Power focusing characteristics of ellipsoidal reflector

Power focusing characteristics of a prolate spheroidal reflector, the aperture of which is parallel to the line connecting the two focuses, are numerically studied. As a transmitting and a receiving antenna, linear wires with finite length are placed at or near the focuses of the reflector. Reflected near fields from the reflector are calculated by the physical optics technique, and transmitting and receiving antenna characteristics are obtained by the method of moments. Calculated results are given for the power focusing characteristics as a function of the eccentricity, the aperture size of the reflector, antenna directions, antenna lengths, and the receiving antenna displacement. Better power focusing characteristics are obtained when the transmitting antenna becomes less directive, or the size of the aperture becomes larger. Measured and calculated results agree very well.     

Radiation pattern synthesis for circular aperture horn antennas

A set of radiation pattern functions, suitable for synthesis of radiation patterns from circular aperture horn antennas, is obtained by assuming an aperture distribution consisting of the fields of cylindrical waveguide modes. A technique is presented for using a linear combination of the radiation pattern functions to approximate a desired radiation pattern. Linear combinations of the radiation pattern functions resulting in maximum secondary gain, when used to illuminate a paraboloidal antenna, are obtained empirically. Using spherical wave theory, maximum performance theoretically obtainable from an antenna is derived as a function of the aperture size of the feed system; the feed efficiency resulting from these theoretical limits on performance is compared to the feed efficiency of patterns obtainable from circular aperture horn antennas, and to experimental results of attempts to realize optimum circular aperture horn patterns.     

Gaussian mode analysis of Cassegrain antenna efficiency

Gaussian mode analysis is a convenient way to characterize long focal length systems, such as the Cassegrain antenna at the secondary focus. We use multimode Gaussian optics to derive several interesting results concerning the aperture efficiencies of Cassegrain antennas fed by corrugated conical horns. The highest efficiency is obtained when the antenna is illuminated by a wide-band, wide-angle horn which has its phase center at the secondary focus, or equivalently by a narrow-band diffraction limited horn with its phase-center located at a confocal tertiary focus. In both cases this corresponds to placing the horn aperture at a position where the incoming fields are frequency-independent and have limited spatial extent. We generalize these results to show that a true image of the aperture distribution can always be formed on the spherical cap in the aperture of any arbitrary scalar horn, by using a single refocusing tertiary lens or mirror. Since both the incoming fields and the horn aperture fields on this surface are frequency independent, it follows that the aperture efficiency is also independent of frequency with either of these arrangements. We suggest that devices for shaping the aperture illumination, such as shaped lenses or dual shaped subreflector systems, should, in general, be installed near an image of the telescope aperture distribution.     

Two-dimensional diffraction by half-planes and wide slits nearradiating apertures

With the complex source-point method used to produce the basis elements of an array of linearly and directionally equispaced two-dimensional (2-D) beams, the fields of any aperture distribution at any range to any degree of accuracy can be obtained. For efficiency a limited number of significant beams and beam directions is required. Approximately twice as many beams as the aperture width in wavelengths, with all beam directions normal to the aperture, is found to be sufficient here for simple uniform and cosinusoidal distributions in apertures of moderate size at ranges outside the evanescent field zone of the aperture. Now the exact solution for the far field of a line source, or here a beam source in the presence of a conducting half-plane, is used as our basis element to give the solution for antenna pattern diffraction by a local half-plane. Antenna pattern diffraction by an aperture near a wide slit is presented as simply a superposition of the solutions for two coplanar half-planes with separated parallel edges. Antenna pattern distortion by various other local obstacles can be obtained similarly     

Analytical form of EM fields radiated by circular aperture antennas of various current distributions

~~Analytical form of EM fields radiated by circular aperture antennas of various current distributions[1] Schelkunoff S. A., Advanced Antenna Theory, John Wiley &Sons, 1952. [2] E|liott R.S., Antenna Theory and Design, Prentice-Hall, Engie-wood Cliffs, N J, 1981. [3] R.W.P. King and G.S. Smith, Antennas in Matter: fundamen-tals, Theory and Applications, MIT Press, Cambirdge, MA,1981. [4] W.L. Stutzman and G.A. Thiele, Antenna Theory and Desigrn,John Wiley & S…     

基于FFT的近场——口径场变换方法

本文提出的近场－口径场变换方法基于探头与被测天线（AUT）的耦合方程,由Fourier变换求出AUT发射场的平面波谱的切向分量,然后由Fourier反变换求出被测天线口径场的切向分量,从而达到诊断天线口径场幅相分布的目的。口径场变换中的大部分计算采用了FFT算法,并且引入空间域的重构法,数值精度好,数据处理效率高,可以达到任意的诊断分辨率。通过数值模拟和诊断实验,说明了该方法的正确性和工程实用性。     

Carrier-to-noise ratio and sidelobe level in a two-laser modeloptically controlled array antenna using Fourier optics

The carrier-to-noise ratio of the microwave signal and the reduction of the antenna sidelobe are discussed for an optically controlled array antenna whose microwave aperture distribution is generated and controlled using Fourier transform and optical/electrical conversion. A two-laser model using two laser diodes whose frequency difference is set to the desired microwave frequency is proposed and compared with a single-laser model using an external modulator. The two-laser model seems to be favorable from the viewpoint of available laser diode output power. The required laser output for the two-laser model can be about 10 dB less than that for the single-laser model. Moreover, relationships between the antenna radiation characteristics and the aperture size of an image mask are investigated. It is clarified that the sidelobe level can be reduced about 20 dB without changing the gain and beamwidth by using a reference optical beam with a tapered amplitude distribution     

Analysis of cavity-backed aperture antennas with a dielectricoverlay

A full-wave analysis of cavity-backed aperture antennas with a dielectric overlay is presented. The theoretical approach uses a closed-form dyadic Green's function in the spectral domain. The aperture equivalent magnetic currents are obtained using the surface equivalence theorem and an integral equation is obtained by matching the fields across the aperture. The moment method applied in spectral domain analysis is employed to solve the integral equation for the equivalent magnetic currents with proper combination of subdomain or entire domain expansion functions. Numerical results include the aperture field distribution and antenna parameters such as input impedance, bandwidth, and efficiency. A set of measurements data is compared with results based on the theoretical work     

Scalar horn with shaped lens improves Cassegrain efficiency

It is shown that the aperture efficiency of a classical Cassegrain antenna can be considerably improved if the radiation pattern of the feed is optimally shaped. The corresponding optimum field distribution Over the aperture of the feed consists of a circular main lobe which is surrounded by concentric sidelobe rings. This optimum field distribution with one sidelobe ring included is realized by shaping of a dielectric lens which is positioned in the aperture of a corrugated horn antenna. The design can provide a theoretical aperture efficiency of 90.5 percent when subreflector diffraction and aperture shadowing are neglected, i.e., an improvement of 0.4 dB compared to an optimum conventional feed. Measurements of a model at 22.8 GHz gave a practical result of 85.6 percent. If the model is used to feed a 30 m radiotelescope the overall antenna efficiency becomes about 71 percent.     

A numerical solution for the near and far fields of an annular ring of magnetic current

A simple method for calculating the near and far zone fields from an annular ring of circumferentially directed magnetic current which may be used to represent coaxial apertures is presented. Near-field contours are given for two ring sizes. The utility of the method has been illustrated by its application in several practical antenna problems where the magnetic ring current serves as the primary source. Among these are the analysis of dipole antennas mounted on a conducting sphere or cylinder, the impedance of a coaxially fed Yagi-Uda antenna, a coaxially driven wire loop, and the radiation from a coaxial aperture at the base of a cone.     

A scattering matrix-based beamformer for wide-angle scanning in hybrid antennas

This paper presents a new approach to beamforming in hybrid antennas. Using a scattering matrix model for the hybrid antenna system, a bidirectional transformation is developed that relates the signals at the hybrid system feed to the signals that would be present in a planar array at the location of the reflector aperture. For example, the received fields at the feed of a hybrid antenna system may be transformed into the fields at the reflector aperture, and these reflector aperture fields may then be processed as if they were received by a planar or linear array. Similarly, the desired field or current distribution across the reflector aperture when transmitting may be transformed into the required field or current distribution at the hybrid system feed. This method allows standard linear or planar array analysis and synthesis techniques to be used with the hybrid system. Examples are provided for transmit and receive weight synthesis.     

Amplitude and phase-shaping effects in beamwaveguides

Results are presented of an investigation on improvements in a geometrical optics design of a beam-waveguide antenna for operation at multiple frequency bands. Improvements might be possible by changing the design of the lower-frequency input pattern to the beam waveguide. The effects of amplitude and phase shaping the input pattern have been studied with an aperture diffraction model. Accurate vector near-field computations were made rapidly with a spherical wave expansion of the input and scattered fields. Numerical results indicate that for aperture sizes of less than 30 wavelengths, there is insufficient control on defocusing due to amplitude and phase shaping. Design tradeoffs on spillover loss and defocusing are possible by changing the amplitude and phase distribution of the input wavefront for larger size apertures     

Copolar and cross-polar radiation of Vivaldi antenna on dielectricsubstrate

The copolar and cross-polar radiation patterns of the Vivaldi antenna on a dielectric substrate are calculated and validated by measurements. The method involves a two-step procedure. The electric field distribution across the antenna slot aperture is calculated first. The radiated fields are then calculated, using Green's functions. The continuous exponential tapered shape is approximated by annular linearly tapered sections. The conical transmission-line theory and a variational method yield the electric field in each section. The radiation calculation is based on closed-form expressions for the dyadic Green's function of an elementary electric field source in a conducting half sheet. Both copolar and cross-polar radiation patterns of the Vivaldi antenna are calculated by integrating the Green's functions weighted by the electric field distribution over the antenna aperture. The effect of lateral truncation is taken into account by defining weighting patterns. The method is validated by original measurements and limitations of the model are discussed. Antenna directivity and sensitivity are calculated     

Measurement of equivalent power density and RF energy deposition inthe immediate vicinity of a 24-GHz traffic radar antenna

In response to reports of alleged health effects associated with the use of hand-held traffic radars, e.g., testicular cancer, a study was undertaken to quantify: (1) the distribution of the electric field (E) in the immediate vicinity of the antenna aperture of a typical 24-GHz hand-held traffic radar; and (2) the relative match and the corresponding depth of penetration of the absorbed energy when the antenna aperture is positioned in contact with material having properties similar to human tissue. The former measurements are important for assessing a large number of measurements reported by others using techniques that are not expected to be reliable indicators of power density at distances small compared with the size of the radiator; the latter are important for verifying predictions that energy from the antenna at 24-GHz will be predominantly absorbed in superficial tissue, i.e., the skin. The results of the first part of the study indicate that the equivalent-plane-wave power density in the plane of the aperture of the radar antenna is approximately one-half of the corresponding values reported in the literature; the results of the second part of the study indicate that the depth of penetration into material simulating human skin is approximately 0.5 mm     

On the safety evaluation of the space in the vicinity of an aperture antenna

In this paper, the safety evaluation of the space in the vicinity of an aperture antenna is presented. The basis of our consideration is the approximate similarity of the contours between uniformly and nonuniformly illuminated apertures. Using the equivalence of either the gain or the beam width, it was possible to derive the uniform aperture with similar contours to the corresponding nonuniform one. In order to avoid radiofrequency (RF) hazards, instead of the isoparametric contours, their envelopes have been studied. Also, in the case where the exact axial field and the radiation pattern of the aperture are known, the contours can be approximately presented. Several examples with different apertures show the procedure of drawing the contours and explain the validity of our method.     

Effects of finite ground plane on the radiation characteristics ofa circular patch antenna

An analytical technique to determine the effects of finite ground plane on the radiation characteristics of a microstrip antenna is presented. The induced currents on the ground plane and on the upper surface of the patch are determined from the discontinuity of the near field produced by the equivalent magnetic current source on the physical aperture of the patch. The radiated fields contributed by the induced current on the ground plane and the equivalent sources on the physical aperture yield the radiation pattern of the antenna. Radiation patterns of the circular patch with finite ground plane size are computed and compared with the experimental data, and the agreement is found to be good. The radiation pattern, directive gain and input impedance are found to vary widely with the ground plane size     

Experimentally based modeling of field sources forthree-dimensional computation of SAR in electromagnetic hyperthermia andtreatment planning

Presents field source-modeling, for applications to hyperthermia, by utilizing experimental data from the paraxial region of a liquid muscle-like phantom irradiated by an aperture antenna. The data are used in an optimization algorithm, applied to a Gaussian beam model (GBM), to determine the source parameters for GBM-computations of specific absorption rates everywhere, accurate to within 1% (relative to the global maximum) of the experimental results. This paper also shows how the aperture and incident fields may be determined accurately by the GBM and links them to the electric-field integral equation (EFIE), as an example, to improve the accuracy of numerical computations of the electric or magnetic fields associated with the EFIE, the magnetic-field integral equation, or any other field formulations. It is further demonstrated that models of plane waves, or approximate source fields, predict power levels with significant, unacceptable errors. Finally, it is concluded that the GBM is a viable tool for characterizing aperture antennas used in hyperthermia for cancer therapy     

Maximization of the directive gain of a parabolic reflector antenna

The directive gain of a parabolic reflector antenna is maximized by optimizing the feed aperture distribution. The feed aperture distribution is specified by a set ofNbasis functions weighted by coefficients to be determined. This approach is different from the conventional method where, given a particular feed, the directive gain is maximized by subjecting the reflector aperture parameters to optimization.     

基于口面场方法的W波段高口面效率单脉冲卡塞格伦天线

本文提出来一种应用于直升机防撞雷达的高口面效率W波段单脉冲双反射面卡塞格伦天线。口面分析方法的提出解决了W波段反射面天线口面相位分布不均匀的缺陷,从而有效的提高了口面的利用效率。利用该方法,本文研究并制作了口径为135mm、焦距为40.5mm的W波段卡赛格伦天线,并且设计了由四个E面多缝隙电桥和四个四分之一波导波长延迟线级联构成的和差网络。经测试,该单脉冲天线在93GHz具有38.6dBi的和波束增益,相应的口面效率为54.7%;差波束的零深优于-22dB,副瓣电平小于-18dB。测试结果与基于口面分析方法的仿真结果吻合,从而证明了本文所研究天线可以应用于高口面效率的W波段单脉冲系统中。     

Range distance requirements for large antenna measurements

The dependence of the radiation patterns and antenna gains on the measurement distance when both transmitting antenna and receiving antenna have the same order of large dimensions is discussed. The antenna under test is the transmitting antenna. The near-zone radiation patterns (Fresnel pattern) and the antenna gains of various kinds of circular aperture distributions vs the measurement distances are calculated for an arbitrary size of the receiving probe antenna. It is found both theoretically and experimentally that the near-zone measurement error of the radiation patterns introduced when the receiving antenna is of comparable or the same dimension as the transmitting antenna is smaller than the error introduced when the receiving antenna is very small. Thus, it is concluded that as a range criterion 2D²_a/λ is more appropriate than 2(D_a+D_b)² /λ for pattern measurements (D_a and D_b are linear dimensions of the transmitting and receiving antennas, and D_a⩾D_b is assumed). It is shown that the gain error for any aperture distribution obtained by using a large receiving probe antenna is greater at any range distance than the error obtained by using a small antenna such as a dipole