A pattern-reconfigurable electrically small antenna (ESA) that radiates circularly polarized broadside pattern and linearly polarized omnidirectional pattern is proposed. The ESA consists of four dipole arms on the top layer and five dipole arms on the bottom layer. The two layers are connected by five vertical shorting pins. The connection states between different dipole arms and pins are electrically controlled by three inserted PIN diodes to generate the equivalent even- and odd-mode states, which correspond to reconfigurable broadside and omnidirectional patterns. The measured results demonstrate that in the odd-mode state, the boresight realized gain (RG) and axial ratio, front-to-back ratio, and radiation efficiency (RE) are 3.22 dBic, 2.57 dB, 6.39 dB, and 65.2%. In the even-mode state, the measured RG, out-of-roundness values, and RE of the omnidirectional pattern are 1.55 dBi, 2.19 dB, and 92.2%. Corresponding to the wavelength of 1.616 GHz, the profile is 0.04λ0, and the cross-section is π × 0.152λ02, i.e., ka = 0.98.

Low-Profile, Pattern-Reconfigurable, Electrically Small Antenna Based on Equivalent Even and Odd Modes

This paper presents the design of a simple dual-band circularly polarized monopole antenna using characteristic mode analysis. First, a dual-band elliptically polarized “L”-shaped monopole antenna with a partial ground is designed; then, a rectangular stub and a parasitic structure on the ground plane are implemented to achieve dual-band CP operation. To enhance impedance bandwidth and generate circular polarization in the upper band, the rectangular stub is attached to the “L”-shaped strip. The parasitic structure is employed for simultaneous dual-band CP radiation. Characteristic mode analysis is undertaken to predict the performance of the antenna before excitation. The modal analysis which is undertaken before excitation shows the natural modes that can be excited by the antenna structure to generate a dual-band CP response. The analysis gives approximate bandwidths that can be achieved by the antenna even before excitation. The overall dimension of the antenna 
 
 is
  
 0.379
 
 
 λ
 
 
 0
 
 
 ×
 0.379
 
 
 λ
 
 
 0
 
 
 ×
 0.015
 
 
 λ
 
 
 0
 
 
 
 , where 
 
 
 
 λ
 
 
 0
 
 
 
 is the corresponding free-space wavelength at 5.7 GHz. The measured -10 dB impedance bandwidth (ZBW) is realized to be 75.9% (4.5 GHz–10 GHz). The measured 3 dB axial ratio bandwidths (ARBW) at the lower and upper bands are 6% (5.6 GHz–5.95 GHz) and 28% (6.65 GHz–8.82 GHz), respectively. The proposed antenna features a simple and compact structure for Wi-Fi, WLAN, WiMAX, and C band applications.

Design and Analysis of a Low-Profile Dual-Band Circularly Polarized Monopole Antenna Based on Characteristic Mode Analysis

This paper introduces a compact three-bent-dipole end-fire array operating at 916 MHz, exhibiting remarkably high realized gain in relation to its electrical size. The optimization process focuses on feeding the array elements to maximize directive properties in a chosen direction while upholding high efficiency as a constraint. Complex excitation coefficients are subsequently transformed into equivalent impedance loads for implementing a parasitic array. Two antennas are designed, fabricated, and characterized. For the first prototype (loaded), the central element is actively fed, and the external elements are loaded with reactive components. A second prototype (load-less) is designed to incorporate the required complex excitations by adjusting the imaginary parts of the dipoles through modifications in their vertical length. Both array versions are matched with a T-match on the active elements. Experimental results from differentially-fed fabricated prototypes reveal outstanding agreement between simulated and measured radiation properties. In the differential mode, a peak realized gain of 8.9 dBi at 910 MHz is achieved for the parasitic loaded array, and 8.3 dBi at 911 MHz for the load-less array.

Loaded and Load-Less Supergain Parasitic End-Fire Arrays

Two 28 GHz broadside-radiating unidirectional mixed-multipole antenna (UMMA) designs are presented. These UMMAs are self-resonant metamorphoses of previously reported systems. Both versions are matched to a $\mathbf{50} \Omega$ source. Details of their directivity, front-to-back ratio, realized gain, and radiation efficiency performance characteristics are reported. It is demon-strated that both of them are superdirective, highly efficient and have interesting bandwidths. Their intrinsic attributes surpass several recognized bounds. These results add further credence to the fact that superdirective systems with useful properties are indeed practical in contradistinction to the general perception that they are not.

Self-Resonant Broadside-Radiating Superdirective Unidirectional Mixed-Multipole Antennas

The fundamental bound on maximum antenna gain is expressed as a sum of lossy characteristic modes, specifically, as a sum of characteristic far fields squared. The procedure combines the favorable properties of Harrington's classical approach to maximum gain and current-density-based approaches. The decomposition into modes makes it possible to study the degrees of freedom of an obstacle, classify its radiation into normal or super-directive currents and determine their compatibility with a given excitation. The bound considers an arbitrary shape of the design region and specific material distribution. The cost in Q-factor and radiation efficiency is also studied. The extra constraint of a self-resonance current is imposed for an electrically small antenna. The example verifies the developed theory, demonstrates the procedure's utility, and provides helpful insight to antenna designers.

pdf/fundamental-bound-on-maximum-gain-as-a-sum-of-lossy-1lwm20rw.pdf

Fundamental Bound on Maximum Gain as a Sum of Lossy Characteristic Modes and Its Feasibility

A low-profile dipole antenna with improved bandwidth and gain is presented in this work. The basic dipole is realized with one parasitic radiation arm, corresponding to high gain at low profile condition. Moreover, a parasitic patch with eight slots etched on it is located above the dipole for bandwidth enhance. Finally, the developed dipole antenna is achieved with a low profile of only 0.068 $\lambda_{0}(\lambda_{0}$ is the operating wavelength in free-space). It exhibits a −10dB impedance bandwidth of 8.5% (0.79-0.86 GHz) and a high gain of 8.1-8.9dBi with a stable gain fluctuation of 0.8 dBi over the operating band.

A Low-Profile Parasitic Dipole Antenna with Improved Bandwidth and Gain

A Compact and Wideband Circularly Polarized Microstrip Patch Antenna Driven by Sequentially Rotated Dipole Array

Electrically small, high-directivity antennas are in demand for a variety of current and future wireless applications. An electrically small directive antenna (ESDA) that requires only one specially-engineered port to excite a set of multipoles is demonstrated in this paper. Four 90° copper sectors are combined with additional structures and fed with a coaxial cable. Two resonant quadrupoles (equivalent to two pairs of resonant electric dipoles) and one magnetic dipole are excited. Both high radiation efficiency and good impedance matching are achieved. Theoretical calculations, numerical simulations, and experimental measurements are shown to be in good agreement. An optimized prototype is designed, fabricated, and tested. The measured results confirm that it is a supergain system. The unidirectional ESDA has a peak directivity of 6.71 dBi, a peak realized gain of 6.31 dBi, a radiation efficiency of 94.5%, and a front-to-back ratio of 14.89 dB at its resonance frequency, 814 MHz. Its height is 0.06 λres and ka = 0.98. These measured realized gain and directivity values exceed both the Harrington and Kildal-Best ka-based upper limits.

Multipole-based Electrically Small Unidirectional Antenna with Exceptionally High Realized Gain

A low-profile circularly polarized (CP) metasurface-based (MS-based) antenna with enhanced axial-ratio bandwidth (ARBW) and stable high gain is developed. The developed antenna has a stacked layout with the MS containing 3 × 3 patch lattices, which are placed above a custom-designed driven element. The driven element comprises a patch with an etched slot and a parasitic ring strip with shorting pins, which can excite the MS to generate multiple resonances and three AR minimum points, thus resulting in enhanced ARBW and stable high gain. Finally, an antenna prototype is designed to confirm the simulation results. The experimental (simulated) results demonstrate that the MS-based antenna is low-profile: 0.03 λ0, where λ0 corresponds to the operating central frequency at 2.63 GHz, achieving a −10 dB fractional bandwidth (FBW) of 20.48 (19.81)%, and 3 dB ARBW of 10.2 (11)% along with a stable gain of 8.1–8.8 (8.0–9.2) dBic, respectively.

A Low-Profile, Circularly Polarized, Metasurface-Based Antenna With Enhanced Bandwidth and Stable High Gain

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A Low-Profile Parasitic Dipole Antenna with Improved Bandwidth and Gain

A Compact and Wideband Circularly Polarized Microstrip Patch Antenna Driven by Sequentially Rotated Dipole Array

Multipole-based Electrically Small Unidirectional Antenna with Exceptionally High Realized Gain

A Low-Profile, Circularly Polarized, Metasurface-Based Antenna With Enhanced Bandwidth and Stable High Gain