Analysis of Miniaturized, Circularly Polarized Antennas for Bidirectional Propagation

Size reduction is necessary to fit the recent demand for small sized communication systems in consumer electronics. Wireless communication systems rely on antennas for long range transmission of signals, so size reduced antennas have been sought after in recent years. Also, not many antennas are designed for use in bidirectional scenarios like subways, tunnels, bridges, etc. Three sized reduced antennas with circular polarization are presented for use in bidirectional communication systems. An electrically small pattern reconfigurable array, an electrically small two-sided printed cross dipole, and a size reduced printed wideband antenna are introduced within this thesis. All antennas’ results are obtained from simulation, with two of the antenna designs being measured to verify their results

Wideband Circularly Polarized Elements and Arrays for Wireless Systems

Circularly polarized (CP) antennas have received increasing interest during recent decades due to their unique features such as the mitigation of multi-path fading, reduction of the "Faraday rotation" effect when signals propagate through the ionosphere and immunity of the polarization mismatching between transmitting and receiving antennas. Due to the requirements of high date rate and large system capacity, CP antennas deployed in various wireless systems are always demanded to have wide bandwidth. Furthermore, other system requirements such as polarization diversity, wide-angle beam scanning and low power consumption impose additional requirements to CP antennas. Therefore, it is becoming a more stringent requirement to design wideband CP antennas with diverse features to fulfil the requirements of various wireless systems. In this thesis, six different types of wideband CP antenna elements and arrays are designed, fabricated and characterized to meet the different demands of wireless systems. Chapters 3-5 investigate three different types of wideband CP antenna elements while Chapters 6-8 investigate three different kinds of wideband CP array antennas. In Chapter 3, an ultra-wideband CP element with a bandwidth of 100% (3:1) is proposed. It over-comes the problem of limited 3 dB axial ratio (AR) bandwidth for single-feed CP antennas and achieves high front-to-back ratio (FBR) by using a novel ground plane with simple configuration, which makes it a good candidate for high-performance Global Navigation Satellite System (GNSS) receivers. Chapter 4 presents a wideband loop antenna with electronically switchable circular polarizations. It solves the issue of narrow overlapped bandwidth under different polarization states for a polarization reconfigurable CP antenna. Because of the available orthogonal polarizations across a wide bandwidth, this antenna can be deployed in wireless communications which implement polarization diversity. The third antenna element investigated in Chapter 5 tackles the difficulty of designing wide-band wide AR beamwidth CP antennas. It achieves wide AR beamwidth within a 42% bandwidth, which is suitable for wideband wide-angle CP beam-scanning applications. The second main part of this thesis focuses on the investigation of wideband CP arrays. In Chapter 6, a dual-CP beam-scanning array is investigated, which can scan its beam independently in right-hand circular polarization (RHCP) and left-hand polarization (LHCP) from 27 GHz to 30 GHz. It tackles the problem of low isolation between the two orthogonally polarized ports across a wide bandwidth at Ka-band. A single-layer high-efficiency CP reflectarray is proposed in the following Chapter. The proposed design solves the issues of bandwidth limitation and low aperture efficiency for single-layer CP reflectarrays. It achieves the widest bandwidth compared with other CP reflectarrays reported in terms of 3 dB AR bandwidth, 3 dB gain bandwidth, larger than 50% aperture efficiency and undistorted radiation pattern bandwidth. In Chapter 8, we investigate the first application of tightly coupled array (TCA) concept into ultra-wideband arrays with CP radiation. Instead of trying to reduce the mutual coupling among the elements, it exploits the strong mutual coupling to improve the bandwidth of a CP array. By using the strong coupling in a constructive way, it overcomes the bandwidth limitation of CP arrays which are constituted by narrow-band elements

Design of a Broadband Circularly Polarized Antenna by Using Axial Ratio Contour

A novel method of using axial ratio (AR) contour to design a broadband circularly polarized (CP) antenna is presented in this letter. By resolving a radiated CP wave into two orthogonal far-field components, the resulted AR will be determined by the radiated power and phase differences between the two orthogonal components. Therefore, a series of AR contours can be obtained for different AR values of the radiated wave. In the AR contour, details of phase-advance, phase-lag, power-advance, and power-lag can be simultaneously observed along with the resulted AR. Based on these analyses, the AR contour is first utilized to directly design a broadband CP antenna instead of using the traditional AR versus frequency curve to achieve optimal AR bandwidth. Compared to the traditional AR curve, the method of using AR contour to design CP antenna is more effective and informative. The designed broadband CP antenna was also fabricated and measured for the final performance verification

An Ultra-Wideband Circularly Polarized Asymmetric-S Antenna With Enhanced Bandwidth and Beamwidth Performance

This paper introduces an ultra-wideband circularly polarized (CP) asymmetric-S antenna with wide axial ratio beamwidth (ARBW) for C-band applications. The proposed antenna is realized by bending a linearly polarized dipole into asymmetric-S shape with variable trace width, which achieves CP radiation. Unlike the reported symmetric-S antenna, the proposed antenna is constituted with two unequal curved arms to enhance the bandwidth and beamwidth performances. Compared with the symmetric-S antenna, the proposed antenna demonstrates much wider AR bandwidth and wider ARBW over broader frequency range. A prototype is fabricated to verify the design principle. The measured and simulated results are very consistent and both indicate that the proposed antenna has a wide impedance bandwidth (VSWR <; 2) of 70.2% (3.58 to 7.46 GHz), and a wide 3-dB AR bandwidth of 84.8% (2.75 to 6.8 GHz). Moreover, maximum ARBW of 153° is achieved, and a 3-dB ARBW of more than 100° is maintained within a wide operation bandwidth of 46.3% (3.65-5.85 GHz)

Advanced Circularly Polarised Microstrip Patch Antennas

The thesis describes outcomes of research on advanced circularly polarised antennas. The proposed designs are intended for integration into small mobile devices, therefore low profile and easy manufacturability are key parameters, along with good CP radiation properties. The designs were validated by simulation and measurement, and are also backed by theory and design guidelines. The primary focus is on the development of planar omnidirectional circularly polarised antennas, which are fabricated using multilayer PCB techniques and thus are lightweight and cost-efficient. Unlike in classical microstrip patch antenna designs, the groundplane of the proposed antenna was substantially reduced. This helps to achieve an omnidirectional circular polarisation pattern and miniaturize the antenna, however at the cost of increased feed circuit complexity. The basic design, its advantages and disadvantages are discussed in Section 3. In the next step, the omnidirectional circularly polarised antenna was extended with additional, advanced features. A miniaturized version is investigated, which offers a 20% footprint reduction by folding parts of the patch underneath itself. Further miniaturization is possible by increasing the dielectric constant of the substrate. A method to adjust the omnidirectional circularly polarised antenna performance by trimming four lumped capacitors is also investigated. Manufacturing inaccuracy in large scale production may cause some of the units to radiate outside of the desired frequencies. By integrating four trimmed capacitors into the antenna it can be precisely tuned to the desired band. Simulated results demonstrate this property by trimming the antenna between GPS L1 band (centre frequency at 1.575 GHz) and Galileo/Beidou-2 E2 band (1.561 GHz). Furthermore, a dual-band omnidirectional circularly polarised antenna is presented, which employs slots and capacitor loading to steer the current path of the first and second resonant mode. The design offers a small frequency ratio of 1.182. The methods to obtain a planar omnidirectional circularly polarised antenna have been further advanced to propose a reconfigurable antenna. The beam reconfiguration is capable of rotating it dipole-like radiation pattern around an axis, thus allowing reception or transmission from any spherical angle. The switching method is simple and does not require any semiconductor devices. Finally, a dual circularly polarised antenna is presented, which achieves dual-polarisation by employing even and odd modes in a coplanar waveguide. This technique allows greater flexibility and size reduction of the feed network, as two signals can be transmitted by a single multi-mode transmission line. Simulated results demonstrate this property by trimming the antenna between GPS L1 band (centre frequency at 1.575 GHz) and Galileo/Beidou-2 E2 band (1.561 GHz). Furthermore, a dual-band omnidirectional circularly polarised antenna is presented, which employs slots and capacitor loading to steer the current path of the first and second resonant mode. The design offers a small frequency ratio of 1.182. The methods to obtain a planar omnidirectional circularly polarised antenna have been further advanced to propose a reconfigurable antenna. The beam reconfiguration is capable of rotating it dipole-like radiation pattern around an axis, thus allowing reception or transmission from any spherical angle. The switching method is simple and does not require any semiconductor devices. Finally, a dual circularly polarised antenna is presented, which achieves dual-polarisation by employing even and odd modes in a coplanar waveguide. This technique allows greater flexibility and size reduction of the feed network, as two signals can be transmitted by a single multi-mode transmission line

Wideband and UWB antennas for wireless applications. A comprehensive review

A comprehensive review concerning the geometry, the manufacturing technologies, the materials, and the numerical techniques, adopted for the analysis and design of wideband and ultrawideband (UWB) antennas for wireless applications, is presented. Planar, printed, dielectric, and wearable antennas, achievable on laminate (rigid and flexible), and textile dielectric substrates are taken into account. The performances of small, low-profile, and dielectric resonator antennas are illustrated paying particular attention to the application areas concerning portable devices (mobile phones, tablets, glasses, laptops, wearable computers, etc.) and radio base stations. This information provides a guidance to the selection of the different antenna geometries in terms of bandwidth, gain, field polarization, time-domain response, dimensions, and materials useful for their realization and integration in modern communication systems

Compact-Size Wideband Antennas and Arrays for Wireless Communications

Polarization is an important parameter for characterizing antenna systems. Dual-polarized and circularly-polarized wideband antennas with compact size are very useful for mobile communications and satellite communications. Due to the multipath propagation and shadowing in urban environment, radio signals received by mobile terminals can become very weak. Dual-polarized antennas can achieve better signal quality in mobile communications by using polarization diversity. Wideband circularly polarized antennas are very important for mobile satellite communications as circularly polarized signals are immune to Faraday rotation effects. Circular polarization also enables mobile satellite communications without strict alignment between transmit and receive antennas. Therefore, dual-polarized antennas and circularly polarized antennas have been drawn increasing popularity in the wireless communication systems. In this thesis, several novel designs of compact, wideband, and specially functioned antennas and arrays are developed for wireless communication applications. First, wideband antennas and arrays are investigated for base station applications with different appealing features, such as compact radiator size, enhanced upper out-of-band suppression, or low pattern sidelobes. They are designed with different novel design concept, such as shared-dipole, electromagnetic dipoles, shorted dipoles, and fourth-order coupling structure. Then, to directly match to the newly emerged differential circuit systems, several wideband differentially fed dual-polarized antennas are proposed for base station applications. They are designed for high common mode suppression, high harmonic suppression, or compact radiator size by using the idea of orthogonal six-port power divider, multi-resonance structure, and crossed open loop resonators. The final designs are two circularly polarized antennas, which have the wide overlapped impedance and axial ratio bandwidth, or dual circularly polarized radiations realized by using crossed open slot-pairs, orthogonal power diver, and phase shift unit cells. The working principles of these different antennas are extensively illustrated with the relevant design theories and detailed structure studies. The performances of these antennas and arrays are evaluated first by the full-wave electromagnetics simulations, and followed by the measurements of the corresponding fabricated prototypes. Good agreements between the simulated and measured results are obtained. With these different features to accommodate different requirements, these antennas and arrays can be the good candidates for the wireless communication systems

Novel Asymmetric T-Shaped Radiating Element for Circularly-Polarized Waveguide Slot Arrays

Waveguide slot arrays are a widely used solution to attain linearly-polarized high-gain antennas. In this paper, an all-metal T-shaped radiating element is conceived with the aim to replace the usual rectangular slots and attain circularly-polarized arrays, preserving their good properties. The polarization conversion is made possible thanks to the inclusion of a parasitic arm perpendicular to the main active slot. Due to the fact that this second arm is not directly fed by the waveguide but coupled to the active slot, the T-shaped element can be used in any array originally formed by metallic rectangular slots. The design of resonant shunt arrays has been addressed here for validation purposes. The experimental results of two sample linear arrays at 30 GHz demonstrate the design accuracy and manufacturing reliability, reporting an axial ratio below 2 dB within a bandwidth of 1.9 GHz and a peak efficiency around 98%.This work was supported by the Spanish Government, Agencia Estatal de Investigación (AEI), under project PID2019-107688RB-C22

Design and Analysis of Circularly Polarized Electrically Small Antennas

A growing need for efficient wireless communication is prevalent in the world in which we live. From cell phones to television to GPS applications, wireless communications are vital in consumer electronics and military applications. In these applications, a miniaturized antenna is sometimes necessary for reducing overall size of the communication system. For many satellite based communication applications, circular polarization in antennas is needed for efficient communication. In this thesis, the miniaturization technique known as T-top loading is utilized on two novel antenna designs. One design is an electrically small, circularly polarized planar cross dipole and the other design is a compact circularly polarized log-periodic dipole array. Both antennas are designed in simulation software with the intent for prototype fabrication for measurement verification of simulation results