. First, a simplified array element consisting of skewed stacked dipoles in the presence of a backing reflector is studied, by means of a dedicated spectral periodic method of moments (MoM). The analysis of the stacked dipoles provides guidelines to design arrays with certain radiation characteristics, by varying the tilt angle and the inter-element spacing. Based on the findings of numerical analysis, we then propose a tilted dipole element design with artificial dielectric loading to implement the pattern asymmetry. The design includes the feeding structure and the balun transition to a 50-Ohm coaxial line.
Conference paper
(2018)
-
Christina Yepes, Ralph van Schelven, Daniele Cavallo, Erio Gandini, Andrea Neto
In this work we present a study to determine theparameters affecting the power radiated in specific directions byantenna arrays with tilted elements. We developed an efficientMethod of Moments (MoM) for the analysis of linear arrays witharbitrarily tilted dipole elements, in free space or in the presenceof a backing reflector. By using this analysis method, we studythe radiation characteristics of arrays of stacked dipoles overa ground plane, highlighting the variation of the patterns as afunction of the inter-element distance and the angle of inclinationof the element. It is shown that, for large arrays (i.e. with morethan 10 elements), the shape of the active element pattern doesnot change significantly as a function of the number of elements,but it depends on the inter-element distance and the skew angleof the array elements. A few examples to show this capabilityare presented.
...
In this work we present a study to determine theparameters affecting the power radiated in specific directions byantenna arrays with tilted elements. We developed an efficientMethod of Moments (MoM) for the analysis of linear arrays witharbitrarily tilted dipole elements, in free space or in the presenceof a backing reflector. By using this analysis method, we studythe radiation characteristics of arrays of stacked dipoles overa ground plane, highlighting the variation of the patterns as afunction of the inter-element distance and the angle of inclinationof the element. It is shown that, for large arrays (i.e. with morethan 10 elements), the shape of the active element pattern doesnot change significantly as a function of the number of elements,but it depends on the inter-element distance and the skew angleof the array elements. A few examples to show this capabilityare presented.
Journal article
(2018)
-
Cristina Yepes, Daniele Cavallo, Erio Gandini, Stefania Monni, Andrea Neto, Frank E. van Vliet
A five-layer frequency selective surface (FSS) composed of subwavelength elements with large harmonic rejection bandwidth is presented. The FSS design is based on an equivalent circuit model, where the interlayer interaction is only described with a single transmission line representing the fundamental Floquet wave. A prototype of the designed FSS is fabricated, and the measured response exhibits good stability over a wide conical incidence range up to 45°. The FSS is combined with a wide-scanning connected array of dipoles to implement a phased array with integrated filtering properties. A dispersion analysis is performed to define the distance between the array and the FSS that avoids the propagation of surface waves between the combined structures, allowing to maximize the radiation efficiency. The performance of the array combined with the FSS is experimentally characterized, showing high-order harmonic rejection better than 17 dB over a large bandwidth.
...
A five-layer frequency selective surface (FSS) composed of subwavelength elements with large harmonic rejection bandwidth is presented. The FSS design is based on an equivalent circuit model, where the interlayer interaction is only described with a single transmission line representing the fundamental Floquet wave. A prototype of the designed FSS is fabricated, and the measured response exhibits good stability over a wide conical incidence range up to 45°. The FSS is combined with a wide-scanning connected array of dipoles to implement a phased array with integrated filtering properties. A dispersion analysis is performed to define the distance between the array and the FSS that avoids the propagation of surface waves between the combined structures, allowing to maximize the radiation efficiency. The performance of the array combined with the FSS is experimentally characterized, showing high-order harmonic rejection better than 17 dB over a large bandwidth.
Conference paper
(2018)
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Cristina Yepes, Erio Gandini, Raymond Van Dijk, Fabien Bruning, Hessel Maalderink, Stefania Morini, Frank E. Van Vliet
A 24 GHz antenna design for miniaturized FMCW radar is presented. The antenna was manufactured with additive manufacturing technology at TNO. The characterization of the polymer material used in the fabrication was carried out and the measurement results of the antenna are presented in this work. A good agreement between simulations and measurements was achieved.
...
A 24 GHz antenna design for miniaturized FMCW radar is presented. The antenna was manufactured with additive manufacturing technology at TNO. The characterization of the polymer material used in the fabrication was carried out and the measurement results of the antenna are presented in this work. A good agreement between simulations and measurements was achieved.
Conference paper
(2017)
-
Cristina Yepes, Daniele Cavallo, Andrea Neto, Erio Gandini, Stefania Monni, Frank E. van Vliet
We present the design of a frequency selective surface (FSS) with large harmonic rejection bandwidth. The FSS comprises five metal layers, each consisting of a periodic array of subwavelength elements. The design is based on an equivalent transmission line model that includes only the fundamental Floquet mode. This model is valid under the condition that the unit cell size is smaller than the distance between layers. The multi-layer FSS has an overall height of one sixth of the free-space wavelength at the central frequency of operation. The simulated frequency response exhibits good stability over a wide conical incidence range, up to 45 degrees. A prototype of the designed FSS is fabricated and combined with a wide-scanning connected array of dipoles to implement an antenna element for phased array applications, with integrated filtering properties.
...
We present the design of a frequency selective surface (FSS) with large harmonic rejection bandwidth. The FSS comprises five metal layers, each consisting of a periodic array of subwavelength elements. The design is based on an equivalent transmission line model that includes only the fundamental Floquet mode. This model is valid under the condition that the unit cell size is smaller than the distance between layers. The multi-layer FSS has an overall height of one sixth of the free-space wavelength at the central frequency of operation. The simulated frequency response exhibits good stability over a wide conical incidence range, up to 45 degrees. A prototype of the designed FSS is fabricated and combined with a wide-scanning connected array of dipoles to implement an antenna element for phased array applications, with integrated filtering properties.
In this work, the problem of enhancing the frequency selectivity property of an antenna array has been studied. A solution based on a multi-layer frequency selective surface (FSS), composed of sub wavelength elements is proposed. An equivalent circuit model is used for the design, under the assumption that the inter-layer interaction is dominated by a single Floquet mode. This approximation remains valid by keeping the unit-cell size comparable to the inter-layer distance. Results for generic oblique incidence are presented. Fast roll-off is achieved by using a 3 pole Chebyshev filter design. Moreover, the FSS is used in combination with a connected slot array antenna. The resulting structure allows to scan up to 45° in both principal planes with a relative bandwidth (S11<; -10 dB) of 15% around the central frequency f0 = 4.8 GHz. Higher order harmonics are rejected up to 18 GHz.
...
In this work, the problem of enhancing the frequency selectivity property of an antenna array has been studied. A solution based on a multi-layer frequency selective surface (FSS), composed of sub wavelength elements is proposed. An equivalent circuit model is used for the design, under the assumption that the inter-layer interaction is dominated by a single Floquet mode. This approximation remains valid by keeping the unit-cell size comparable to the inter-layer distance. Results for generic oblique incidence are presented. Fast roll-off is achieved by using a 3 pole Chebyshev filter design. Moreover, the FSS is used in combination with a connected slot array antenna. The resulting structure allows to scan up to 45° in both principal planes with a relative bandwidth (S11<; -10 dB) of 15% around the central frequency f0 = 4.8 GHz. Higher order harmonics are rejected up to 18 GHz.