Variable beamwidth dual reflector antenna

Boresight error (BSE), the angular difference between a target's actual and radar-indicated position, is influenced by protective radomes used on airborne platforms. Research results have demonstrated a reliable computer modeling technique for predicting the BSE of electrically large radar-radome systems. This technique, based on a ray-trace receive formulation using geometric optics (GO), was extended to investigate the dependance of radome-induced BSE on various combinations of aperture scan angle, element polarization, and incident wave polarization. The results obtained compare very well with available empirical, published, and measured data for the specific scan angles and polarization cases considered. Generally, the BSE exhibits less dependance on incident wave polarization when the aperture elements are linearly polarized and a higher degree of sensitivity when aperture elements are circularly polarized.

Circularly symmetric, dual-reflector, high-gain antenna systems often require feeds placed off the system's axis because of the need for multiple feeds to use the reflector antenna. Also, the constraint requiring the hyperboloid or shaped subreflector to remain circularly symmetric is sometimes added. In a Cassegrainian system, the subreflector and feed may be rotated off-axis around the paraboloid focus and retain main reflector focusing. However, substantial spillover results in considerable noise in a high-gain/low-noise temperature system. In a shaped system, the tilt of the shaped subreflector and feed together results in substantial defocusing as well as spillover noise. If the subreflector is tilted approximately one-half the angle of the feed tilt in either the Cassegrainian or the dual-shaped reflector antenna, it is found that spillover and noise are substantially reduced with tolerable defocusing. An extensive numerical analysis of these effects was conducted to determine the characteristics of a 70-meter, dual-shaped reflector versus Cassegrainian antenna and to gain some understanding of the cause of the observed effects.

Recent terrorist attacks and possibilities of such actions in future have forced to develop security systems for critical infrastructures that embrace sensors technologies and technical organization of systems. The used till now perimeter protection of stationary objects, based on construction of a ring with two-zone fencing, visual cameras with illumination are efficiently displaced by the systems of the multisensor technology that consists of: visible technology – day/night cameras registering optical contrast of a scene, thermal technology – cheap bolometric cameras recording thermal contrast of a scene and active ground radars – microwave and millimetre wavelengths that record and detect reflected radiation. Merging of these three different technologies into one system requires methodology for selection of technical conditions of installation and parameters of sensors. This procedure enables us to construct a system with correlated range, resolution, field of view and object identification. Important technical problem connected with the multispectral system is its software, which helps couple the radar with the cameras. This software can be used for automatic focusing of cameras, automatic guiding cameras to an object detected by the radar, tracking of the object and localization of the object on the digital map as well as target identification and alerting. Based on “plug and play” architecture, this system provides unmatched flexibility and simplistic integration of sensors and devices in TCP/IP networks. Using a graphical user interface it is possible to control sensors and monitor streaming video and other data over the network, visualize the results of data fusion process and obtain detailed information about detected intruders over a digital map. System provide high-level applications and operator workload reduction with features such as sensor to sensor cueing from detection devices, automatic e-mail notification and alarm triggering. The paper presents a structure and some elements of critical infrastructure protection solution which is based on a modular multisensor security system. System description is focused mainly on methodology of selection of sensors parameters. The results of the tests in real conditions are also presented.

In preparation for the future AMSU-B mission, measurements of clear-air radiative transfer at 89 and 157 GHz were made using a radiometer operating on the C-130 aircraft of the UK Meteorological Research Flight. Observations of water vapour and oxygen absorption in arctic, middle latitude and tropical atmospheres were obtained. Four different empirical models of continuum absorption are compared and tested; three are found to be more representative of the observations in middle-latitude conditions. In the tropical Atlantic, large model deficits are found for these models. The fourth is more representative of observations in the tropics but over-estimates absorption at middle and arctic latitudes. The results confirm observations taken during other atmospheric field experiments, and laboratory measurements, that a model absorption deficit at high humidity exists in the microwave and millimetre wavelength regions.

For a small earth station, a detailed comparative study of three types of dual offset reflector antenna is presented. These are the shaped dual-reflector antenna, the sector-shaped paraboloidal reflector antenna and the spherical offset reflector antenna. The efficiency, the radiation patterns and the complexity of fabrication are some of the aspects considered.

The design, construction and measured performance is described of an offset parabolic reflector antenna which employs a reflectarray subreflector to tilt the focused beam from the boresight direction at 94 GHz. An analysis technique based on the method of moments (MoM) is used to design the dual-reflector antenna. Numerical simulations were employed to demonstrate that the high gain pattern of the antenna can be tilted to a predetermined angle by introducing a progressive phase shift across the aperture of the reflectarray. Experimental validation of the approach was made by constructing a 28 times 28 element patch reflectarray which was designed to deflect the beam 5deg from the boresight direction in the azimuth plane. The array was printed on a 115 mum thick metal backed quartz wafer and the radiation patterns of the dual reflector antenna were measured from 92.6-95.5 GHz. The experimental results are used to validate the analysis technique by comparing the radiation patterns and the reduction in the peak gain due to beam deflection from the boresight direction. Moreover the results demonstrate that this design concept can be developed further to create an electronically scanned dual reflector antenna by using a tunable reflectarray subreflector.

Large apertures are of great benefit to applications that are prime powered limited as is found on aerostat and other airborne platforms. Electronically scanned array antennas are often proposed for these applications. However, increasing the aperture area with conventional array technology is met with prohibitive cost, weight, and prime power increases because of the dense spacing of phase shifters and/or active T/R modules. This paper discusses the recent development of RF MEMS (microelectromechanical system) switch technology and the use of these switches in a Radant lens configuration for arrays of approximately 10m2 at X-band. A proof-of-concept 0.4m2 MEMS electronically steerable antenna (ESA) containing 25,000 MEMS switches has been successfully designed fabricated and tested. The 0.4m2 MEMS ESA was then integrated with an AN/APG-67 radar system to form the MEMS demonstration radar system. The MEMS Demonstration Radar system successfully detected both airborne and ground moving targets during a series of extensive radar demonstrations. This is believed to be the first large scale employment of MEMS switches in a scanning antenna and radar system. The low-cost, lightweight and low power technology demonstrated here can enable weight and power constrained platforms with electronic steering.

A design method for an offset-fed, dual reflector antenna (Cassegrain type or Gregorian type) system with an axisymmetric main reflector is presented. Geometrical optics (GO) and the geometrical theory of diffraction (GTD) are used to find the surface-current density on the main reflector. A modified Jacobi-Bessel series (JBS) method is used to find the far-field pattern for the physical optics (PO) integral. In the defocused mode of operation, a new technique is developed to find the reflection point on the subreflector corresponding to the defocused feed and a general field point on the main reflector. Two sample systems are designed.

A simultaneous X-band and wide-band Ka-band horn is designed for reflector antenna satellite communication applications. The theoretical performance of a 2.4m offset parabolic reflector, a 13.5m shaped Cassegrain reflector and a compact 1.3m dual reflector antenna are calculated as a proof of concept.

Dual reflector antennas are considered as pencil beam antennas that can produce radiation identical to searchlight.Cassegrain Reflector Antenna Design consists of various effects caused by blockage by primary feed or by the subreflector and its effect on overall performance.The objective of the paper is to provide the overview of the designing approach that is used to design the axially symmetric cassegrain antenna.This paper also provides the reader the overview of the various challenges and limitation that are faced by the designer while designing the axially symmetric dual reflector cassegrain system.

In some applications construction of reflector antennas require assembling smaller panels and conforming them to reflector surfaces, a process, which naturally leave gaps between the panels. Consequently, characterization of performance from these gaps, though small in terms of wavelength, becomes an important issue in antenna analysis and design. In this paper, the dual reflector antenna performance is quantified in terms of gain reduction, sidelobe structures, surface currents, etc., in order to visualize the effects of gaps on the antenna patterns.

Active reflectors are often used to compensate the surface distortion caused by environmental factors that degrade the electromagnetic performance of large high-frequency reflector antennas. This is crucial for maintaining high gain operation in antennas. A distortion compensation method for the active reflector of a large dual-reflector antenna is proposed. A relationship is established between the surface deformation and the optical path difference for the primary reflector by geometric optics. Subsequently, employing finite element analysis, a polynomial fitting approach is used to describe the impact of adjusting points on the reflector surface based on the coordinates of each node. By standardizing the positions of various panels on the reflector, the fitting ns can be applied to the reflector panels of similar shapes. Then, based on the distribution characteristics of the primary reflector panels, the adjustment equation for the actuators is derived by the influence matrix method. It can be used to determine the adjustment amount of actuators to reduce the rms of the optical path difference. And, the least squares method is employed to resolve the matrix equation. The example of a 110 m aperture dual-reflector antenna is carried out by finite element analysis and the proposed method. The results show that the optical path difference is reduced significantly at various elevation cases, which indicates that the proposed method is effective.

This paper presents advanced antenna system suitable for multi-band communications satellites that could simultaneously support multiple services such as direct broadcast, Ka broadband and military communications. A multi-aperture reflector antenna system that could generate multiple spot beams simultaneously over five discrete frequency bands that are spread over more than an octave band is presented. A smooth-wall high efficiency multi-mode horn that operates over 12.3 GHz to 30.0 GHz has been developed. Results of the multi-beam antenna system supporting uplink and downlink frequencies for forward DBS, reverse DBS, and Ka broadband services are discussed.

This work considers the synthesis and analysis of dual reflector antennas designed to radiate omnirectional radiation patterns. The reflectors are circularly symmetric concentric surfaces. Here, the case of the main reflector obtained from a circular generating curve is considered. By applying Geometrical Optics approximations, The main reflector is kept fixed and the subreflector is shaped to produce different tilt angle of the main lobe radiation pattern over the horizon line, considering a uniform phase distribution on a conical aperture placed in front of the main reflector. Depending on the type of ray mapping between the reflectors, two kinds of configurations are identified. By employing the aperture method, the performance of these antennas is predicted and a full wave analysis based on Method of Moments analysis is employed to validate these results.

A distributed charge transfer model has been developed to analyze solid oxide fuel cells and electrolyzers operating in H2-H2O and CO-CO2 atmospheres. The model couples mass transport based on the dusty-gas model, ion and electron transport in terms of charged species electrochemical potentials, and electrochemical reactions defined by Butler-Volmer kinetics. The model is validated by comparison to published experimental data, particularly cell polarization curves for both fuel cell and electrolyzer operation. Parametric studies have been performed to compare the effects of microstructure on the performance of SOFCs and SOECs operating in H2-H2O and CO-CO2 gas streams. Compared to the H2-H2O system, the power density of the CO-CO2 system shows a greater sensitivity to porosity and tortuosity. Analyses of the effects of the pore diameter suggest the H2-H2O and CO-CO2 systems are affected by changes in pore diameter in a similar manner. However, the concentration losses of the CO-CO2 system are significantly higher than those of the H2-H2O system for the pore sizes analyzed. While both systems can be shown to improve in performance with higher porosity, lower tortuosity, and larger pore sizes the results of these parametric studies imply that CO-CO2 systems would benefit more from such microstructural changes. These results further suggest that objectives for tailoring microstructure in solid oxide cells operating in CO-CO2 are distinct from objectives for more common H2-focused systems.