The diagonal loading method is a simple and efficient method to improve the robustness of beamformers. However, how to determine the ideal diagonal loading level has not been adequately addressed. In this paper, it is observed in the simulation that the peak of the main beam is moved with the diagonal loading level when there exists a Direction of Arrival (DOA) estimation error. Based on the observation, a novel diagonal loading method is proposed, and a tradeoff exists between the robustness and the interference suppression capability by controlling the peak location of the main beam. As long as the DOA estimation error is less than the half of the width of main beam, the proposed beamformer will not suppress the Signal of Interest (SOI) as interference. Numerical experiments prove the effectiveness of the proposed method.
In order to use power splitter for communication and real world applications (e.g., telephony performances, antenna designs, wireless communications, digital communications, optical communications, CATV transmission systems, airborne systems), studying the nature and characteristics of the splitter is important. The purpose of this paper is to review and discuss various techniques aimed to develop the power splitters component and remove its interference. This paper further focuses on the review of future implementation techniques and performance comparison along with their applications. Some of the applications are illustrated at the end of the paper, and recommendation for further study is also outlined. This review serves as a comparative studies and reference beneficial for power splitter researchers and for future implementation of the technology. This review paper opens a corridor for researchers to perform future comparative studies between different architectures and models as a reference point for developing more powerful, flexible and efficient applications.
In this paper several Defected Microstrip Structure (DMS) are used to suppress the first and second spurious responses in microstrip hairpin filters. The DMSs are integrated in filter structure, and therefore this method keeps the filter size unchanged. The DMS interconnection disturbs the current distribution only across the strip, thereby giving a modified microstrip line with certain stop band. The undesired spurious harmonics are suppressed through multiple transmission zeros which are added at these frequencies by designed DMSs. Experimental results verify that 25 dB suppression for the first harmonic and 40 dB suppression for the second harmonic, respectively, without affecting the main passband response. There is a good agreement between the simulated and measured results.
A single feed compact rectangular microstrip antenna is presented in this paper. A triangular slot is introduced at the upper edge of the patch to reduce the resonant frequency. A small piece of triangular patch is grown within the triangular slot to improve the gain bandwidth performance of the antenna. The antenna size has been reduced by 46.2% when compared to a conventional square microstrip patch antenna with a maximum of 160 MHz bandwidth and -27.36 dB return loss. The characteristics of the designed structure are investigated by using MoM based electromagnetic solver, IE3D. An extensive analysis of the return loss, radiation pattern, gain and efficiency of the proposed antenna is presented. The simple configuration and low profile nature of the proposed antenna leads to easy fabrication and make it suitable for the applications in Wireless communication system. Mainly it is developed to operate in the WiMax frequency range of 3.2--3.8 GHz.
A microstrip-fed slot antenna is proposed for short-range UWB communication. First, the characteristics of a circular monopole UWB antenna, as a representative of a class of UWB antennas seen in the literature, are examined in time (pulse-shape) and frequency (reflection and transmission coefficients) domains. From these measurements, certain limitations of this class of antennas are brought out, which are not widely recognized. We then demonstrate that with proper optimization the traditional microstrip-fed slot antenna overcomes these defects and is an excellent candidate for UWB communication systems. This claim is justified with measurements in time domain and frequency domain.
Filter prototypes derived from Gegenbauer polynomials can represent a useful trade-off between amplitude and phase behavior. This paper discusses the main features of this prototype through a comparison with the more classical Chebyshev and Butterworth solutions; it shows, in the case of an X-band waveguide realization, how its intermediate characteristics, with respect to both amplitude and phase responses, can be very useful in satisfying particular filter performance requirements without increasing filter order.
The moment method technique has been improved to investigate the scattering properties of high Tc Superconducting circular antennas with anisotropic substrate in multi-layered configuration. In this method, the electric field integral equation for a current element on a grounded dielectric slab of infinite extent was developed by basis functions involving Chebyshev polynomials. An improved analytical model is presented taking into account anisotropic substrate, superconducting material for the circular patch and multilayered structure. To validate the theoretical results, an experimental study has been performed for a perfectly conducting circular patch on a single layer, with and without air gap. Good agreements were obtained between our theory and measurements. Effects of temperature and thickness of a superconducting film are also reported and discussed. The performances of high Tc superconducting circular antennas were improved by the use of uniaxial anisotropy substrate and multilayer configuration.
Novel circularly polarized antennas with a circular radiating aperture for broadband characteristics are presented in this paper. The vertical and horizontal components of the L-shaped probe are separated and placed at the front and back side of the substrate. The antennas are excited by a microstrip line which is connected to the vertical component of the L-shaped probe and electromagnetically couples the signal to the horizontal component of the L-shaped probe. A novel concept of placing stub in the slot of a planar antenna, by observing the electric field vector behaviour in the slot, is proposed to enhance the axial ratio (AR) bandwidth by around 10%. Unidirectional patterns can be obtained by having a cylindrical cavity of height λg/4 behind the antenna and is effective when no stubs are placed in the slot. A < 3 dB AR bandwidth of 39.5% with cavity and 41.18% without cavity but with stub in the slot is obtained in simulation and the results well match with the measurement.
The performance of a short dipole antenna closely located above a finite High-Impedance Surface (HIS) is addressed. The antenna behavior is thoroughly analyzed in the frequency range up to the HIS resonance within the region where the propagation of the TE surface waves is not allowed. In the first part of the paper the analysis of a dipole antenna above a grounded dielectric slab is considered, and then it is extended to the case of a substrate with a frequency selective surface printed on it. For all configurations, the radiation pattern of the structure and Front-to-Back Ratio (FBR) are reported and compared. It is shown that the presence of a suitable frequency selective surface, regardless of the shape of the periodic elements, guarantees the antenna matching but does not influence the behavior of the radiation patterns and the front-to-back ratio in the frequency range where only TM modes are allowed to propagate. The front-to-back ratio has been found to be maximum when the size of the generic HIS is around 0.8λg (with λg the TM guided surface wave wavelength). All the speculations are supported by simulated and measured results.
This paper deals with the design of a reconfigurable antenna that resembles a monolithic UWB bow-tie antenna for Ground Penetrating Radar (GPR) applications. In particular, the attention is focussed on the design of the balun system able to work in the frequency band 0.3--1 GHz; the effectiveness of the design is shown by examining the behaviour of the scattering parameters S11 for both the reference monolithic antenna and the designed reconfigurable antenna. Also, an analysis of the radiation pattern of both the monolithic and reconfigurable antennas is presented and confirms the effectiveness of the designed balun system.
In this paper, a novel compact butterfly shaped printed monopole antenna for ultra-wideband (UWB) applications is presented. The proposed antenna is designed with a standard printed circuit board (PCB) process for suitable integration with other microwave components. The antenna prototype is designed then fabricated and tested experimentally. The calculated impedance bandwidth of the proposed antenna ranges from 3 GHz to 13 GHz for a 10 dB reflection coefficient (S11) while the measured impedance bandwidth ranges from 3 GHz to 10.8 GHz covering the whole UWB frequency range. The measured antenna radiation patterns show relatively stable radiation patterns with almost constant gain over the whole frequency band of interest. By introducing a slit ring resonator (SRR) in the feedline, a bandstop of 830 MHz from 5.0 to 5.83 GHz for band rejection of wireless local area network (WLAN) can be achieved. So, the proposed antenna is considered a good candidate for future UWB communication systems.
In this paper, a traveling wave Koch dipole antenna is proposed. The antenna is an amalgamation of traveling wave antennas that require large elctrical lengths and fractal curves that are known for excellent form factor characteristics. The antenna is analyzed using a Mom code. The antenna exhibits an impedance bandwidth that is more than 10:1 for VSWR < 3:1. A comparision of simulated and measured results are presented. The traveling wave fractal antenna has many potential applications in communications and electronics warfare.
In this contribution we propose the design of an inductive Frequency Selective Surface (FSS) with double resonant elements aimed at the achievement of a simple well-performing, dielectric-free, space filter screen able to separate the Ku band from the Ka band. The FSS performance is compared to that of a typical double ring FSS which major drawback is the use of a dielectric substrate that leads to unavoidable additional transmission losses and makes the dichroic mirror more complex with respect to a simple single perforated screen. For all applications in which the FSS is asked to be as simple as possible and the transmission losses specifications are severe, the Inductive FSS Double Resonant Elements here proposed turns out to be an interesting alternative to typical Double Ring FSS.
An effective procedure is developed in this paper to compensate the probe positioning errors when using a near-field to far-field transformation technique with helicoidal scanning for long antennas. It is based on a prolate ellipsoidal modelling of the antenna under test and makes use of an iterative scheme to retrieve the uniformly distributed helicoidal near-field data from the irregularly spaced acquired ones. Once these data have been recovered, those required to perform a standard near-field--far-field transformation with cylindrical scanning are efficiently determined via an optimal sampling interpolation algorithm. Some numerical tests are reported to assess the accuracy of the approach and its robustness with respect to random errors affecting the data. At last, the validity of the developed technique is further confirmed by the experimental tests performed at the Antenna Characterization Lab of the University of Salerno.
In this paper, we propose an optimization method based on Gravitational Search Algorithm (GSA) for design of reconfigurable dual-beam concentric ring array of isotropic elements with phase only control of five-bit digital phase shifters. The problem is to find a common radial amplitude distribution using four-bit digital attenuator that will generate three different types of broadsided beam pair in vertical plane: a pencil/pencil beam pair, a pencil/flat-top beam pair and a flat-top/flat-top beam pair sector pattern) with desired value of side lobe level, first null beam width and ripple. The two patterns differ only in radial discrete phase distribution while sharing a common discrete radial amplitude distribution. The optimum sets of four-bit discrete radial amplitude distribution generated by four-bit digital attenuators and five-bit discrete radial phase distribution generated by five-bit digital phase shifters for obtaining dual radiation patterns are computed by Gravitational Search Algorithm.
In this paper, a modified square slot antenna with modified radiating patch, for UWB applications is proposed. The proposed antenna consists of a square radiating patch with a double fed structure and a ground plane with a pair of L-shaped slots which provides a wide usable fractional bandwidth of more than 140% (3--18 GHz). By optimizing the L-shaped slots dimensions and rectangular slot width, the total bandwidth of the antenna is greatly improved. The designed antenna has a small size of 35×35 mm2.
In the present paper a dual frequency resonance antenna is achieved by introducing half U-shaped slot in semicircular disk. It is analysed by using circuit theory concept. The resonance frequency is found to be 1.50 GHz and 2.32 GHz, and the bandwidth of the proposed antenna for lower and upper resonance frequency is found to be 5.96% and 11.08% respectively. It is found that the resonance frequency depends inversely on the slot length and feed point, while it increases with increasing the slot width and coaxial probe feed radius. The frequency ratio is found to be 1.54. The theoretical results are compared with IE3D simulation as well as reported experimental results, and they are in good agreement.
A bandwidth improvement method in reflectarray antennas by using closely space elements, i.e. unit-cell sizes smaller than λ/2, has been investigated both numerically and experimentally in this paper. A new definition of phase error has been introduced to analyze the broadband mechanism of closely spaced phasing elements. Through full wave EM simulations, it is revealed that closely spaced elements achieve a smaller phase error over the band. Based on these theoretical studies two Ka-band reflectarrays were fabricated and their performance was measured across the frequency range of 30 to 34 GHz. It is demonstrated that the reflectarray designed with closely spaced elements achieves a notable improvement in gain bandwidth performance.
We propose a circularly polarized microstrip patch antenna using artificial magneto-dielectric (AMD) substrate. The proposed antenna has a square patch with dual feeding point using Wilkinson power divider in order to achieve circularly polarized radiation pattern. The AMD substrate is constructed by single sprit ring resonators, which are arrayed in both x and y directions. The designed antenna has the characteristics of the axial ratio less than 1.7 dB, the peak gain of 0.55 dBic, and the right handed circular polarized radiation pattern at Korea UHF RFID service band. The sizes of the antenna and radiating patch are 100 mm×100 mm×5.2 mm and 65 mm×65 mm (0.199λ0×0.199 λ0), respectively.
A thick metal microstrip diplexer is presented. The circuit is based on compact folded half wavelength resonators and uses a source/load-multi-resonator coupling method providing improved performance and greater design flexibility. Source/load coupling with multiple resonators introduces additional transmission zeros, and this coupling is enhanced by using high-aspect-ratio metal structures. Tall, narrow metal arms connected to the ports and extended to the non-adjacent resonators provide effective multi-resonator bypass coupling. The high-aspect-ratio diplexer fabricated using polymer-based deep X-ray lithography and 0.22 mm thick metal electroplating demonstrates the advantages of thick metal structures for coupled resonator applications.
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