Numerical modeling of ultrawide-band dielectric horn antennas using FDTD

A detailed characterization of the input impedance of ultrawide-band (UWB) dielectric horn antennas is presented using the finite-difference time-domain (FDTD) technique. The FDTD model is first validated by computing the characteristic impedance of two conical plate transmission lines (including planar bow-tie antennas) and comparing the results to analytical solutions. The FDTD model is next used to calculate the surge impedance of dielectric horn antennas using the conical plates as launchers. Design curves of the surge impedance for different choices of geometries and dielectric loadings are provided. The modeled antennas are particularly attractive for applications such as UWB ground penetrating radars (GPR) applications.     

A deep parametric study of resistor-loaded bow-tie antennas for ground-penetrating radar applications using FDTD

Resistor-loaded bow-tie antennas are analyzed thoroughly to find out their performance on ground-penetrating radar (GPR) applications. The analysis is done with the finite-difference time-domain (FDTD) technique. The antenna is pulse driven and enclosed in a rectangular conducting cavity. The ability to detect a buried conducting sheet using two such identical antennas for transmitting and receiving is investigated. Simulations are carried out for various antenna parameters like end resistor values, flare angle, and antenna length. The gap between the two antennas and their height above the ground are also varied. Moreover, the results are obtained for different sizes, depths, and positions of the buried sheet. It is studied how the broadband impedance characteristics and better target discrimination with low clutter can be achieved by optimally selecting these antenna parameters. Also, it is shown that apart from the total parallel end resistance, the individual end resistor values and the number of resistors connected have no significant effect on the input impedance and the received signal.     

Modal impedances of planar, non-complementary, N-fold symmetric antenna structures

Planar N-arm rotationally symmetric antennas are commonly used in applications that demand wideband, multiple-mode, dual-polarization, and conformal geometries. Antennas included in this category are spirals, log-periodic dipole arrays, bow-tie antennas, and sinuous structures. An analytic expression for the modal impedances of these complementary planar structures has been available for many years. This paper extends the theory to non-complementary planar /V-fold symmetric antennas, and provides equations valid for any arm-to-gap width ratio, arm width, expansion rate, and low-loss electrically thin substrate materials. The equations are based on quasi-TEM coplanar stripline theory, and provide a band-averaged approximation of the input impedance of the antenna for each characteristic mode of operation. Predicted modal impedances are compared to spiral antenna measurements. In addition, a procedure for accurately measuring the dielectric constant of the substrate material is discussed.     

Measuring the impedance of balanced antennas by an S-parametermethod

The input impedance of a balanced antenna is conventionally measured by using a balun that forces opposite currents in each part of the radiator. In this paper, balanced antennas are viewed as two-ports, the S-parameters of which are determined by standard network-analyzer techniques. Simple formulas then lead to the differential input impedance. The method is illustrated by measurements on a dipole antenna, which are compared to the results for monopoles over a ground plane     

一种工作于UHF频段的宽频带互补天线设计

本文应用互补天线的原理,通过采用蝶形平面天线作为电偶极子,采用垂直壁与接地面作为等效磁偶极子,设计了一种新颖的蝶形宽频带天线.该天线实现了62%的相对带宽(1.52-2.89GHz,S11<-10dB),在工作频段内具有稳定的波瓣图,优于-30dB的交叉极化和超过18dB的前后比.     

Modeling of a quasi-optical Josephson oscillator

The authors have developed a computer model of a Josephson tunnel junction embedded in a general circuit with frequency-dependent impedance using the harmonic balance method. This model has been applied to the analysis of a two-dimensional Josephson junction array with integrated coupling structures, called a quasi-optical Josephson oscillator. Simulations are done for a junction with dipole, slotline, and bow-tie antennas. The results show that the junction with a bow-tie antenna gives the best performance, and the output power from an array of 4000 junctions can reach 25.7 μW at a frequency as high as 1091 GHz for niobium junctions deposited on a 0.207-mm-thick quartz substrate     

标签天线弯曲对射频识别系统性能影响的研究

探讨了工作在微波段的无源射频识别(Radio frequencyIdentification, RFID)系统中标签天线的弧形弯曲对RFID系统性能的影响。分析比较了几种典型的半波振子标签线天线弧形弯曲的情况。通过理论分析确立标签天线的方向系数与输入阻抗为RFID系统性能的主要影响因子。弯曲振子天线特性参数由线天线分析软件NEC WinBasic数值模拟得到。分析结果表明,标签天线的弧形弯曲使得无源RFID系统的工作距离明显变短。因此,为避免此类问题有必要在RFID系统中考虑对于弯曲具有低敏感性的天线结构。     

一种新型探地雷达天线的设计分析

该文研究了一种近似椭圆结构的超宽带偶极子天线。通过设计沿着振子向末端方向渐变的导体臂,可以有效地减小天线末端电流的反射,改善天线的输入阻抗特性,拓宽天线的带宽。由于没有采用任何加载措施,此天线相比电阻加载宽带天线具有更高的效率。采用三维电磁仿真软件对天线进行了分析和设计,根据设计结果实际制作了一副天线样机,并对其电压驻波比和辐射特性进行了测试,测试结果与仿真设计结果吻合良好。仿真结果和实验测试结果表明,该天线在很宽的工作频带内具有良好的阻抗特性和辐射特性。     

Terahertz Yagi-Uda Antenna for High Input Resistance

This paper presents the design of terahertz Yagi-Uda antennas with high input resistance. The proposed Yagi-Uda antennas can achieve much higher input resistance than existing THz antennas by using a full-wavelength straight-line dipole and a full-wavelength U-shaped dipole as driver elements. Current leakage into the bias line was minimized by applying photonic bandgap structures at the bias line. By designing the antennas on a thin substrate, end-fire radiation patterns with high antenna resistance can be obtained even when the substrate has a high dielectric constant. We expect that the proposed Yagi-Uda antennas can achieve increased terahertz output power by improving their capacity for impedance matching with a photomixer.     

Bow-tie slot antenna fed by CPW

A bow-tie slot antenna fed by CPW is introduced. It is designed to work in the Ku-band around 15.5 GHz. The proposed antenna is analysed both theoretically and experimentally. The presented results include the return loss and the radiation patterns. Two different geometries of the bow-tie antenna are presented and compared. The proposed antenna has several advantages such as a very large bandwidth, good control of its input impedance, and ease of fabrication. The proposed antenna has a dipole like radiation pattern which makes it suitable for mobile communication systems designed to work in the Ku-band     

Measurement of impedance transformation on practical dipoles

The measured input impedance of a practical dipole antenna is shown to be related to the theoretical input impedance of a delta-gap cylindrical dipole through a bilinear transformation. In order to determine the three complex constants which specify the transformation, the impedance of a dipole in front of a ground plane is measured and compared with the theoretical impedance. The comparison is performed by a curve-fitting procedure for bilinear transformation on a complex plane. The same measurement yields also the receiving efficiency of the antenna and its feeding network. Experiments show that the input impedance of a practical folded dipole, incorporating a balun transformer, may be predicted with an accuracy of better than four percent.     

Input impedance, radiation pattern, and radar cross section ofspiral antennas using FDTD

A finite-difference time-domain (FDTD) analysis of spiral antennas is performed to calculate input impedance, antenna gain, and scattering. A semicircular spiral mounted on a dielectric substrate was simulated for computing the input impedance versus frequency. The gain and scattering computations were performed on a square Archimedean spiral mounted in a ground plane with a cavity backing. Total-field FDTD calculations are used to compute the impedance and gain patterns, while a specially modified scattered-field approach for aperture antennas in infinite ground planes is used for the scattering results. Comparisons are made with published impedance measurements and gain and scattering calculations done with a finite element method. Good results were obtained for impedance, radiation, and scattering     

Bow-tie antennas on high dielectric substrates for MMIC and OEICapplications at millimetre-wave frequencies

A two-element bow-tie dipole antenna and a single-element bow-tie slot antenna fabricated on a high dielectric constant (ε_r=10.2) substrate are introduced for applications at millimetre-wave frequencies. The former antenna provides 2 GHz bandwidth at 35 GHz and the latter 1.3 GHz at 32.7 GHz. With a broadband match, these antennas would yield significantly higher bandwidths     

The properties of antennas in plasmas

The last five years have witnessed remarkable progress in the theory and measurement of both the radiation and impedance properties of antennas in plasmas. Increased motivation for research in this area has been provided by the Space Shuttle program and by the prospect of nuclear fusion. The focus of attention has been on resonance cones in linear, anisotropic plasmas, including radiation patterns, wave interference, pulse propagation, reflections from boundaries and inhomogeneous media effects. Under nonlinear conditions, the focussed field of a resonance cone can significantly depress the plasma density. Under both linear and nonlinear conditions, the input impedance of dipole and loop antennas has been studied extensively, for both anisotropic and isotropic plasmas. A continuing challenge has been the as-yet-not-fully-explained experimental observation of linear, non-collisional enhanced resistivity of the sheath region around an antenna. Numerical impedance calculations employing simplified velocity distributions have shown particular promise. Ion and electron wave radiation patterns for various antenna shapes have been calculated and checked experimentally. The response of both single antennas and pairs of antennas to plasma fluctuations has been studied and found to have applications to plasma diagnostics.     

折叠偶极子天线的设计

射频识别(RFID)应用中的天线设计需考虑的最重要因素是低价位、小剖面和小型化,而为了最大功率传输,天线的输出阻抗必须和其后的芯片的输入阻抗匹配。本文介绍一种新颖的简单结构折叠偶极子天线,所需的输入阻抗能通过选择合适的几何参数轻易获得,这对设计特殊阻抗的天线非常有用。     

Self- and mutual impedances of traveling-wave linear antennas

A traveling-wave distribution of current can be produced on a conventional linear antenna, such as a dipole, through resistive loading. The self- and mutual impedances of parallel traveling-wave linear antennas are measured and compared with those of the corresponding conventional linear antennas. The effects of coupling on the input impedance of an array element is discussed, and it is shown that these effects are much less pronounced for traveling-wave antennas, which may be advantageous for some array applications     

Effect of disc reflectors on radiation impedance of short-backfire antenna

Using near-field analysis, the influence of large and small disc reflectors used in short-backfire antennas on the radiation impedance of a dipole feed element has been investigated. The effect of each reflector is evaluated separately, and the overall change in the radiation impedance of the dipole is predicted.     

Analysis of Eccentered Dipole Antenna for Borehole Radar

A dipole antenna in a borehole may be placed away from the center of the borehole, and this eccentricity produces additional complicated electromagnetic fields, which may influence borehole radar signals. In this paper, we analyze an eccentric dipole antenna in a borehole for borehole radars. Our approach is an extended version of the pseudoanalytical formulation that was previously applied for analyzing an induction logging tool and a dielectric logging tool. The proposed equations are suitable for modeling the finite dipole antenna, utilizing a method-of-moments. The eccentricity of the radar sonde has two consequences. First, it modifies the antenna characteristics such as its impedance, and second, it changes the electromagnetic field inside the borehole, owing to the change of the structure of the cylindrical layers. These two changes are evaluated by the proposed algorithm and two types of experiments. The first one is a measurement of the input impedance of a monopole antenna which is in cylindrical layers on a ground plane in air. The second one is about the transmission between two antennas, one of which is eccentered in cylindrical layers. This experiment was done in granite at a field test site and on a ground plane in air. In the field experiments, we controlled the position of the radar sonde in a borehole and made the antenna eccentered artificially. We found that a 4-cm displacement resulted in changes of 4 dB in amplitude and 4 ns in time delay. Comparing the experimental and calculated data obtained by the present method shows the validity of the proposed analytical formulation.      

闪电电磁脉冲卫星接收天线设计初步探索

针对卫星轨道闪电电磁脉冲(LEMP)的信号特征以及接收环境的限制，提出了卫星轨道接收LEMP的天线设计参数，并据此设计了对称振子天线和对数周期振子阵天线。计算出了两种天线的辐射阻抗，输入阻抗，增益曲线，驻波比等参数，还给出了对数周期振子阵天线的各参数曲线。最后对两种天线进行了比较。对称振子的特点结构简单适合于当前应用。对数周期天线的接收性能优于对称振子天线，更适合于专门的电磁脉冲探测试验卫星。     

Fundamental transmitting properties of carbon nanotube antennas

Fundamental properties of dipole transmitting antennas formed by carbon nanotubes are investigated. Since carbon nanotubes can be grown to centimeter lengths, and since they can be metallic, the properties of carbon nanotubes as antenna elements are of fundamental interest. In this paper, dipole carbon nanotube antennas are investigated via a classical Hallen's-type integral equation, based on a quantum mechanical conductivity. The input impedance, current profile, and efficiency are presented, and the radiation pattern is discussed, as are possible applications.