On the Effects of Driven Element L/D Ratio and Length in VHF-SHF Yagi-Uda Arrays

While the Yagi-Uda array has been studied for decades, one issue appears to have received less attention than perhaps it should, namely, the effects on performance of the array’s driven element length and its length-to-diameter ratio. This paper looks at that question. It shows that decreasing the L/D ratio increases impedance bandwidth, but it may shift the IBW band sufficiently far from the design frequency that other parameters such as gain and front-to-back ratio probably are adversely affected. It also shows that array performance is not relatively independent of element diameters. This paper also investigates the effect of lengthening the driven element, which can substantially improve IBW. Several iterations of a 3-element prototype and improved arrays are modeled with the Method of Moments and discussed in detail. A five step design procedure is recommended and applied to a Genetic Algorithm-optimized 3-element Yagi at 146 MHz. This array exhibits excellent performance in terms of gain, front-to-back ratio, and especially impedance bandwidth (nearly 14% for voltage standing wave ratio ≤ 2:1 with two frequencies at which 50 ? is almost perfectly matched). While the analysis and recommended design steps are applied to cylindrical array elements, which commonly are aluminum tubing for stand-alone VHF-SHF Yagis, they can be applied to other element geometries as well using equivalent cylindrical radii, for example, Printed Circuit Board traces for planar arrays. One consequence of lengthening the driven element while reducing its L/D ratio is that some reactance is introduced at the array feedpoint which must be tuned out, and two approaches for doing so are suggested.

A TRIBAND SWASTIKA SHAPED PATCH ANTENNA WITH REDUCED MUTUAL COUPLING FOR WIRELESS MIMO SYSTEMS

A novel compact Swastika shaped patch antenna is designed in the present work,which can be used for Multiple Input Multiple Output(MIMO) systems.The proposed two element MIMO system resonates at a triband of 3.3 GHz,5.8 GHz,and 7.1 GHz with an improved impedance bandwidth of 37% and a reduced mutual coupling of-33 dB.These results are better compared to a normal E shaped patch antenna designed with same size and thickness,achieved without using any additional decoupling methods.A 2×2 MIMO system employing the Swastika shaped patch antennas is analyzed using computational electromagnetic ray tracing software for an indoor environment.The results show an improvement in the capacity compared to a 2×2 MIMO system developed with dipole antennas.The proposed antenna is a good choice for MIMO systems operating for several Ultra WideBand(UWB) applications.

An ultra-wideband pattern reconflgurable antenna based on graphene coating

An ultra-wideband pattern reconfigurable antenna is proposed.The antenna is a dielectric coaxial hollow monopole with a cylindrical graphene-based impedance surface coating.It consists of a graphene sheet coated onto the inner surface of a cylindrical substrate and a set of independent polysilicon DC gating pads mounted on the outside of the cylindrical substrate.By changing the DC bias voltages to the different gating pads,the surface impedance of the graphene coating can be freely controlled.Due to the tunability of graphene＇s surface impedance,the radiation pattern of the proposed antenna can be reconfigured.A transmission line method is used to illustrate the physical mechanism of the proposed antenna.The results show that the proposed antenna can reconfigure its radiation pattern in the omnidirectional mode with the relative bandwidth of 58.5% and the directional mode over the entire azimuth plane with the relative bandwidth of 67%.

Six-Element Yagi Array Designs Using Central Force Optimization with Pseudo Random Negative Gravity

A six-element Yagi-Uda array is optimally designed using Central Force Optimization (CFO) with a small amount of pseudo randomly injected negative gravity. CFO is a simple, deterministic metaheuristic analogizing gravitational kinematics (motion of masses under the influence of gravity). It has been very effective in addressing a wide range of antenna and other problems and normally employs only positive gravity. With positive gravity the six element CFO-designed Yagi array described here exhibits excellent performance with respect to the objectives of impedance bandwidth and forward gain. This paper addresses the question of what happens when a small amount of negative gravity is injected into the CFO algorithm. Does doing so have any effect, beneficial, negative or neutral? In this particular case negative gravity improves CFO’s exploration and creates a region of optimality containing many designs that perform about as well as or better than the array discovered with only positive gravity. Without some negative gravity these array configurations are overlooked. This Yagi-Uda array design example suggests that antennas optimized or designed using deterministic CFO may well benefit by including a small amount of negative gravity, and that the negative gravity approach merits further study.

A Compact Rhombus Shaped Antenna with Extended Stubs for Ultra-Wideband Applications

Ultra-wideband(UWB)is highly preferred for short distance communication.As a result of this significance,this project targets the design of a compact UWB antennas.This paper describes a printed UWB rhombusshaped antenna with a partial ground plane.To achieve wideband response,two stubs and a notch are incorporated at both sides of the rhombus design and ground plane respectively.To excite the antenna,a simple microstrip feed line is employed.The suggested antenna is built on a 1.6 mm thick FR4 substrate.The proposed design is very compact with overall electrical size of 0.18λ×0.25λ(14×18 mm2).The rhombus shaped antenna covers frequency ranging from 3.5 to 11 GHz with 7.5 GHz impedance bandwidth.The proposed design simulated and measured bandwidths are 83.33%and 80%,respectively.Radiation pattern in terms of E-field and H-field are discussed at 4,5.5 and 10 GHz respectively.The proposed design has 65%radiation efficiency and 1.5 dBi peak gain.The proposed design is simulated in CST(Computer Simulation Technology)simulator and the simulated design is fabricated for the measured results.The simulated and measured findings are in close resemblance.The obtained results confirm the application of the proposed design for the ultra-wide band applications.

The Dual-Polarized Staggered Stacked Patches Antenna

A new dual-polarized staggered and stacked patches antenna with wide impedance band-width and high isolation is proposed.The antenna consists of two groups of radiation patches,in 7 layers,and uses the orthogonal adjacent coupling structure on staggered layer to excite a pair of linear polarization modes.Thanks to the staggered feeder mode,it has increased the isolation performance be-tween ports and compressed the transverse size of the antenna.As a result of the combination of staggered stack-up between the patches and the stepped gradient shape of the main radiating patches,it has effectively expanded the impedance bandwidth of the antenna.The proposed antenna is simulated,fabricated and measured.The staggered feeding structure effectively reduces the cross-sectional area of the antenna,and greatly improves the isolation between feeding ports.The measurement results show that the impedance bandwidths for vertical and horizontal polarization modes are 40.2%(638-960 MHz)and 40.0%(645-968 MHz)respectively when the return loss is lower than-10 dB,and the isolation between feeding ports is better than-30 dB.Meanwhile,the antenna has a stable and symmetrical radiation pattern across the working band,therefore making it suitable to be used as antenna and antenna array element of mobile wireless communication base stations.

A Low-Profile Cylindrical Dielectric Resonator Antenna with Bandwidth-Enhancement Under Triple-Mode Resonance

A low-profile cylindrical dielectric resonator antenna(CDRA)with enhanced impedance bandwidth is proposed via sharing a single triple-mode dielectric resonator.Initially,the resonant frequencies of TM01d and TM02d modes of a conventional CDRA are maintained far away from each other under central probe feed.Next,a circular disk with an annular ring is loaded around the conventional CDRA,aiming to excite an additional mode between these dual modes.The electric fields demon-strate that the previous TM_(02δ) mode is transformed into the TM03d mode for the modified CDRA,and an additional TM_(02δ) mode is successfully excited between the TM_(01δ) and TM_(03δ) modes.As a result,the resonant frequencies of these three radiative modes are reallocated in proximity to each other,thus achieving the desired bandwidth enhancement.In addition,considering the small input impedance of the CDRA,a microstrip feeding line is introduced underneath the central probe for good impedance matching.With these arrangements,the resultant antenna can generate an extended bandwidth under simultaneous radiation of TM_(01δ),TM_(02δ),and TM_(03δ) modes.Finally,the proposed CDRA is designed,fabricated,and measured to validate the predicted per-formance.The simulated and measured results show that the impedance bandwidth(|S11|<-10 dB of the antenna is dramatically extended from 8%to about 62%(2.835.36 GHz),while keeping a stable conical radiation pattern.In particular,a low profile property of about 0.15 free space wavelength is achieved as well.

Artificial neural network approach for analyzing mutual coupling in a rectangular MIMO antenna

A wideband rectangular patch antenna resonat- ing at 3.5 GHz and 8 GHz frequencies is developed on a flexible substrate, which can be used for wearable applications. The proposed antenna gives a wide impe- dance bandwidth of 116%, operating from 2.SGHz to 9.5 GHz, covering most of the ultra-wideband （UWB） operating frequency range. A two-element multiple-input multiple-output （MIMO） system is developed using the proposed antenna, and the mutual coupling between the two antennas for various separations and frequencies is analyzed by using artificial neural networks （ANNs）. The neural structure is trained by using different ANN algorithms and a comparative study is made between them. It is shown that, quasi-Newton （QN） and quasi- Newton multi layer perceptron （QN-MLP） algorithms are better in terms of training, testing errors, and correlation coefficient.

A waveguide overloaded cavity kicker for the HLS Ⅱlongitudinal feedback system

In the upgrade project of Hefei Light Source（HLSⅡ）,a new digital longitudinal bunch-by-bunch feedback system will be developed to suppress the coupled bunch instabilities in the storage ring effectively.We design a new waveguide overloaded cavity longitudinal feedback kicker as the feedback actuator.The beam pipe of the kicker is a racetrack shape so as to avoid a transition part to the octagonal vacuum chamber.The central frequency and the bandwidth of the kicker have been simulated and optimized to achieve design goals by the HFSS code.A higher shunt impedance can be obtained by using a nose cone to reduce the feedback power requirement.Before the kicker cavity was installed in the storage ring,a variety of measurements were carried out to check its performance.All these results of simulation and measurement are presented.