A novel amplifying antenna array using patch-antenna couplers-design and measurement

This paper proposes a novel amplifying antenna array using the patch-antenna coupler formed by placing one or two open-ended microstrip lines (coupled lines) near and along the nonradiating edge(s) of a patch antenna. An X-band five-element array with broadside 25-dB Chebyshev radiation is demonstrated. When the input signal is fed to the center element, with most of the power radiating from the antenna, part of it is tapped to the coupled lines, amplified by an FET amplifier, and fed to the next antenna element. This process is repeated after all the antenna elements are fed with suitable power. The amplitude distribution of the fields radiated from the antennas is controlled by the coupling coefficient from the patch to the coupled line, which, in turn, is governed by the coupling length and gap between the patch and line. The measured return loss of the designed five-element array is -27 dB at the center frequency of 10 GHz with 2% 10-dB bandwidth. The radiation pattern possesses a transmitting gain of 15.9 dB, a half-power beamwidth of 17/spl deg/, and a sidelobe level of -22 dB.     

机载天线辐射方向图研究

本文采用一致性几何绕射理论分析了某型载机机体对其上安装的单极子天线辐射方向图的影响，计算了天线经机体散射后的场，编制了计算程序，调用该程序可计算机载天线任意面方向图及立体方向图。其结果可用于天线间耦合特性分析和机载多天线配置优化研究。     

Antenna coupling and near-field sampling in plane-polar coordinates

A probe-corrected vector transmission formula and a rigorous sampling-reconstruction theorem for near-field antenna measurements in plane-polar coordinates are derived from three fundamental theorems of antenna theory: the mutual coupling function between two antennas satisfies the homogeneous wave equation; a receiving antenna can be represented as a differentiator of the incident field; and the mutual coupling function is virtually bandlimited. The rigorous sampling equations are applied to compute the far fields of a circular-aperture antenna sampled in the near field at half-wavelength radial spacing     

THE LIMITATION OF MUTUAL IMPEDANCE PRECISION TO THE SIDELOBE LEVEL OF ARRAY ANTENNA

Accurate mutual coupling correction is necessary for an array antenna to reach ultra-low sidelobe level.If the mutual impedance or mutual coupling coefficient matrix of an array isperfectly known,theoretically,one can compensate the effects of mutual coupling completelyand realize the desired low sidelobe level.However,the mutual impedance matrix obtainedwhether by calculation or by measurement has limited precision,which limits the effectiveness ofcompensation.This paper deals with the requirements on the precision of mutual impedance forcompensation in ultra-low sidelobe array antennas.The relationship between mutual impedanceerrors and the amplitude and phase errors of an array is derived,by which the relationship betweenthe mutual impedance errors and the sidelobe level is obtained.     

Antenna diversity in mobile communications

The conditions for antenna diversity action are investigated. In terms of the fields, a condition is shown to be that the incident field and the far field of the diversity antenna should obey (or nearly obey) an orthogonality relationship. The role of mutual coupling is central, and it is different from that in a conventional array antenna. In terms of antenna parameters, a sufficient condition for diversity action for a certain class of high gain antennas at the mobile, which approximates most practical mobile antennas, is shown to be zero (or low) mutual resistance between elements. This is not the case at the base station, where the condition is necessary only. The mutual resistance condition offers a powerful design tool, and examples of new mobile diversity antennas are discussed along with some existing designs.     

Performance of a modified feedback loop adaptive array with TVROsatellite signals

Performance of an experimental adaptive antenna array system is evaluated using television receive-only (TVRO) satellite signals. The experimental system is a sidelobe canceller with two auxiliary channels. Modified feedback loops are used to enhance the suppression of weak interfering signals. The modified feedback loops used two spatially separated antennas, each with an individual amplifier for each auxiliary channel. Thus, the experimental system uses five antenna elements. Instead of using five separate antennas, a reflector antenna with multiple feeds is used to receive signals from various TVRO satellites. The details of the earth station are given. It is shown that the experimental system can null up to two signals originating from interfering TVRO satellites while receiving the signals from a desired TVRO satellite     

Coupling between crossed-dipole feeds

Various effects of coupling between crossed-dipole antennas are analyzed and by using an arbitrary feeding network some generality is preserved. With one cross excited and another cross acting as a parasitic loaded antenna, coupling losses and gain and polarization losses are presented as a function of orientation and feeding network properties. The antennas are used as feeds for a parabolic reflector, and the effect of coupling on the secondary fields is analyzed. Especially significant is the polarization loss and it may, to some extent, be reduced by a proper choice of feeding network.     

Exact derivation of aperture errors in antenna measurements

Antenna-pattern measurements are generally undertaken by placing the antenna under test (usually the receiving antenna) sufficiently far from the transmitting antenna that the receiving antenna is illuminated with a plane wave. Gain measurements, whether by substitution, paired horns, or integration of the area under the beam pattern, depend upon the same criterion. A true plane wave is only achieved by separating the two antennas by an infinite distance, but it has been shown by Milne (1950) that small departures from a plane wave do not usually seriously degrade the measurement. The authors consider three aspects of the fields across a linear aperture. The finite spacing between the antennas produces a phase deviation and an amplitude taper. If we are only interested in the apparent gain loss, we need to combine both tapers into an illumination efficiency. It is surprising how little the illumination loss is when the phase deviation and amplitude taper are large. The formulation is given in terms of linear apertures, and can be easily extended to circular or rectangular apertures by adjusting the limits of the integrals     

HF RFID Reader Antenna with Loop Switch for Avoiding Magnetic Coupling

This paper studies the magnetic coupling between two adjacent loop antennas that are parallel to each other in a plane and presents a new practical method to avoid the resulting magnetic coupling interference. The study focuses on the high frequency radio‐frequency identification (RFID) system for casino applications, where several loop antennas are closely built into a game table to monitor gaming chips. In this case, neighboring loop antennas may severely interfere with each other by magnetic coupling, which leads to the malfunction of the RFID system. In this paper, we present a practical loop antenna with a new loop switch circuit for avoiding magnetic coupling. The loop switch circuit is integrated with a matching circuit and automatically operated by using an interrogating signal from a reader. We verified the validity of the proposed design by showing that an RFID reader with the proposed antenna can exactly and separately read the gaming chips placed on the different betting zones of a game table.     

On an algorithm for analysis of the radiation patterns of dual reflector and segmented reflector antennas

Large reflector antennas, from cost and weight considerations, appear to be the best configurations for achieving the very large apertures needed for antennas mounted on orbiting spacecraft. Radiation pattern calculations for such antennas are, in general, quite costly both in terms of computer time and the considerable memory required to perform the large surface integrations. The method presented here treats the large aperture as a set of small subapertures for which the radiation fields are computed separately, stored and then added with due regard to phase to yield the radiation pattern of the large reflector. Numerical methods developed to provide the illumination of each subaperture to avoid overlap of the aperture fields of adjacent subapertures and to simplify the surface integrations are discussed. The algorithm is straightforward and has considerable intuitive appeal. The methods of geometrical optics (GO) are used to calculate the aperture plane tangential field components; and electric vector potential is then used to compute the antenna radiation fields. The algorithm includes a set of options for different reflector surfaces. If high accuracy of the far sidelobe levels is required, a subroutine accounting for edge diffraction should be added to the algorithm given here. Calculations made with this algorithm are compared with calculations made by other methods and with measured patterns.     

基于阵元局部分布特征的MIMO雷达阵列设计方法

MIMO雷达采用多发多收的工作模式,减少了天线和收发模块的数量,降低了系统的成本和复杂性,被广泛应用于城市反恐、灾后救援等领域。针对现有二维MIMO雷达阵列存在旁瓣高、成像质量差的问题,本文提出了一种基于各天线单元局部特性的二维MIMO阵列设计方法,设计了等效阵元投影局部密度和二维局部密度两个优化对象,以及边界约束和最小间距约束两个约束条件,通过对方形均匀MIMO阵列进行迭代优化,得到旁瓣更低的阵列排布形式。仿真和实测实验结果表明,本文所设计的二维MIMO阵列的成像结果有更低的旁瓣,且在生命体征检测和人体姿态重构等雷达应用领域都有很好的探测性能,为人体目标的多域信息感知提供了可能。     

Signal Ratio Detection and Approximate Performance Analysis for Ambient Backscatter Communication Systems with Multiple Receiving Antennas

Recently, ambient backscatter attracts much attention since it can utilize ambient radio frequency signals to enable battery-free devices to communicate with others. Most existing studies about ambient backscatter assume that the reader is equipped with one receiving antenna. In practice, the reader can utilize multiple antennas to overcome channel fading. In this paper, we investigate the problem of signal detection for ambient backscatter systems with multiple receiving antennas. Specifically, we formulate a new transmission model where the reader is equipped with at least two antennas and propose a ratio detector that exploits the ratio of the signal strength received at each antenna. It is shown that the closed-form expression of the optimal detection threshold for this detector is difficult to derive. Therefore, we derive a reasonable approximate expression for the optimal detection threshold. Moreover, we obtain the closed-form expression for approximate bit error rate (BER). Furthermore, we propose an antenna selection scheme if the reader is equipped with more than two antennas. The selection scheme is investigated through the BER performance. It is found that the largest gain in BER can be achieved when the antenna number increases from two to three, and that much less gain is obtained from enlarging the antenna number when the reader already has four antennas. Finally, simulation results are provided to corroborate our theoretical studies.     

OFDM MIMO radar waveform design for targets identification

In order to obtain better target identification performance, an efficient waveform design method with high range resolution and low sidelobe level for orthogonal frequency division multiplexing (OFDM) multiple-input multiple-output (MIMO) radar is proposed in this paper. First, the wideband CP-based OFDM signal is transmitted on each antenna to guarantee large bandwidth and high range resolution. Next, a complex orthogonal design (COD) is utilized to achieve code domain orthogonality among antennas, so that the spatial diversity can be obtained in MIMO radar, and only the range sidelobe on the first antenna needs suppressing. Furthermore, sidelobe suppression is expressed as an optimization problem. The integrated sidelobe level (ISL) is adopted to construct the objective function, which is solved using the Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm. The numerical results demonstrate the superiority in performance (high resolution, strict orthogonality, and low sidelobe level) of the proposed method compared to existing algorithms.     

复射线理论应用于天线一体化设计

分析自由空间天线辐射特性的方法已十分成熟,如口径场积分法、平面波谱积分法等,它们大都是行之有效的,但要求对天线及天线罩进行一体化分析时,现有的经典方法往往不能迅速而准确地给出解答,如果用复射线理论这一新方法则可以对某些问题圆满地给出结果.根据复射线理论,利用复源点远场具有的高斯波束特性,无论是在远区或是近轴区域内,复源点场和高斯函数有很接近的相同分布,理论上论证了对于具有高斯波束分布的口径场可以用复源点场来近似模拟.     

基于超表面的天线多波束偏转技术研究北大核心CSCD

5G移动通信对天线提出了多极化、多频段、多波束等更高的技术需求。超表面具有很强的电磁调控能力,利用这一特性可以实现天线多波束偏转。本文首先提出新型的基于行波激励网络的双波束偏转方法,利用超表面实现定向波束可控,设计了单频低副瓣高增益的双波束超表面天线。其次,提出基于多相位自由度理论的双频行波激励网络,结合双频超表面,设计了双频双波束偏转角可独立控制的超表面天线。在此基础上,提出基于正相位响应理论的稳定波束偏转角的方法,设计了具有稳定波束的±45°双极化超表面天线。与传统多波束方法相比,本方法省略了复杂的波束形成网络,设计简单、结构紧凑,而且能够实现双频、双极化等特性,可为新一代移动通信天线的研制提供技术思路。     

Study of Mutual Coupling Between Circular Stacked-Patch Antennas on a Sphere

A rigorous mathematical analysis is given of spherical stacked-patch arrays with emphasis on the physical interpretation of mutual coupling mechanisms present in doubly- curved convex structures. The analysis method is based on electromagnetic field representation in terms of spherical harmonics where each harmonic has the same angular variation as the spectral source component. To obtain the spectral representation the vector-Legendre transformation is applied to currents and fields. A novel approach to the mutual coupling calculation within the method of moments analysis of spherical arrays is applied. By expressing the patch current in terms of two suitable potential-like auxiliary functions, it is possible to avoid the use of Euler's formulas for coordinate system rotation and the related lengthy integrations. Instead, the rotation of antenna elements and corresponding current distributions can be done in closed form with the help of Vilenkin's addition theorem for associated Legendre functions. It is shown that the new approach results in significant acceleration and improved accuracy of the analysis of spherical patch antenna arrays. The algorithm is successfully tested against a commercially available electromagnetic software and measurements performed on the developed laboratory model, confirming its accuracy for both input impedance and mutual coupling calculation and with only a small difference between the predicted and measured resonant frequencies, due to limitations in the experimental model. The influence of the structure parameters on mutual coupling level is extensively investigated, including all coupling mechanisms and leakage of energy due to curvature of the structure. It is shown that stacked-patch antennas can have reduced coupling level comparing to single patch antennas with possible deep nulls above the antenna resonant frequency.      

Impact of transmit antenna spacing on 2x1 Alamouti radio transmission

It is a common belief that multiple transmit antennas have to be placed far apart in order to avoid undesired effects such as mutual coupling, changing beampatterns, antenna mismatching losses, and correlation. The following radio communication setup is considered: two lambda/4-monopole ground-plane transmit antennas, a single receive antenna, 4QAM modulation, Alamouti space-time coding, uncoded single carrier transmission at 2.4 GHz, indoor scenario. By measurements, the impact of very low transmit antenna spacing on the average uncoded BER over transmit-power performance of such a radio link, therefore accounting for all losses in the transmission chain, is examined. The common belief that multiple transmit antennas have to be placed far apart cannot be confirmed     

Experimental Study on the Capacity of Compact MIMO Antennas for Portable Terminals

This paper presents the relationship between antenna structures and the performance of two kinds of compact MIMO antennas in order to find critical factors that affect the capacity of MIMO systems. The relationship between the channel capacity and some factors (antenna efficiency, mutual coupling, correlation) are analyzed based on experimental data under indoor Rayleigh fading environment. Antenna elements mounted in two different configurations (common and separated ground plane) with antenna spacing varying, were investigated at the frequency of 2.6 GHz band experimentally. The good characteristics in the case of separated ground plane show that the proposed antennas, even with small spacing, can still achieve high capacity to combat multipath fading and deliver higher data rates. It demonstrates that multiple antennas could be mounted onto small terminal devices without much loss of capacity. It is also found that mutual coupling has positive impact which could reduce channel correlation; negative effect which could degrade antenna efficiency. In the indoor multipath-rich environment, the negative effect is dominant.     

An FDTD/MoM hybrid technique for modeling complex antennas in thepresence of heterogeneous grounds

Calculating the current distribution and radiation patterns for ground-penetrating radar antennas is a challenging problem because of the complex interaction between the antenna, the ground, and any buried scatterer. Typically, numerical techniques that are well suited for modeling the antennas themselves are not well suited for modeling the heterogeneous grounds, and visa versa. For example the finite-difference time-domain (FDTD) technique is well suited for modeling fields in heterogeneous media, whereas the method of moments (MoM) is well suited for modeling complex antennas in free space. This paper describes a hybrid technique, based upon the equivalence principle, for calculating an antenna's current distribution radiation pattern when the antenna is located near an air-ground interface. The original problem is decomposed into two coupled equivalent problems: one for the antenna geometry and the other for the ground geometry, with field information passing between them via a rapidly converging iterative procedure. The fields in each region may be modeled using numerical techniques best suited to them. Results for several test cases are presented, using FDTD to model the ground problem and MoM for the antenna problem, that demonstrate the accuracy of this hybrid technique