Backscattering-Based Measurement of Reactive Antenna Input Impedance

A scattering technique for measuring reactive antenna input impedance is described. The antenna scattering is measured with three different loads: an open circuit, a conjugate match, and a reactive match. The load reactances tune the antenna into resonance at the measurement band. Theory and error considerations are presented, as well as measurement results of two ultra high frequency radio frequency identification antennas. The measurements were performed in a gigahertz transverse electromagnetic mode cell. The measured impedances are within about 10% of the simulated values for a dipole-like antenna. The results of a planar inverted-F antenna are somewhat more complex, but also supported by the presented simulations and the coaxial impedance measurement results.     

Experimental study on direction dependency of reflectioncoefficients of microwave electromagnetic anechoic chamber

For measurements of reflection coefficients in an anechoic chamber by the free-space voltage standing wave ratio (VSWR) method, the composite antenna technique, using a combination of two antennas, is proposed. Reliable data on reflection coefficients can be obtained by this technique, with respect to reflected waves from arbitrary directions. Using the measurement results, the direction dependency of reflection coefficients is studied in relation to specular reflection and to the directivity of a transmitting antenna     

利用线极化天线快速测量圆极化天线轴比的方法

针对轴比测试需要连续旋转线极化天线极化轴,在高频段旋转关节难以消除高速旋转带来的电缆抖动所引起的幅度和相位不一致性,提出了一种快速、精确测量圆极化天线轴比的测试方法。基于椭圆极化波的正交分解理论,该方法利用线极化天线对待测圆极化天线进行两组正交的线极化幅度测量,通过计算得到圆极化天线的轴比等极化椭圆参数的信息。在此基础上又提出了只用三个线极化分量测量圆极化天线轴比的方法。通过在微波暗室中对圆极化天线进行多次测试,验证了该方法的有效性。该方法提高了圆极化天线的测试效率,降低了测试难度,对于工程应用有重要的应用价值。     

Antenna Effects in Depolarization Measurements

The depolarization of electromagnetic energy scattered from natural terrain has been of interest to experimenters and theoreticians for many years. However, the reported measurements have not agreed with theoreticaly predicted results. Boresight axial ratio has commonly been used as a figure of merit to describe the polarization isolation properties of the measurement system antennas. Since depolarization measurements from terrain involve extended targets which fill the full antenna beam, boresight axial ratio is often an inadequate measure of antenna polarization purity. This paper describes antenna isolation characteristics which impact the quality of the depolarized measurements. Representative calculations demonstrate the effects of nonideal antenna systems on the observed measurements. A criterion for antenna specification is given which will provide high-quality cross-polarized measurements.     

Wire antennas in the presence of a dielectric/ferrite inhomogeneity

A moment method solution for treating thin-wire antennas in the presence of an arbitrary dielectric and/or ferrite inhomogeneity is presented. The wire is modeled by an equivalent surface current density, and the dielectric/ferrite inhomogeneity is modeled by equivalent volume polarization currents. The conduction currents on the wire and the polarization currents in the dielectric/ferrite inhomogeneity are treated as independent unknowns and determined in the moment method solution. The method is applied to the problem of a loop antenna loaded with dielectric or ferrite. Numerical results are presented, and are in good agreement with measurements and previous calculations.     

Accurate measurement of antenna gain and polarization at reduced distances by an extrapolation technique

A new technique is described for determining power gain and polarization of antennas at reduced range distances. It is based on a generalized three-antenna approach which, for the first time, permits absolute gain and polarization measurements to be performed without quantitative a priori knowledge of the antennas. The required data are obtained by an extrapolation technique which includes provisions for rigorously evaluating and correcting for errors due to proximity and multipath interference effects. The theoretical basis provides a convenient and powerful approach for describing and solving antenna measurement problems, and the experimental method employed illustrates the utility of this approach. Examples of measurements are included which exhibit errors in gain as small aspm0.11dB (3sigma).     

Maritime VHF communications with polarization diversity over a Rician channel

In this paper, we propose a polarization diversity scheme to enhance maritime VHF communications. Multiple antenna systems require an antenna separation of 5–10 wavelengths to keep the correlation coefficient below 0.7 to permit the realization of space diversity. However, this may be difficult to implement in a maritime VHF system, because the maritime VHF is working in the frequency range from 156 to 174 MHz, with wavelength of approximately 2 m. On the other hand, the polarization device technique allows two co‐located antennas by using a micro‐strip technique. For this reason, the polarization technique is a practical method of attaining diversity. However, the theoretical model of polarization is very difficult and complex. Therefore, we express the cross correlation of each polarization antenna and the cross polarization discrimination of multiple polarization antennas with a simple model. Simulations show that the polarization diversity of a maritime VHF system shows better BER performance than that of the conventional reception diversity on the basis of vertical antenna with the antenna separation of 10 wavelengths. Copyright © 2012 John Wiley & Sons, Ltd.     

Fully Probe-Corrected Near-Field Far-Field Transformation Employing Plane Wave Expansion and Diagonal Translation Operators

Near-field antenna measurements combined with a near-field far-field transformation are an established antenna characterization technique. The approach avoids far-field measurements and offers a wide area of post-processing possibilities including radiation pattern determination and diagnostic methods. In this paper, a near-field far-field transformation algorithm employing plane wave expansion is presented and applied to the case of spherical near-field measurements. Compared to existing algorithms, this approach exploits the benefits of diagonalized translation operators, known from fast multipole methods. Due to the plane wave based field representation, a probe correction, using directly the probe's far-field pattern can easily be integrated into the transformation. Hence, it is possible to perform a full probe correction for arbitrary field probes with almost no additional effort. In contrast to other plane wave techniques, like holographic projections, which are suitable for highly directive antennas, the presented approach is applicable for arbitrary radiating structures. Major advantages are low computational effort with respect to the coupling matrix elements owing to the use of diagonalized translation operators and the efficient correction of arbitrary field probes. Also, irregular measurement grids can be handled with little additional effort.     

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.     

Phaseless bi-polar planar near-field measurements and diagnosticsof array antennas

Antenna near-field measurements typically require very accurate measurement of the near-field phase. There are applications where an accurate phase measurement may not be practically achievable. Phaseless measurements are beginning to emerge as an alternative microwave antenna measurements technique when phase cannot be directly measured. There are many important aspects for successful implementation of a phaseless measurement algorithm. This paper presents appropriate phaseless measurement requirements and a phase retrieval algorithm tailored for the bi-polar planar near-field antenna measurement technique. Two amplitude measurements and a squared amplitude optimal sampling interpolation method are integrated with an iterative Fourier procedure to first retrieve the phase information and then construct both the far-field pattern and diagnostic characteristics of the antenna under test. In order to critically examine the methodologies developed in this paper, phaseless measurement results for two different array antennas are presented and compared to results obtained when the near-field amplitude and phase are directly measured     

Integratable wide-band dual polarized antennas with rear field cancellation

Two variations of an integratable coplanar waveguide fed aperture stacked patch antenna are presented, which are capable of generating wideband dual polarized radiation. One of the antennas displays the desired characteristics for reducing polarization loss between an antenna remote unit (ARU) and mobile units at arbitrary angles. The other has a dual input structure and low cross-polarization useful for polarization diversity applications, or it can also produce circular polarization with the addition of a 90/spl deg/ hybrid. Back radiation concerns are addressed with the use of reflector patch elements. Results indicate that the rear directed radiation of the two slot coupled printed antennas mounted on small ground planes can be reduced across a wide bandwidth with the addition of a reflector element.     

ANSERLIN. a broad-band, low-profile, circularly polarized antenna

A technique is presented for improving the performance of low-profile, circularly polarized (CP) radiating-line antennas. The essential elements of the technique pertain to establishing a single traveling wave on an annular sector of conducting strip that is positioned parallel and close to a large conducting surface. The antenna is named the annular sector, radiating-line (ANSERLIN) antenna. Using this technique, the impedance bandwidth becomes so large that it no longer has any bearing on the operating bandwidth. Instead, the pattern shape and axial ratio become the factors that limit the bandwidth. Both senses of circular polarization can be transmitted or received from the same structure. Experimental results for two-port ANSERLIN antennas are presented. The method of exciting a traveling wave on the annular sector is discussed. Design parameters are given for an antenna that will produce a broadside axial ratio that is less than 0.5 dB at a specified frequency     

A figure of merit for signal processing reflector antennas

In recent years a new class of reflector antennas utilizing array feeds has been receiving attention. An example of this type of antenna is a reflector utilizing a moveable array feed for beam steering. Due to the circuitry required to adjust the weights for the various feed array elements, an appreciable amount of loss can be introduced into the antenna system. One technique to overcome this possible deficiency is to place low noise amplifiers with sufficient gain to overcome the weighting function losses just after each of the feed elements. In the evaluation of signal processing antennas that employ amplifiers the standard antenna gain measurement will not be indicative of the antenna system's performance. In fact, by making only a signal measurement, the antenna gain can be made any arbitrary value by changing the gains of the amplifiers used. In addition, the IEEE Standard Test Procedures for Antennas does not cover the class of antennas where the amplifier becomes part of the antenna system. There exists a need to establish a standard of merit or worth for multi-element antenna systems that involve the use of amplifiers. A proposed figure of merit for evaluating such antenna systems is presented.     

散射矩阵同时测量的一种新方法

该文提出了散射矩阵同时测量的一种新方法频域分割法。这种方法的基本原理是:发射时水平极化天线和垂直极化天线同时发射两个中心频率不同、调频斜率相同的线性调频信号,中心频率的差值等于或略大于信号带宽,接收时上述的两个天线同时接收,那么每个天线接收的回波是两个不同极化组合回波的叠加,我们利用两个回波信号频谱的差异,将之分离,从而实现散射矩阵的同时测量。这种方法避免了测量不同时以及极化开关造成的极化耦合而引起的测量误差,与散射矩阵同时测量的信号编码法相比,更容易实现高极化隔离度。     

Gain measurements of vertically polarized antennas over imperfect ground

A technique is described for measuring the absolute gain of vertically polarized HF antennas. A horizontal half-wave dipole, located near the antenna being measured (test antenna), is used as a reference in a comparison type of gain measurement. A rotatable, linearly polarized, transmitting antenna is mounted on an airplane and used to illuminate the test-reference antenna location. With the airborne antenna vertical, the response of the test antenna is measured. The airborne antenna is quickly rotated to a horizontal position and the response of the reference dipole is measured. When the airplane is in the main lobe of the test and reference antennas, these two measurements yield the gain of the test antenna relative to that of the reference dipole. The gain of a horizontal half-wave dipole is presented graphically, versus main lobe elevation angle, for various heights above imperfect ground. Mutual impedance variations were considered in the computations for these graphs. Model antenna measurements were made at 400 MHz to verify the principle of the technique. On-site HF measurements could give gain figures accurate to within a few tenths of a dB of their true value.     

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.     

Impedance matrix of an antenna array in a quasi-optical resonator

The power from numerous millimeter-wave solid-state sources can be efficiently combined using quasi-optical techniques. One technique is to place an array of active radiating sources within a quasi-optical resonator. The driving point impedance of each antenna is strongly affected by the presence of all other active antennas as well as by the mode structure and Q of the resonator. The impedance matrix for an array of antennas radiating into a plano-concave open resonator is determined here through use of the Lorentz integral. The resulting expressions include the effect of diffraction loss and are valid for arbitrary reflector spacing, source frequency, array location and geometry. The result can be used for impedance matching of each active source to its antenna, facilitating design of an efficient power combining system. Simulations using the impedance matrix in conjunction with an antenna impedance model are compared with two-port measurements     

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     

An antenna pattern measurement technique for eliminating the fieldsscattered from the edges of a finite ground plane

A novel antenna pattern measurement technique is presented to eliminate the effects of the finite ground plane and anechoic chamber wall reflections which significantly perturb antenna pattern measurements. This technique consists of the measurement of the edge-diffracted fields and their subsequent subtraction from the original pattern. A simple theoretical model is developed to introduce the subtraction technique, and comparisons are made which show the excellent agreement between theoretical (assuming an infinite ground plane) and `corrected' experimental antenna patterns. Experimental results are given for open-ended waveguide, microstrip patch, and monopole antennas mounted on circular and square ground planes of various sizes. Time gating techniques are illustrated as well