Principles of Impedance Matching in Photoconductive Antennas

The principles of impedance matching in photoconductive antennas in comparison with conventional antennas are described. Because of the optical nature of the input signal in photoconductive antennas and the dependence of photoconductor conductance on the optical pump power, the optimum photoconductor impedance is not necessarily determined by the complex conjugate of antenna impedance. Using the equivalent circuit model of photoconductive antennas, the photoconductor impedance optimization criteria are evaluated according to the photoconductive antenna structure and operational settings.     

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.     

细同轴天线的宽频带技术

运用同轴天线的原理,引入附加阻抗,通过同一传输线变压器的不同阻抗变换作用,改善天线的阻抗特性,展宽细同轴天线的频带,同时使天线尺寸大为减小。     

Wide-band slot antennas with CPW feed lines: hybrid and log-periodic designs

In microwave and millimeter wave applications, slot antennas fed by coplanar waveguide (CPW) lines are receiving increasing attention. These antennas have several useful properties, such as a wider impedance bandwidth compared to microstrip patch antennas, and easier integration of solid-state active devices. In this paper novel CPW-fed wideband slot antennas are presented. The design procedure of CPW-fed hybrid slot antennas (HSA) having impedance bandwidths (VSWR<2) up to 57% is described. Theoretical and measured results are shown. We also describe the design procedure of a CPW-fed log-periodic slot antenna (LPSA). The impedance matching and the radiation characteristics of these structures were studied using a method of moment technique. Simulated and measured results for different dielectrics are presented.     

A numerical technique for calculating mutual impedance and element patterns of antenna arrays based on the characteristics of an isolated element

A numerical technique for calculating mutual impedance and element patterns of antenna arrays based on the characteristics of an isolated element is presented. The basis for this technique is the theory of minimum-scattering (MS) antennas and, in particular, the interpretation of the mutual impedance between two canonical minimum-scattering (CMS) antennas as the first term in a perturbation series of the mutual impedance of arbitrary antennas. For the computation of the mutual impedance via the CMS approximation this pattern must be continued analytically into the complex domain. However, numerical codes provide radiation patterns only for real observation angles. To overcome this problem, the numerically calculated patterns are expanded in terms of spherical modes and the computation over complex angles is carried out analytically. Numerical results for collinear and linear arrays of parallel electric dipole antennas and rectangular probe-fed patch antennas are presented and a comparison is made with direct calculations using the WIPL-D code. Results presented show the good agreement between the CMS approximation and the WIPL-D code.     

Input impedance of arc antennas and short helical radiators

The problem of the input impedance of curved wire antennas is generally formulated in terms of an integral equation. A stationary formula is used in evaluating the input impedances of arc antennas and short helical radiators of which the cylindrical antenna is a special case. The computational results are presented in graphical form. The impedance characteristics of these curved antennas are discussed.     

面向方向图保形的共口径基站天线去耦技术

为了节省基站天线所占空间资源,降低其运营成本,现代移动通信需要将多个天线阵列紧密地排列在一起,构成多频共口径基站天线。多天线的紧密排列势必造成天线间强烈的耦合,不仅使得天线阻抗失配、隔离度变差,同时也造成辐射方向图的严重变形。因此,近年来多频共口径基站天线的去耦成为工业界和学术界研究的热点,早期的去耦技术主要是面向改善天线的阻抗匹配和隔离度,而对面向方向图保形的去耦技术研究相对较少。文章在简要介绍面向方向图保形的基站天线去耦技术的研究现状的基础上,重点阐述了褚庆昕教授天线射频团队基于感应电流抵消、耦合场抵消和辐射阻断等原理,提出的几种新型的去耦技术。这些技术不仅可以有效地改善基站天线的阻抗匹配和隔离,更重要的是很好地实现了天线方向图保形。仿真和测试结果验证了原理的正确性和设计的可行性。一些技术已被用于5G基站天线产品。     

Theory of mutual coupling among minimum-scattering antennas

A novel and rigorou formulation for mutual impedance among a class of idealized but realizable antennas is presented. Unlike past treatments of mutual coupling, which have proceeded from structurally specified antennas, the present treatment deals with a class of antennas, all the electromagnetic properties of which are rigorously expressed explicitly in terms of their radiation patterns alone. Employing a network formulation based on a scattering representation of electromagnetic fields in terms of spherical (or cylindrical) modes, the mutual impedances between such antennas are computed exactly. Several alternate representations for the mutual impedance are derived, one of which is an integral representation involving the power pattern function for both real and complex angles. In special cases, these exact results agree with published results obtained by applying well-known approximate techniques to structurally specified antennas. An important and illuminating example is provided by an infinite planar array of antennas radiating into a half-space for which the present formulation yields the well-known grating-lobe series representation of the active impedance without an explicit reference to structural properties of the radiating elements.     

EMP response of aircraft antennas

The responses of aircraft antennas to a broad-band electromagnetic wave such as the nuclear electromagnetic pulse (EMP) are analyzed. For convenience of analysis the antennas are divided into five classes: blades, loops, slots, bowls, and long wires. From each class a few specific antennas are selected as examples for detailed discussion. For each exemplary antenna the detailed equivalent circuit, input impedance, and effective height at the antenna's connector are given. Measurements of input impedance on some antennas are compared with the corresponding calculated results.     

FDTD结合变分法计算线天线间耦合度

线天线的自阻抗、互阻抗及耦合度是工程应用中迫切需要但又很难计算准确的。该文将时域有限差分法和变分原理相结合,求解了线天线之间的互阻抗,从而得到了耦合系数。其结果与FEKO,NEC仿真均吻合良好,表明该文方法可以获得比传统FDTD计算方法更加准确的计算结果。     

Independent control of resonant frequency and input impedance of amicrostrip patch by individually biased varactor diodes

Individually biased varactor diodes fitted to the radiating edges of microstrip patch antennas enable independent control of the resonant frequency and input impedance. The technique has been successfully applied to polarisation-agile patch antennas and it affords a solution to the problem of the change in input impedance when switching between polarisations     

Signal correlation between two normal-mode helical antennas for diversity reception in a multipath environment

The spatial correlation coefficient of signals received by two normal-mode helical antennas in a multipath environment characterized by Clarke's model is rigorously obtained. A new formula relating the signal correlation coefficient to the mutual impedance is derived. The mutual impedance is defined and calculated in a new manner. Our findings show that: 1) the envelope correlation coefficient of two helical antennas is greater than that of two monopole antennas; 2) the envelope correlation coefficient is dependent on the antenna terminal load and the mutual impedance and an optimal antenna load can be determined to produce the smallest correlation coefficient; and 3) the frequency response of the envelope correlation coefficient resembles that of the voltage standing-wave ratio.     

螺旋倒相型天线的分析

本文应用反应积分方程，考虑任意倾斜导线间的互阻抗，建立起广义阻抗矩阵，采用矩量法求出该天线的电流分布，进而获得辐射特性及输入阻抗等。计算结果与实测值吻合较好，计算精度令人满意，本文的分析结果对螺旋例相型天线的设计、改进、提高是很有用的     

Analysis of higher order regular polygonal loop antennas

Higher order regular polygonal loop antennas are analyzed. The input impedance of higher order antennas varies gradually when the operating frequency or the order of polygon changes. This behavior of input impedance provides a wide range of input impedance and many resonant points to antenna designers. The higher order regular polygonal loop antenna generally possesses higher directivity but poorer broadband impedance property. As the operating frequency becomes higher its directivity increases remarkably and the broadband property becomes more prominent. To achieve the desired properties one may select one of the many design data sets with a trade-off between broadband property and directivity     

Effective length and input impedance of the NIST standard dipoles

Computed values for the effective length and input impedance for a set of standard dipoles used at the National Institute of Standards and Technology are presented. These dipoles are used as calculable reference antennas in the calibration of dipole-like electromagnetic interference antennas in an open-area test site. Estimates for the uncertainty in the computed values of the effective length and antenna impedance are also given     

Ground influence on the input impedance of transient dipole and bow-tie antennas

In this paper, the influence of a lossy ground on the input impedance of dipole and bow-tie antennas excited by a short pulse is investigated. It is shown that the ground influence on the input impedance of transient dipole and bow-tie antennas is significant only for elevations smaller than 1/5 of the wavelength that corresponds to the central frequency of the exciting pulse. Furthermore, a principal difference between the input impedance due to traveling-wave and standing-wave current distributions is pointed out.     

FDTD analysis of CPW-fed folded-slot and multiple-slot antennas onthin substrates

Folded slot antennas are attractive for active arrays due to relatively large bandwidth, fabrication simplicity, and ease of integration with devices. Currently, there is little design information for these antennas, especially on thin substrates. The finite-difference time-domain (FDTD) method is applied to the analysis of CPW-fed folded-slot antennas. The paper describes the problems encountered in the analysis, compares the theoretical results and measured data, and provides some design information for folded slots. In addition, it explores the manipulation of input impedance through the use of additional slots, yielding antennas with a broadband 50 Ω input impedance     

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     

Superconducting microstrip antennas: an experimental comparison oftwo feeding methods

The recent discovery of high-temperature superconductors (HTSs) has generated a substantial amount of interest in microstrip antenna applications. However, the high permittivity of substrates compatible with HTS causes difficulty in feeding such antennas because of the high patch edge impedance. Two methods for feeding HTS microstrip antennas at K- and Ka-band are examined. Superconducting microstrip antennas that are directly coupled and gas-coupled to a microstrip transmission line have been designed and fabricated on lanthanum aluminate substrates using Y-Ba-Cu-O superconducting thin films. Measurements from these antennas, including input impedance, bandwidth, efficiency, and patterns, are presented and compared with published models. The measured results demonstrate that usable antennas can be constructed using either of these architectures, although the antennas suffer from narrow bandwidths. In each case, the HTS antenna shows a substantial improvement over an identical antenna made with normal metals     

Broadband conical microstrip antenna

Conical patch antennas, excited on TE11-mode are studied and their resonance frequencies are computed. These antennas present a broader bandwidth compared to classical microstrip antennas and, furthermore, good impedance matching can be easily obtained.