An equivalent focused reflector feed in place of any generalised defocused or extended feed

A procedure is presented whereby generalised reflector feeds with near-field and/or defocused properties can be replaced by an equivalent point-source feed located at the origin. The equivalent feed induces identical reflector currents and consequently yields identical scattered fields. Thus all physical optics reflector analysis can be carried out in terms of a focused point-source illumination function.     

Single reflector synthesis using an analytical gradient procedure

A very efficient iterative procedure for the synthesis of a single reflector antenna from far-field data under the assumptions of physical optics (PO) is discussed. A key feature of the approach is that an analytical gradient expression is used to modify the reflector shape in each iteration and a geometrical optics (GO) method is used to provide an initial solution     

Studies of the focal region of a spherical reflector: Stationary phase evaluation

The geometric optics and polarization properties of a spherical reflector are used to develop an integral representation of its focal region fields. These integrals are evaluated by the extended method of stationary phase for field points off the caustics, on the axial caustic, on the caustic surface, and at the paraxial focus. The contributions to the field at a field point are shown to arise respectively from three ordinary stationary points: a stationary ring and a stationary point at the vertex; an ordinary stationary point and a caustic type stationary point; and a fourth-order stationary point. The resulting formulas are used to compute the value of the focal region fields. The computed results are then compared to measured data.     

Comparison of induced current and aperture field integrations for an offset parabolic reflector

The fields predicted by integration of the physical optics induced currents on the reflector (ICM) and the geometrical optics aperture fields (AFM) on the surface that caps the reflector are compared numerically for offset parabolic reflectors. It is found that the AFM solutions are very close to ICM solutions for the main beam and the first few sidelobes but discrepancy remains for the far-out sidelobes. This discrepancy is found to come mainly from the difference of polarization matrix for each method.     

Analysis of aperture blockage in reflector antennas by usingobstacle-located blockage currents

An improved method is presented to account for blockage effects in the analysis of reflector antennas. Commonly this is done by introducing shadows on the reflector surface according to the location of the obstacles when performing the physical optics integration. By using physical optics blockage currents located at the blocking obstacle instead of at the main reflector surface, the effect of the different locations in the axial direction is accurately accounted for. This can easily be included by a single phase factor in existing computer programs based on physical optics integration     

Bicollimated Gregorian reflector antenna

A bicollimated Gregorian reflector is structurally similar to a classical confocal Gregorian reflector, but its surfaces are shaped to have better scan capability. A geometrical optics procedure is used in designing the reflector surfaces. A three-dimensional ray tracing procedure is used in analyzing the aperture phase errors as the beam is scanned to different angles. The results show that the bicollimated configuration has about 45 percent greater angular scanning range than the equivalent confocal Gregorian reflector antenna.     

A design procedure for classical offset dual reflector antennaswith circular apertures

A geometrical optics procedure for designing electrically optimized classical offset dual reflector antennas with circular apertures is presented. Equations are derived that allow the size and spacing of the main and subreflectors of the antenna system, along with the feed horn subintended angle, to be used as input variables of the design procedure. The procedure, together with these equations, yields an optimized design, starting from general system requirements. The procedure is demonstrated by designing both an offset Cassegrain and an offset Gregorian antenna, and is validated by analyzing their radiation patterns using physical optics surface current integration on both the main and subreflectors     

A resistive sheet approximation for mesh reflector antennas

A simplified method of estimating the equivalent surface resistance of a reflecting mesh is presented. The equivalent resistance is obtained from the approximate mesh reflection coefficients, which are based on averaged boundary conditions. This resistance approximation allows an integral equation solution for the mesh reflector that is a simple extension of that for the perfectly conducting reflector. Paraboloid radiation patterns using physical optics in conjunction with the reflection coefficients are compared to an E-field integral equation solution for a resistive surface. The agreement is excellent for low to moderate resistance values, even in the sidelobe regions     

Optimisation of cross-polarisation performance for tri-reflectorCATR with spherical main reflector

For a tri-reflector compact antenna test range (CATR) with spherical main reflector and shaped subreflectors, the cross-polarisation performance is optimised, based on the beam-mode expansion method. For optimal designs, physical optics (PO) calculations of the quiet zone fields are presented, and it is shown that a tradeoff has to be made between a low cross-polarisation level and a low amplitude ripple in the quiet zone     

Analytical solution for early-time transient radiation frompulse-excited parabolic reflector antennas

A closed-form analytical solution is developed for predicting the early-time transient electromagnetic fields which are generated by a perfectly conducting parabolic reflector antenna when it is illuminated by a transient step spherical wave due to an elemental Huygen's source located at the focus. This closed-form time-domain solution, which is valid both near and far from the reflector (and anywhere in the forward region) can be used via the convolution theorem to efficiently obtain the early-time transient fields generated by the same parabolic reflector antenna when it is illuminated by a realistic finite-energy pulse which emanates as a spherical wave from the focus. The transient solution is developed here by analytically inverting, in closed form, the corresponding frequency-domain solution in terms of a radiation integral that employs an asymptotic high-frequency geometrical optics (GO)-based approximation for the fields in the aperture. Numerical results are presented for the transient fields both near and far from the reflector. The fields on boresight exhibit an impulse-like behavior similar to that of the impulse radiating antenna (IRA) introduced by Baum et al. (1989, 1993)     

预测反射面天线馈源毁伤效应的混合算法

提出了一种预测反射面天线系统在馈源毁伤状态下辐射特性的混合算法.采用时域有限差分法(FDTD)分析了波导开口辐射器馈源穿孔毁伤时的初级辐射特性,利用物理光学法(PO)和物理绕射理论(PTD)分析了反射面天线的次级辐射特性.并采用非场分裂式完全匹配层(UPML)吸收边界条件、共形网格(Conformalmesh)技术提高计算精度和效率,取得了良好的效果,充分验证了混合算法的有效性.     

A parabolic reflector with asymmetric low sidelobes

A technique for generating low sidelobes on one side of the main lobe for a parabolic reflector antenna is presented. The reflector system that generates such an asymmetric sidelobe pattern consists of a central parabola with two sections of offset parabolas situated at the top and bottom of the central parabola. By adjusting the positions and dimensions of the side reflectors, considerable suppression of the first few sidelobes can be obtained. Further, by similar adjustments it is possible to achieve suppression of sidelobes on both sides of the main beam, though this suppression is relatively less. The technique, though explained in detail with reference to a cylindrical geometry, is extended for a paraboloid of revolution. The analysis program utilized for the optimization of the side reflectors is based on physical optics current integration.     

Effect of an arcjet plume on satellite reflector performance

The effect of an arcjet plume on the performance of satellite reflector antennas is studied. The arcjet plume is modeled as a weakly ionized plasma. The spatial permittivity distribution of the plume is approximated using the measured electron density profile and a cold plasma model. Geometrical optics is applied to determine the ray paths as well as the transmitted fields through the inhomogeneous plume. The ray optics results are compared against several exact solutions for scattering from inhomogeneous dielectrics, and good agreement is observed for sufficiently large scatterer size. The far-field antenna patterns of the reflector in the presence of the plume are calculated from the transmitted ray fields using a ray-tube integration scheme. For arcjet prototypes in the 1-kW class, the plume effect on the antenna performance is small. As the electron density increases, the main beam and sidelobe level gradually degrade. The main beam also tends to squint away from the plume region     

Analysis of reflector antenna systems with arbitrary feed arraysusing primary field superposition

In contrast to secondary pattern superposition, where the fields reflected from the main reflector arising from each element are superimposed in the far field of the reflector, the approach presented here sums the primary fields at the reflector surface before the physical optics radiation integral is performed. The method allows each feed array element to have arbitrary position, orientation, pattern, and excitation (magnitude and phase). In addition, it is inherently efficient because evaluation of only one time-consuming radiation integral is required, rather than one per feed element as in secondary superposition. The method allows for accurate calculation of the power radiated from the feed, permitting the reflector gain and spillover efficiency to be determined within the context of a single computer program. The accuracies and characteristics of this method are demonstrated with several examples     

Stationary phase method for far-field computation of defocused reflector antennas

Computation of the far field of a defocused reflector antenna using the stationary phase method is described. The positions of the stationary phase points are determined, to eliminate one of the two numerical integrations of the radiation integral. Examples of scan patterns and computational time saving are presented.     

Analysis and characterization of multilayered reflector antennas:rain/snow accumulation and deployable membrane

There are important engineering issues in designing reflector antennas that cannot be addressed by simply assuming a perfect electric conductor (PEC) reflector surface. For example, coatings may exist on antenna surfaces for protection, rain or snow can accumulate on outdoor reflectors, and the deployable mesh or inflatable membrane antennas usually do not have solid PEC reflector surfaces. Physical optics (PO) analysis remains the most popular method of reflector analysis owing to its inherent simplicity, accuracy, and efficiency. The conventional PO analysis is performed under the assumption of perfectly conducting reflector surface. To generalize the PO analysis to arbitrary reflector surfaces, a modified PO analysis is presented. Under the assumptions of locally planar reflector surface and locally planewave characteristic of the waves incident upon the reflector surface, the reflection and transmission coefficients at every point of the reflector surface are determined by the transmission-line analogy to the multilayered surface structure. The modified PO currents, taking into account by the finite transmissions of the incident waves, are derived from the reflection and transmission coefficients. Applications on the analyses of the rain and snow accumulation effects on the direct-broadcast TV antennas and the effects of finite thickness and finite conductivity of the metal coating on a 15-m inflatable antenna are described and results are presented     

Compact antenna test range reflector edge treatment

The physical optics technique is used to compare the performance of single offset compact antenna test ranges with different reflector edge treatments and rim shapes. A comparison between reflector edge taper and rim serrations in controlling edge diffraction is demonstrated     

ICARA: induced-current analysis of reflector antennas

The aim of this work is to present the ICARA (induced-current analysis of reflector antennas) software, which is able to predict the behavior of reflector antennas using the physical optics method. The software offers different options for antenna configurations, single and array feed models, and far-field or aperture-field analysis.     

Secondary pattern and focal region distribution of reflector antennas under wide-angle scanning

A new numerical method, Fourier-Bessel series techniques, has been developed to investigate the far-field pattern and focal region distribution of reflector antennas under wide-angle scanning. In this Fourier-Bessel series technique, the current on the reflector surface is first expanded in terms of elementary sinusoidal functions via the well established fast Fourier transform (FFT) algorithm and the surface integration involved in physical optics integration is then carried out analytically. The derivation of Fourier-Bessel series and its convergence as applied to parabolic reflectors are described. The secondary patterns and focal region distributions of a parabolic reflector withF/D = 0.48and scanning up to 48 beamwidths are presented.     

Equivalence of surface current and aperture field integrations for reflector antennas

The "extinction theorem" is used to prove that the fields of reflector antennas determined by integration of the current on the illuminated surface of the reflector are identical to the fields determined by aperture field integration with the Kottler-Franz formulas over any surfaceS_{a}that caps the reflector. As a corollary to this equivalence theorem, the fields predicted by integration of the physical optics (PO) surface currents and the Kottler-Franz integration of the geometrical optics (GO) aperture fields onS_{a}agree to within the locally plane-wave approximation inherent in PO and GO. Moreover, within the region of accuracy of the fields predicted by PO current or GO aperture field integration, the far fields predicted by the Kottler-Franz aperture integration are closely approximated by the far fields obtained from aperture integration of the tangential electric or magnetic field alone. In particular, discrepancies in symmetry between the far fields of offset reflector antennas obtained from PO current and GO aperture field integrations disappear when the aperture of integration is chosen to cap (or nearly cap) the reflector.