S. B. Sharma

Improving the Cross-Polar Performance of an Offset Parabolic Reflector Antenna Using a Rectangular Matched Feed

This letter presents the improved cross-polar performance of a matched feed with an offset parabolic reflector antenna of low focal-length-to-diameter (F/D) ratio and higher offset angle. In the pyramidal horn, the higher order mode (TE11) has been added in correct amplitude and phase with respect to the dominant mode (TE01) . Parametric study on the secondary radiation pattern has been carried out for different values of offset angle. The numerical results have been presented on radiation pattern for both linearly polarized and circularly polarized rectangular matched feed. The design of a rectangular matched feed and its simulated secondary patterns have also been included.

Cross-Polarization Cancellation in an Offset Parabolic Reflector Antenna Using a Corrugated Matched Feed

This letter presents the design of a dual-mode corrugated matched feed to cancel the unwanted high cross polarization and beam squinting in an offset parabolic reflector antenna. The matched feed concept has been implemented in a cylindrical corrugated structure by adding a higher order HE21 mode to the fundamental HE11 mode. The designed feed was then used to illuminate a linearly, as well as a circularly, polarized offset parabolic reflector antenna, and the secondary radiation patterns were computed. Significant improvement in the cross polarization was observed when the results were compared to that of a conventional corrugated horn illuminated offset reflector antenna.

Performance comparison of a matched feed horn with a potter feed horn for an offset parabolic reflector

The present paper deals with the comparison of matched feed performance with that of the conventional potter horn when used with an offset parabolic reflector. The effect of TE21 mode amplitude on the cross polarization has been studied in order to ascertain the suitable TE21 modal amplitude. Followed this, the behavior of change of TE21 modal amplitude with offset angle (thetas0) has been investigated. The effect of cross-polarization variation as a function of (F/D) ratio has been studied for both the matched feed and the potter horn. Finally, the beam squinting phenomenon in case of circular polarization has been studied for the matched feed and the results were compared with that of the potter horn.

Historical and Planned Uses of Antenna Technology for Space-Borne Microwave Radiometers

This paper reviews the historical and future uses of antenna technology for space-borne microwave radiometers. The radiometer antennas are discussed, starting from the Dicke-type radiometer, as used in the Microwave Sounder Unit (MSU), to the present-day total-power radiometer. The antennas cater to the need for very high beam efficiencies, of the order of 95% to 98%. The paper also summarizes the specifications, design issues, and the performance of various radiometer antennas.

Cut-off wave number and dispersion characteristics of eccentric annular guide with dielectric support

The paper presents a rigorous analysis of a uniform waveguide with circular outer conductor and eccentric inner conductor supported by dielectric slab between the inner and outer conductors. A vector finite element approach in which non-physical spurious solutions do not appear has been followed to study the cut-off frequencies of higher-order modes as well as dispersion characteristics of this guiding structure. Numerical data on cut-off wave numbers and dispersion characteristics have been presented.

Square Coaxial Beam Forming Network for Multilayer Microstrip Antenna

This paper presents the design of square coaxial beam former and its transition from square coaxial line to microstrip line. Impedance matching has been achieved by creating a groove discontinuity in inner conductor. A cone has been provided for smooth impedance transformation for required broad banding. The measured performance of the beam former has been obtained by feeding it to feed layer of the electromagnetically coupled multilayer patch antenna. Equal and unequal power divider has been designed with proper impedance transformer by changing dimensions of the inner conductor keeping same outer conductor dimensions. Smooth linear taper has been provided at impedance transformer junction. High power calculations are also presented for SCL to handle several kW of power.

Design of a novel corrugated matched feed for an offset parabolic reflector antenna

The antennas for space-borne microwave radiometers have several specific requirements to achieve the specified radiometric accuracies, sensitivity and the desired performance of the radiometer in the space. These include high beam efficiency (≫ 95 %), high gain and low losses, low cross-polarization, low side-lobe levels, etc. [1]. The offset parabolic reflector antennas fulfills most of these requirements due to their inherent advantages of reduced aperture blockage, isolation between the reflector and the feed, lesser spurious radiation and suppressed side-lobes [2]. However, the performance of offset reflectors is satisfactory only when the larger F/D (F/D ≫1) configuration is selected. Generally, in spacecraft antennas, the available space for the antenna structure is limited, and hence the reflector with a large F/D may not be preferred. Alternatively, the use of an offset reflector antenna with a smaller F/D ratio results into high cross-polarization (when illuminated by linearly polarized feed), and beam squinting (when illuminated by a circularly polarized feed) [2]. This high cross-polarization and beam squinting degrade the overall performance of the system and restricts the use of offset reflectors for many practical applications. Thus, to overcome these limitations, an effective cross-pol suppression technique as well as beam squinting minimization technique are to be explored.

Tri-mode horn array fed multiple-beam offset reflector antenna with low cross-polarization and improved side lobes

In the present paper, the cross polarization reduction and improvement in the sidelobes have been discussed for a multiple beam offset parabolic reflector antenna illuminated by an array of uniformly excited tri-mode horns. The simulated secondary patterns have been obtained for a single tri-mode horn fed offset reflector and also for a tri-mode horn array fed multibeam offset reflector. The results are compared with a conventional potter horn and the significant improvement in the cross-polarization and sidelobes are observed.

Computation of cross-polarization radiated by an elliptical feed horn

Elliptical waveguides and horns find various applications, such as feeds for shaped reflector antennas [1–6], in microwave resonator [7], filter [8], polarizer [9], etc. As a radiating feed, the elliptical waveguide/horn can generate different beamwidths in two-orthogonal planes. Such feeds are preferred for space-borne reflector antennas to obtain an elliptical footprint on earth [4]. Generally, for radar, remote sensing as well as for satellite applications it is preferred that the overall cross-polarization in the secondary radiation pattern should be as small as possible. As, one of the sources of overall cross-polarization is the cross-polarization of the feed that illuminates the reflector antenna, it is worthwhile to first estimate the cross-polarization of the primary feed. In this context, the present paper is focused on the computation of cross-polarization radiated by an elliptical waveguide and the elliptical horn.

Analysis of focal region field of the offset-parabolic reflector for space-borne radiometers

The paper presents the analysis of the focal region field of an offset parabolic reflector antenna. The current induced on the reflector surface by an incident plane wave has been used to compute the field at the focal region. The analysis has been carried out for a reflector of diameter 1.8 meters by varying the f /d ratio and offset angle. The simulated results are presented. The validity of the analysis has been justified through comparison with the professional antenna design software.