Keyvan Bahadori

A Miniaturized Elliptic-Card UWB Antenna With WLAN Band Rejection for Wireless Communications

An elliptic-card ultra-wideband (UWB) (3-11 GHz) planar antenna is designed and miniaturized. It consists of an elliptic radiating element and a rectangular ground plane. A novel feeding mechanism is proposed to feed the antenna by using a microstrip line on the other side of the substrate and connecting the line to the elliptic element by a via. The structure of the antenna is miniaturized by optimizing its elliptic profile and the required ground plane to obtain only 22 times 40 mm dimension. The antenna is then modified to possess band rejection at the wireless local area network (5.1-5.8 GHz) band by adding two slits within the elliptic element. Critical antenna characteristics are verified by measurements including the antenna transfer function. Housing effects on the antenna performance are also studied. The satisfactory overall performance with such a simple structure and small size makes this antenna a viable candidate for UWB wireless communications applications.

Advanced precipitation Radar antenna: array-fed offset membrane cylindrical reflector antenna

As part of the overall NASA earth science technology research effort, a half-size prototype model of light-weight, dual-frequency and wide-swath scanning antenna for the next generation of spaceborne precipitation radar has recently been developed. It operates radar channels at both 13.6 and 35 GHz for improved rainfall retrieval accuracy. The antenna for proposed spaceborne precipitation radar is an offset parabolic cylindrical reflector fed by two linear arrays. This design is adopted, instead of a double-curved offset reflector, because it provides the required wide scan angle in the cross-track plane at both the Ku and Ka band operating frequencies. To demonstrate the technological readiness of the concept, the focus is on a half-scale model of 2.65 m operating at Ku and Ka band. In this paper, the technology development status on this Advanced Precipitation Radar Antenna will be presented. Design and characterization of the 2.65 m antenna is detailed by presenting the features of various components of the antenna, including simulation of the performance, 2/spl times/166 Ku and 4/spl times/166 Ka element feed array designs and evaluation and the design concept of the membrane reflector. Measurement results of the entire antenna system using a compact range facility will also be summarized.

Characterization of effects of periodic and aperiodic surface distortions on membrane reflector antennas

The focus of this paper is to characterize the effects of periodic and aperiodic surface distortions on the performance of membrane reflector antennas. Since the surface of this class of reflector antennas is very thin, it is susceptible to various types of periodic and aperiodic distortions. The particular antenna dimensions used for this study are similar to the specifications for the JPL/UCLA half scale model of second generation precipitation radar (PR-2) mission reflector. Analytical expressions are introduced to model periodic and aperiodic surfaces and based on these models the effects of distortions on the radiation performance of the antenna are simulated. Aperiodic distortions are more realistic cases of distortions due to the fact that the period of the distortions is not constant through out the reflector surface. For each case, far-field patterns of the reflector are simulated and it is shown that closed-form expressions can then be derived which result in a very efficient computational method to predict some of the unique features of these patterns including location and level of observed grating lobes. Furthermore, based on spatial Fourier analysis of the surface distortion, it is shown that deviation from periodicity in the distortions of reflector surface results in lowering these grating lobes. Parametric studies have been performed to provide design guidelines for acceptable surface behavior for large deployable membrane reflector antennas for future space borne missions.

Tri-Mode Horn Feeds Revisited: Cross-Pol Reduction in Compact Offset Reflector Antennas

The design of a tri-mode horn feed with novel structure is revisited in this communication. This design allows a significant reduction in the cross-pol level in the far-field patterns of linearly polarized offset parabolic reflector antennas with low F/D. This is, in particular, advantageous in spaceborne applications where the space and weight is limited. It is demonstrated that TE21 mode in conical horns possesses a similar aperture field distribution as the focal plane field of an offset reflector antenna and can reduce the cross-pol level if the mode coefficients are optimized. The complex mode coefficients are optimized for a particular application of a 4.5 m offset reflector antenna operating at 10 GHz with F/D = 0.4. It is shown that the cross-pol level is reduced dramatically (~28 dB over 3% of bandwidth) when the reflector is fed by the tri-mode horn with optimum coefficients. The results suggest that this design could be very useful for future narrow band spaceborne remote sensing applications.

A Tri-mode Horn Feed for Gravitationally Balanced Back-to-Back Reflector Antennas

The concept of tri-mode feeds was used to lower the cross-polarization level in far-field pattern of offset parabolic reflector antennas. First, the complex mode coefficients of a tri-mode horn are optimized by using particle swarm optimization (PSO) technique. Using optimized complex mode coefficients, the performance of the reflector is simulated to verify the effectiveness of the method. The horn is then designed to provide the calculated optimum mode coefficients. The performance of the reflector is then simulated when it is fed by the designed horn and the results are compared with PSO optimized horn case. In addition, a simulation was performed to evaluate the bandwidth in which the horn can lower the level of cross polarization for at least 10 dB. The acceptable level of cross polarization verifies the possibility of utilizing these gravitationally balanced back-to-back reflectors fed by matched feed horns for narrowband applications in future spaceborne antenna missions requiring spinning platforms

An array-compensated spherical reflector antenna for a very large number of scanned beams

There are many stringent demands imposed on the applications of spaceborne antenna systems. One of the most challenging demands is the generation of multiple beams with the ability to scan a very large number of beamwidths. Since the parabolic reflectors have limitations in this application, a 35-m spherical reflector antenna is proposed for a geostationary radar antenna at Ka-band (35.6 GHz) due to its inherent capability of scanning the beams to very large number of beamwidths. The utility of using planar array feeds for correcting spherical phase aberrations is investigated to overcome the performance degradation effects. Two different methodologies are developed for the array excitation coefficients determination based on phase conjugate matching and the results are compared. Using the compensating feed array, the radiation characteristics of the compensated spherical reflector are simulated for no scan and large scan cases and the results are compared with the uncompensated case to show performance improvement. In order to demonstrate the technological readiness of the concept a 1.5-m breadboard model is designed to be built for experimental measurements. Some important mechanical design tolerances and realistic array feed topologies are investigated. The antenna concept developed in this paper is advocated to be used in the next generation of geostationary satellite antenna systems for remote sensing radar applications.

Back-to-Back Reflector Antennas With Reduced Moment of Inertia for Spacecraft Spinning Platforms

A back-to-back reflector antenna system with reduced moment of inertia is proposed in order to address the demanding problem of supporting large reflector antennas on spinning platforms. The configuration provides additional potential advantages, such as reducing the spinning speed by half for a given sampling rate when both back-to-back reflectors are utilized. Geometrical parameters of the reflector are determined such that the moment of inertia of the rotating system is reduced. It is shown that these back-to-back reflectors suffer from a high cross-pol level in the asymmetrical plane due to the large feed offset angle. Two different methods are explored to alleviate the high cross-pol level problem. In the first method, a sub reflector is utilized to minimize the cross-pol level by satisfying the Mizugutchi condition. In the second method, a tri-mode matched feed horn is suggested to achieve a similar result. The suppressed cross-pol level puts forward the gravitationally balanced back-to-back reflector antenna systems as a potential candidate for future spacecraft antennas on spinning platforms.

An elliptic-card UWB antenna for wireless communications

An elliptic card UWB planar antenna is designed and miniaturized in this paper. A novel feeding mechanism is proposed to feed the antenna by using a microstrip line on the other side of the substrate and connecting the line to the elliptical element by a via. The structure of the antenna is miniaturized to have minimum length. All antennas characteristics are verified by measurement results. These overall results propose the antenna to be a good candidate for UWB wireless communications.

Stacked Microstrip-Patch Arrays as Alternative Feeds for Spaceborne Reflector Antennas

The feasibility of using stacked microstrip-patch arrays as feeds for offset reflector antennas is investigated in this paper. It is shown that patch arrays can be used as alternatives to the conventionally used horn feeds, which tend to be bulky. In particular, patch arrays can be of interest for spacecraft applications where reduced size and light-weight feeds are highly desirable. In this paper, patch arrays were tailored to provide radiation characteristics similar to those of horn feeds by varying the element spacing and excitation. A reduction in weight was mainly realized by the planar construction of the patch arrays. A full-wave analysis of the feed array, using finite-difference time-domain (FDTD) and PO-based UCLA reflector-analysis codes, was used to test the results of the proposed feed, operating at 1.413 GHz for radiometer applications, and 1.26 GHz for radar applications. A dual-polarized and dual-frequency stacked microstrip-patch element was fabricated and tested. It was then demonstrated that a seven-element hexagonal array design seemed to be the best match to the horn feeds for a 12 m offset-reflector antenna.

Ku/Ka bands precipitation radar antenna: half-scale offset cylindrical reflector model [spaceborne]

Precipitation radar provides new and exciting data on 3D rain structures, for a variety of scientific uses. The second generation spaceborne precipitation radar is a proposed profiling radar that will enhance the capabilities aboard the TRMM satellite. The proposed radar system antenna is a 5.3 m offset parabolic cylindrical reflector antenna which has dual frequency operation at 13.6 GHz and 35 GHz. In order to demonstrate the technological readiness of the concept, this paper focusses on a half-scale model of 2.65 m operating at Ku and Ka bands, with an evaluation of the antenna feed and reflector antenna system.