William A. Imbriale

Design of a dichroic Cassegrain subreflector

The design of a dichroic subreflector for a dual-frequency reflector antenna is described. This antenna consists of a Ku -band Cassegrain feed requiring the subreflector surface to be highly reflective at 13-15 GHz and a primary focus S -band feed requiring the subreflector to be transparent at 2.0-2.3 GHz. Such a performance is achieved by a surface of crossed dipoles printed on a dielectric sheet. The influence of parameters, dipole length, width and spacing, and the dielectric constant and thickness of the sheet on the reflection and transmission coefficients is experimentally evaluated. An analysis based upon the Floquent mode theory is shown to correctly predict the experimental results. The construction of a hyperbolic subreflector using the selected surface parameters is briefly described. As compared to a solid subreflector of identical shape, this dichroic subreflector produced a negligible loss (less than 0.1 dB) over a 13-15 GHz band. At the S band the loss was less than 0.2 dB over narrow selected bands and the axial ratio deterioration was also no more than 0.2 dB.

RF MEMS Actuated Reconfigurable Reflectarray Patch-Slot Element

This paper describes the design of a reconfigurable reflectarray element using commercially available radio frequency micro-electromechanical system (RF MEMS) switches. The element consists of a microstrip patch on the top surface and a slot with an actuated variable length in the ground plane. RF MEMS switches are mounted on the slot to electronically vary the slot length by actuating the switches and thus obtaining the desired phase response. Waveguide measurements and high frequency structure simulator (HFSS) simulations are used to characterize the reflectarray element. The four MEMS switches element gives 10 independent states with a phase swing of 150 deg and a loss variation from 0.4 dB to 1.5 dB at 2 GHz (more switches can provide larger phase shift). The loss is mainly attributed to the dielectric loss and the conductor loss, which occur due to the relatively strong electric fields in the substrate region below the patch and the large currents on the top surface of the patch, respectively, close to the patch resonance. Detailed analysis is performed to characterize the effect of the switches by taking into consideration the switch model and wire bonding effects.

The two-dimensional inverse scattering problem

It is demonstrated that the knowledge of the incident field and the scattered far fields at one frequency may be employed to determine the size, shape, and location of a perfectly conducting scatterer. The reconstruction of the scattering body is accomplished via an analytic continuation procedure that generates the fields in the neighborhood of the scatter from the specified far-field distribution. The geometry of the body is then determined by locating a closed surface for which the total tangential electric field, i.e., the sum of the tangential components of the incident and scattered field, is zero. Whereas exact knowledge of the entire far field is sufficient to determine the scatterer, a technique is also given for size and shape determination when only part of the far field is available. Numerical examples of several different geometries are given for ranges of ka (a the largest dimension of the body) from 0.2 to 10. Geometries considered were elliptic and circular cylinders, conducting strips, and two cylinders. Plots of the fields reconstructed from the far-field data are compared to the known solutions, and the accuracy of the procedure is demonstrated. The effects of noise in the far-field pattern is also considered, and it is shown that even with noise levels of -20 dB the scattering geometry can be recovered.

On the theory of the synthesis of single and dual offset shaped reflector antennas

Since Kinber (Radio Technika and Engineering-1963) and Galindo (IEEE Trans. Antennas Propagat.-1963/1964) developed the solution to the circular symmetric dual shaped synthesis problem, the question of existence (and of uniqueness) for offset dual (or single) shaped synthesis has been a point of controversy. Many researchers thought that the exact offset solutions may not exist. Later, Galindo-Israel and Mittra (IEEE Trans. Antennas Propagat.-1979) and others formulated the problem exactly and obtained excellent and numerically efficient but approximate solutions. Using a technique similar to that first developed by Schruben for the single reflector problem (Journal of the Optical Society-1973), Brickell and Westcott (Proc. Institute of Electrical Engineering-1981) developed a Monge-Ampere (MA) second-order nonlinear partial differential equation for the dual reflector problem. They solved an elliptic form of this equation by a technique introduced by Rall (1979) which iterates, by a Newton method, a finite difference linearized MA equation. The elliptic character requires a set of finite difference equations to be developed and solved iteratively. Existence still remained in question. Although the second-order MA equation developed by Schruben is elliptic, the first-order equations from which the MA equation is derived can be integrated progressively (e.g., as for an initial condition problem such as for hyperbolic equations) a noniterative and usually more rapid type solution. In this paper, we have solved, numerically, the first-order equations. Exact solutions are thus obtained by progressive integration. Furthermore, we have concluded that not only does an exact solution exist, but an infinite set of such solutions exists. These conclusions are inferred, in part, from numerical results.

Array feed synthesis for correction of reflector distortion and vernier beamsteering

An algorithmic procedure is described for the synthesis of a planar-array for paraboloidal reflectors to provide simultaneously electronic correction of systematic reflector surface distortions as well as a vernier electronic beamsteering capability. Several f/D ratios and distortion models were examined that are typical of large paraboloidal reflectors. Numerical results are presented showing that, for the range of distortion models considered, significant on-axis gain restoration can be achieved with a one-ring (seven-element) array. However, with seven elements, the array parameters that maximize system gain, do not provide uniform beam-steering (+or-1 BW) and an additional ring (19 elements) is required. For arrays either 7 or 19 elements, the results indicate that the use of high-aperture-efficiency elements in the array yields higher system gain than can be obtained with elements having lower aperture efficiency. Contour plots of the focal-plane fields are also presented for various distortion and beam-scan-angle cases, showing the dynamic nature of the problem. >

On the reflectivity of complex mesh surfaces (spacecraft reflector antennas)

Poorer than expected surface reflectivity was observed in an early Tracking and Data Relay Satellite System antenna utilizing a tricot mesh weave. This poor reflectivity was determined to be caused by inadequate electrical contact at wire crossover points. A proper mathematical and numerical approach to assess the impact of wire junctions on reflectivity performance is developed. A mathematical method is presented for computing the surface reflectivity of complex mesh configurations like those on unfurlable-type spacecraft antennas. The method is based on the Floquet mode expansion to establish an integral equation for mesh wire currents. The equation is solved using the method of moments with triangular basis functions. It is observed that it is necessary to give special attention to the junction treatment among different branches of the mesh configurations. A vector junction current approach that resulted in satisfactory solutions for the current is described. The results of numerical simulations are compared against measured data and excellent agreement is observed. >

Circular Quadruple-Ridged Flared Horn Achieving Near-Constant Beamwidth Over Multioctave Bandwidth: Design and Measurements

A circular quadruple-ridged flared horn achieving almost-constant beamwidth over 6:1 bandwidth is presented. This horn is the first demonstration of a wideband feed for radio telescopes which is capable of accommodating different reflector antenna optics, maintains almost constant gain and has excellent match. Measurements of stand-alone horn performance reveal excellent return loss performance as well as stable radiation patterns over 6:1 frequency range. Physical optics calculations predict an average of 69% aperture efficiency and 13 K antenna noise temperature with the horn installed on a radio telescope.

Subreflectarrays for Reflector Surface Distortion Compensation

With the increasing interest in the applications of large deployable reflector antennas operating at high frequencies, the requirement on the reflector surface accuracy becomes more demanding. Thermal effects inevitably cause certain reflector surface distortions, thus degrading the overall antenna performance. This paper introduces a novel reflector surface distortion compensation technique using a subreflectarray and presents detailed discussions. A microstrip reflectarray is used as a subreflector, illuminated by a primary feed. By properly adjusting the additional phase shift provided by the subreflectarray, the aperture phase errors caused by the main reflector surface distortions are compensated, resulting in a considerably improved antenna performance. As an example, a distorted 20-m offset parabolic reflector antenna operating at X-band is successfully compensated by a subreflectarray, and the simulation results are compared with those obtained by array feed and shaped subreflector compensation techniques. The microstrip subreflectarray is low-profile, lightweight, and cost-effective. Only one primary feed is required, and a reconfigurable design can be achieved if electronically reconfigurable reflectarray elements are adopted.

On numerical convergence of moment solutions of moderately thick wire antennas using sinusoidal basis functions

Wire antennas are solved using a moments solution where the method of subsectional basis is applied with both the expansion and testing functions being sinusoidal distributions. This allows not only a simplification of near-field terms but also the far-field expression of the radiated field from each segment, regardless of the length L . Using sinusoidal basis functions, the terms of the impedance matrix obtained become equivalent to the mutual impedances between the subsectional dipoles. These impedances are the familiar impedances found using the induced EMF method. In the induced EMF method an equivalent radius is usually used in the evaluation of the self-impedance term to reduce computation time. However, it is shown that only for very thin segments that the correct equivalent radius is independent of length. When the radius to length ratio ( a/L ) is not small, an expansion for the equivalent radius in terms of a/L is given for the self-impedance term. The use of incorrect self-term, obtained by using a constant equivalent radius term, is shown to be responsible for divergence of numerical solutions as the number of sections is increased. This occurrence is related to the ratio of a/L of the subsections and hence becomes a problem for moderately thick wire antennas even for a reasonably small number of segments per wavelength. Examples are given showing the convergence with the correct self-terms and the divergence when only a length independent equivalent radius is used. The converged solutions are also compared to Kings second- and third-order solutions for moderately thick dipoles.

Design and measurements of dual-polarized wideband constant-beamwidth quadruple-ridged flared horn

A quad-ridged, flared horn achieving nearly constant beamwidth and excellent return loss over a 6∶1 frequency bandwidth is presented. Radiation pattern measurements show excellent beamwidth stability from 2 to 12 GHz. Measured return loss is > 10 dB over the entire band and > 15 dB from 2.5 to 11 GHz. Using a custom physical optics code, system performance of a radio telescope is computed and predicted performance is average 70% aperture efficiency and 10 Kelvin of antenna noise temperature.