Aperture Integration Method Research Articles

Effect of Panel Misalignment Error Distribution on the Radiation Pattern of Leighton Chajnantor Telescope’s Antenna

The Leighton dish formerly at the Caltech Submillimeter Observatory will be reassembled to become the Leighton Chajnantor Telescope (LCT). It will be required to demonstrate high surface accuracy and controlled radiation patterns at high frequency, which are largely limited by surface error due to panel misalignment on the primary reflector. Not only the surface rms error but also the error distribution has effect on the radiation pattern of the antenna. Therefore, analyzing the effect of error distributions resulting from various panel misalignments on the radiation pattern of the antenna will facilitate better compensation of surface errors during the reassembly process. To acquire the radiation pattern of LCT’s antenna with panel misalignments, we first propose a simulation method based on physical optics and the physical theory of diffraction, which offers precise results but is time-consuming, especially at high frequencies. Then, we propose a rapid computation method that combines the method of calculating the optical path difference (OPD), the OPD fitting method, and the aperture integration method for segmented reflectors. Experimental results demonstrate that the rapid computation method is highly efficient and accurate compared to the simulation method. In order to show the effect of error distributions due to two typical panel misalignments (i.e., piston and tip-tilt) on radiation patterns, experiments are conducted using the two proposed methods for various error distributions. These experiments indicate that for the same surface rms error, smaller panel errors at smaller normalized aperture radii are more conducive to achieving improved characteristics of the radiation patterns, such as reduced peak gain losses and lower sidelobe levels. Additionally, comparison experiments also reveal that variations in piston error have a greater impact on the radiation patterns than variations in tip-tilt error under the same surface rms error.

Novel Metamaterial-Element-Based FSS for Airborne Radome Applications

A novel metamaterial (MTM) element for radome applications is presented in this paper, which shows negative permittivity and permeability characteristics at distinct frequency ranges. The MTM element is composed of tightly coupled Swastika-shaped conductors printed on both sides of the dielectric substrate. The electromagnetic (EM) analysis of the proposed structure is carried out using the full-wave method-based standard software package and then validated with the experimental results. The MTM structure is further modified by introducing shorting strips at its edges to improve bandpass FSS response with flat-top and sharp roll-off characteristics in microwave and millimeter-wave frequency regimes. In view of airborne radome application of MTM-FSS structure, a multilayered hemispherical radome is designed by embedding it in the mid-plane of the radome wall. The EM performance of the antenna–radome system is analyzed using 3-D ray tracing geometrical optics in conjunction with aperture integration method, incorporating the transmission characteristics of MTM-FSS-based radome wall using CST Microwave Studio. The MTM-FSS-based radome exhibits superior EM performance characteristics as compared to the conventional optimized monolithic radome over the frequency range of 8.5–10.3 GHz, with high transmission efficiency (~0.30) and minimal boresight error (~4 mrad) for the entire beam steering range.

Electromagnetic Performance Analysis of Graded Dielectric Inhomogeneous Streamlined Airborne Radome

Streamlined nosecone radomes for airborne applications have to cater to high-end electromagnetic (EM) performance requirements of fire control radar antenna system. In this regard, the EM performance analysis of an ogival radome based on novel graded dielectric inhomogeneous wall structure is presented. The radome wall considered here consists of seven dielectric layers cascaded in such a way that the middle layer has maximum dielectric parameters (dielectric constant and electric loss tangent) and on either side, dielectric parameters of the layers decrease in a graded (or stepwise) manner. Further, the outer surface of the radome wall is coated with an antistatic and antierosion radome paint. The EM performance parameters of the radome enclosing an X-band slotted waveguide planar array antenna (center frequency: 10 GHz; bandwidth: 1 GHz) are computed based on 3-D ray tracing in conjunction with aperture integration method. The study shows that the proposed graded dielectric inhomogeneous streamlined radome is an excellent choice for airborne applications as compared to airborne radomes based on conventional constant thickness radome designs and variable thickness radome (VTR) designs. Further, it circumvents the constraints on fabrication that occur in streamlined VTR designs.

TEMPERATURE-DEPENDENT ELECTROMAGNETIC PERFORMANCE PREDICTIONS OF A HYPERSONIC STREAMLINED RADOME

Nosecone radomes of hypersonic flight vehicles show degradation of electromagnetic (EM) performance characteristics due to variations in the dielectric parameters (dielectric constant and electric loss tangent) of the radome wall resulting from heating due to extreme aerodynamic drag. It is indicated that the EM performance predictions based on conventional monolithic half-wave wall based on average dielectric parameters corresponding to temperature ranges in hypersonic conditions may not be accurate. This necessitates the radome wall under hypersonic conditions to be modeled as an inhomogeneous dielectric structure for accurate EM performance predictions. In the present work, the hypersonic radome is considered as an inhomogeneous dielectric radome such that the cross-section of the radome wall in each EM window region is considered as an inhomogeneous planar layer (IPL) model with stacked layers of varying dielectric parameters. The material considered is RBSN Ceralloy 147-010F (an alloy of silicon nitride), which has excellent thermal shock resistance, dielectric and mechanical properties required for hypersonic radome applications. The EM modeling of a section of the radome wall in hypersonic conditions (i.e., IPL structure) is based on Equivalent Transmission Line Method. A comparative study of basic EM performance parameters of the radome wall (power transmission, power reflection, and insertion phase delay) for both the IPL model and conventional monolithic half-wave model are carried out over a range of incidence angles corresponding to the antenna scan ranges in each EM window region of the radome. Further the study is extended to compute the EM performance parameters of an actual tangent ogive nosecone radome (made of RBSN Ceralloy 147-010F) enclosing an X-band slotted waveguide planar array antenna, in a hypersonic environment. The antenna-radome interaction studies are based on 3-D Ray tracing in conjunction with Aperture Integration Method .I t is observed that the EM performance analysis based on conventional monolithic radome wall design cannot accurately predict the radome performance parameters in actual operating conditions during hypersonic flight operations. The current work establishes the efficacy of Inhomogeneous Dielectric Radome model for better EM performance predictions of streamlined airborne radomes in hypersonic environments.

Dielectric concentrator for Cherenkov radiation.

We report on a dielectric target that concentrates Cherenkov radiation into a small spatial area. In contrast to traditional devices, this target can focus almost all of the radiation without using additional lenses or mirrors. We consider the case where radiation is produced by a point charge moving along the axis of a cylindrical channel inside an axially symmetrical target. The specific form of the target is determined using the laws of ray optics. The field is calculated using an aperture integration method that can determine the field near the focus. Typical field plots and the spatial distribution of the field outside the target are presented. We demonstrate that at terahertz frequencies, this concentrator can increase the field magnitude by up to at least 2 orders of magnitude relative to that on the surface of the target.

IMPACT OF FINITE GROUND PLANE EDGE DIFFRACTIONS ON RADIATION PATTERNS OF APERTURE ANTENNAS

In this study, the impact of flnite ground plane edge difiractions on the amplitude patterns of aperture antennas is examined. The Uniform Theory of Difiraction (UTD) and the Geometrical Optics (GO) methods are utilized to calculate the amplitude patterns of a conical horn, and rectangular and circular waveguide apertures mounted on square and circular flnite ground planes. The electric fleld distribution over the antenna aperture is obtained by a modal method, and then it is employed to calculate the geometrical optics fleld using the aperture integration method. The UTD is then applied to evaluate the difiraction from the ground planes' edges. Far-zone amplitude patterns in the E and H planes are flnally obtained by the vectorial summation of the GO and UTD flelds. In this paper, to accurately predict the H-plane amplitude patterns of circular and rectangular apertures mounted on square ground planes, the E-plane edge difiractions need to be included because the E-plane edge difiractions are much more intense than those of the H-plane edge regular and slope difiractions. Validity of the analysis is established by satisfactory agreement between the predicted and measured data and those simulated by Ansoft's High Frequency Structure Simulator (HFSS). Good agreement is observed for all cases considered.

Electromagnetic Performance Analysis of a Novel Monolithic Radome for Airborne Applications

A monolithic half-wave hybrid variable thickness radome (hy-VTR) is proposed here for airborne applications. The 3-D ray-tracing (in conjunction with aperture integration method), which is adapted here to evaluate the radome performance parameters, facilitates efficient analysis of antenna-radome interactions. The optimized power reflection hy-VTR design is established to be superior to the conventional constant thickness radome (CTR) designs in terms of the EM performance.

Boundary diffraction waves at multiple Fresnel-Kirchhoff diffraction

The theory of boundary diffraction wave is generalized to the case of multiple Fresnel-Kirchhoff diffraction on several sequentially situated holes (screens). The generalized boundary wave is the sum of boundary diffraction waves with different multiplicities described by integrals whose maximal multiplicity is equal to the number of obstacles. It is shown that the proposed generalized boundary wave method provides a multiple time gain in calculations, as compared to the aperture integration method. Results of numerical calculations agree with experimental dependences.

FDTD-PWS Scheme and Its Application to Analysis of Focusing Properties of Dielectric Lens

The focusing properties of lens are studied by a hybrid scheme that involves the finite-difference time-domain (FDTD) method for the aperture-plane field computation and the plane wave spectrum (PWS) method for the transformation from the aperture-plane field to the focal-plane field. To demonstrate the accuracy and efficiency of this new scheme, a Ka-band lens antenna is simulated and fabricated as an example. Subsequently, the near-field-region and the Fresnel-region fields of this lens antenna are measured by the planar-near-field (PNF) method. It is found that the results obtained by the FDTD-PWS scheme are in good agreement with the measured data. Furthermore, the numerical results and the computational requirements of this new scheme are compared with those of the near-field to near-field (NF-NF) transformation, the aperture integration (AI) method, the geometrical optics (GO)-physical optics (PO) method, and the full FDTD simulation. Both the central processing unit (CPU) time and memory requirement can be reduced efficiently. Finally, the effects of several fundamental issues, including spatial sample parameters, lens material, operating frequency, and designed focal length, on the accuracy of the proposed scheme and on the focusing properties of the lens are investigated.

Novel A-sandwich radome design for airborne applications

A novel A-sandwich hybrid variable thickness radome design, based on optimised power reflection, is presented. The radome performance parameters are evaluated accurately by a 3D ray-tracing procedure in conjunction with the aperture integration method. This design is demonstrated to be a better choice when multiple performance parameters need to be satisfied simultaneously.

Numerical modeling of a nonuniform grating with FDTD

The need for a numerical analysis of a nonuniform grating arises from antenna testing facilities. A hologram type of compact antenna test range (CATR) is a recently proposed method for antenna testing at millimeter wavelengths. Simulations of the quiet-zone field are done with a two-dimensional finite-difference–time domain method (FDTD), combined with an exact near-field aperture integration method [physical optics (PO)]. © 1997 John Wiley & Sons, Inc. Microwave Opt Technol Lett 15: 134–139, 1997.

Analysis of radiation characteristics of a polyhedron‐approximate reflector antenna using the aperture integration method and the current distribution method

AbstractIn this paper, the radiation characteristics and its analysis method are discussed for a polyhedron‐approximated reflector antenna in which the reflector is made of several planar facets as in a large deployable antenna. Based on the current distribution method considering the law of reflection and the effect of diffraction, the aperture integration method using the ray tracing containing an approximation in the law of reflection is used for the electric field distribution of a polyhedron‐approximated antenna. It is found that the electric field of the present antenna becomes maximal at the joint between the facets. It is shown that the far‐field characteristics of the antenna derived by the aperture integration method based on the approximate ray tracing and by the current distribution method are identical. the relationship was found between the equivalent aperture plane where the field distributions derived by the two methods are identical. Further, by means of the rigorous ray tracing method, the three dimensional ray behavior is shown. It is found that the aperture integration method based on the rigorous ray tracing method is inappropriate for the calculation of the radiation characteristics of the present antenna.

Investigation of radiation characteristics of large deployable antennas using a model antenna

AbstractThis paper clarifies the radiation characteristics of a tension truss‐type deployable antenna based on measurements made on its solid model and presents its experimental method. The main reflector of the model antenna is constructed by many triangular plates approximating a paraboloid. As the aperture shape of a tension truss‐type deployable antenna is a hexagon, the difference between the circular aperture of the model antenna and the hexagon is covered with radiowave absorbers. The distribution of mechanical reflector errors which are projected on the aperture plane is formulated and their relations to electrical errors is shown. Data on reduction of gain and grating lobes in the radiation pattern arising from periodic reflector errors are obtained. The measured results are compared with calculated values of radiation characteristics and good agreement is obtained. It is shown that radiation characteristics can be estimated by the aperture integration method by neglecting the disturbance of current distribution at seams of facets.

A versatile reflector antenna pattern computation method: Shooting and bouncing rays

AbstractBased on shooting and bouncing geometrical optics rays, this paper describes an aperture integration method for computing the secondary pattern of reflector antennas. The method is versatile and flexible. Within the geometricdl optics accuracy, there is virtually no restriction on the number of reflectors, the coating material, radome, or the feed configurations. Pattern computations are shown for an antenna with six reflectors, coated with dielectric material and covered by a spherical radome.

Performance and design procedure of dual parabolic cylindrical antennas

The radiation characteristics of dual parabolic cylindrical antennas are studied, and the dependence of the principal plane beamwidths and the peak cross-polarization on their geometrical parameters is determined. The antenna aperture is rectangular in shape and generates an elliptical beam pattern, with a beamwidth ratio that can be controlled by the main and subreflector focal lengths. The far-field patterns are determined by an extended aperture integration method that includes the contributions of the reflected and the main diffracted rays. It is found that the cross-polarization depends of the offset angle between the axis and the direction of the normal to the subreflector surface and can be minimized by optimizing the relative angle between the reflectors. Other pattern characteristics are controlled by the antenna geometrical parameters and the feed illumination. A procedure for the design of these antennas and the expressions for determining the reflector geometries are provided. >

Properties of dual parabolic cylindrical reflectors

Two parabolic cylinders having the focal line of one coincident with the directrix of the other form an optical system that focuses the focal line of the first reflector to a point at infinity. The operation principle of such a system is studied, and its far diffracted field is determined. The unit is assumed to be illuminated by a directional point source, simulating a feed horn, and a ray-tracing approach is used to obtain the reflected and main diffracted fields over its aperture. The far-field patterns in the vicinity of the main beam direction are then computed by an aperture-integration method. The effects of various system parameters, on the far-field patterns, are also determined.