Quasi-optics-inspired low-profile endfire antenna element
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In this paper, the design, operational principle and experimental validation of an endfire antenna element that is inspired by the energy-focusing characteristics of graded-index optical fibre are presented. The antenna operates with a bandwidth of 1.5 GHz centred around 14 GHz. It has a gain of around 5.5 dBi along the band of interest and a good pattern stability over the band. The antenna derives its unique nature from the arrangement of the arc-shaped parasitics, that couple onto the antenna´s driver dipole. An electromagnetic refractive index retrieval mechanism is used to guide the placement of the parasitics to enhance the gain. In addition to the design principle, a parametric study of the main parameters and their influence on the antenna behaviour is presented. The antenna is a potential candidate for use in multi-user massive MIMO antenna arrays for 5G communications where space is premium and in antenna array applications where a low-profile antenna element with a high gain is a necessity.
High-frequency structure simulator
SubMiniature version A
Scenarios of high data-rate transmissions of several gigabits per second require the use of wide bandwidth and high directivity beams. The wide bandwidth is attained by shifting to a higher millimetre wave frequency, and to obtain high directivity beam, antennas need to be designed to focus the beam with high front-to-back (F/B) ratio. When employed in an array, such an antenna further enhances the directivity offered from the array. Recently there has been an increasing interest in the 14–15-GHz band [1, 2, 3]. A printed antenna configuration is attractive owing to the simplicity of manufacturing, scalability and the reduced cost. The conventional patch or printed dipole antennas generally have a low directivity omnidirectional pattern. Among the techniques used to enhance this directivity, the use of parasitic elements coupled to the printed dipole is appealing. Accordingly, an endfire antenna element based on parasitics aided quasi-Yagi and fibre-optics principle is proposed in this paper. Following the analogy to the graded refractive index optical fibre energy-focusing principle, the designed antenna element has its gain and directivity enhanced—mainly by a unique arrangement of parasitics around the driving sources, aided by reconfigured refractive index—referred to as quasi-optic approach.
Endfire antenna topologies such as the helical antenna, antipodal Vivaldi antenna and the Yagi-Uda antenna have been used in applications such as ultra-wideband, phased array, radars and microwave imaging. Printed endfire antenna elements have been used for high-gain applications. These are generally fed by a half wavelength resonant driver, and the coupling between the driver element and the director element is set to maximize the achievable gain. The compactness of resonant type antennas and the broad bandwidth characteristics were both simultaneously achieved in a single Yagi antenna  where a microstrip to CPS transition balun was used to add to the profile length. With the compactness and broad bandwidth addressed by antennas in [4, 5], papers [6, 7] focus on improving the antenna gain using techniques of a microstrip-fed inset patch as the driver, with parasitic patches added as directors to enhance the gain. The dipole feeding point in this antenna design is based upon , where the antenna operates in the 2–4-GHz band with a gain of around 5 dBi over the band. Since Yagi-Uda antennas or the endfire antennas in general are protrude-out-of-wall configurations, it adds immensely if this dimension of profile length could be minimized while retaining the high-gain characteristics. The novelty of the antenna that is presented in this paper is threefold: it offers the characteristics of [4, 5, 6, 7], proposes a feeding structure to minimize the profile length and adds around 3 dBi to the gain of a basic Yagi antenna, by incorporating a specific, variable refractive index-guided novel arrangement of arc-shaped parasitics, in front of the driver dipole. This launches the surface waves in such a way as to aid their transformation into directional far-field components. A prototype of the low-profile antenna element which measures a square centimetre across has been fabricated and experimentally validated.
The rest of the paper is organized as follows: the antenna element design principle is introduced in Section 2, Section 3 discusses the antenna fabrication and experimental validation of the antenna element with the measurement results and Section 4 concludes.
2 Antenna design principle
The design philosophy adopted rests on the use and integration of three basic elements that form the antenna design: the driver dipoles, the printed reflector and the parasitics. The antenna was brought into operation over an evolved sequential design following first the principles. In the following, the design is discussed for each of the three constituent basic elements.
2.1 Driver dipoles
2.2 The printed reflector
2.3 The parasitics
In the following, the design of the arc-shaped parasitics is discussed. The shape of the parasitics was chosen as an arc, to channel the electric field obliquely. In addition, it also maximizes the coupling by maximizing the amount of exposed surface area that couples onto the next parasitic. The fibre-optic principle was used to position the parasitics along the antenna plane in front of the square resonators. The design derives its inspiration by analogy to a graded-index fibre-optic cable, whose core can be considered analogous to a certain cuboidal volume region, lying directly below the endfire central axis of the antenna, where the graded real refractive index is set to a maximum. The outward displacement towards the edges can be considered to represent the cladding—where there is a gradient decrease of the real refractive index moving away from the centre. This is achieved by the methodical placement of the arc-shaped parasitics in front of the driver dipoles over the antenna plane. The procedure to arrive at this is discussed next.
2.4 Effect of antenna backplate
3 Measurements and discussion
A low-profile endfire antenna element inspired on fibre-optic principles is presented. A gradient-refractive index-based theoretical framework of the design, supported by parametric variation, is discussed. The antenna prototype has been fabricated and experimentally validated. It has an operation over 1.5 GHz of bandwidth centred around 14 GHz with a directional high-gain radiation pattern. The inherent layered structure when incorporated in an array aids to the better cooling of the antenna front end. The antenna element can find potential use in multi-user massive MIMO antenna arrays as well as in multibeam switching antenna arrays for 5G. The prototype has been adopted in a massive MIMO array and its performance has been validated. These findings are presented in  dedicated to the design and implementation of the massive MIMO array and its multi-beam characteristics. Moreover, a performance evaluation of the fabricated antenna in a unique reconfigurable array setup applied to practical scenarios using spatial modulation is presented in .
VB proposed the main idea, completed the experiments, analysed the results and wrote the paper. BB contributed in the pattern measurements. LC and JB read the paper and contributed critical constructive reviews. All authors read and approved the final manuscript.
This work was supported by the 5G wireless project that has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 641985.
The authors declare that they have no competing interests.
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