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.

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

Phased array feeds (PAFs) for large diameter single dish radio telescopes offer the capability for multiple simultaneous beam formation and wide field-of-view surveys An L-band PAF front end design is presented for the Five-hundred-meter Aperture Spherical Telescope (FAST), which is currently under construction and when complete will be the largest single dish radio telescope in the world A broadband PAF element at L-band was designed in conjunction with the FAST optics and focal plane field distribution, fabricated, and tested The simulated performance of the array meets the FAST PAF performance requirements for field of view, efficiency, sensitivity, and other key parameters A unique aspect of the FAST PAF is the need for feed cabin vibration compensation A study of the use of adaptive beamforming to compensate for lateral movement is given

Design Study of an L-Band Phased Array Feed for Wide-Field Surveys and Vibration Compensation on FAST

This research presents the design and main electromagnetic characteristics of the novel compact microwave waveguide polarizer. The developed device's structure is based on a circular waveguide and anti-phase conducting posts located in it. The application of a circular waveguide instead of a square one allowed simple integration of a polarizer with corrugated horn antennas. The obtaining of required phase difference using a posts-based design provides the possibility of fine tuning of the developed circular waveguide polarizer after its manufacturing process. In addition, simultaneous adaptive operation at orthogonal circular and linear polarizations is available in the antenna system including a polarizer with posts. An automatic control of posts height must be applied in a waveguide in order to provide this option. Numerical simulation, optimization, manufacture and measurements of the polarizer's characteristics were carried out in the operating band 7.4-8.5 GHz. Measured characteristics proved that the suggested circular waveguide polarizer provides good matching and polarization performance. Developed circular waveguide polarizer can be integrated in corrugated horn antenna systems and applied in modern weather radars and satellite information systems.

/pdf/circular-waveguide-polarizer-for-weather-radars-and-jum0s3c51b.pdf

Circular Waveguide Polarizer for Weather Radars and Satellite Information Systems

This paper presents the design and development of a multifrequency probe-coupled orthomode transducer (OMT) using a circular waveguide as the primary waveguide and a rectangular waveguide as the secondary waveguide. Design is presented for a common OMT operating at 6.6, 10.65, 18, and 21 GHz using four cascaded circular waveguide sections with different cross-sectional dimensions. An innovative design technique is used to minimize the inter-port coupling and to maximize the power in the dominant mode to get the required radiation performance at all the frequency bands using a common radiating aperture. The simulated and measured parameters of the OMT and the horn fed by this OMT have been presented.

Multifrequency waveguide orthomode transducer

Conjugate-matched primary feeds for offset reflector antennas are reviewed and investigated for both x- and y-polarized cases. Tapered dual-mode circular waveguide feeds are analytically modeled to support a ${\rm {TE}}_{11}$ -type mode with unequal beamwidths and the ${\rm {TE}}_{21}$ -type mode for the two orthogonal polarizations. First, required mode content factors and tapering numbers of the ${\rm {TE}}_{11}$ mode are addressed for the y-polarized case in offset reflectors with different focal length-to-diameter ratios. Then, the model is appropriately tailored for the x-polarized primary feeds. It is shown that when the polarization orientation changes, the corresponding beamwidths of the ${\rm {TE}}_{11}$ mode should be swapped in the principal E- and H-planes. This is particularly important to simultaneously realize low cross-polarization levels at both asymmetry and diagonal planes, when a dual-polarized matched feed is utilized in offset reflector antennas.

Analytical Models of Dual-Polarized Primary Matched Feeds for Offset Reflector Antennas With Low Cross-Polarization Properties at Both Asymmetry and Diagonal Planes

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.

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

The design and development of a tri-mode conjugate matched feed, to suppress the unwanted high cross-polarisation and beam squinting in an offset parabolic reflector antenna, are presented. In a smooth-walled cylindrical waveguide, TE 11 , TM 11 and TE 21 modes are combined in proper amplitude and phase to configure a tri-mode conjugate matched feed. The performance of the proposed tri-mode horn was evaluated, and its simulated far-field radiation pattern is found to be in close agreement with the measured radiation pattern. This horn was then used as a primary feed device to illuminate the linearly polarised offset parabolic reflector antenna, and significant reduction in the cross-polarisation has been achieved in the secondary radiation pattern. Similarly, by illuminating the offset reflector with a circularly polarised matched feed, a squint-free radiation pattern has been achieved.

Design and development of a conjugate matched feed for an offset parabolic reflector antenna

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.

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

This paper presents the analysis of scattering characteristics of a radially directed thin post in a circular waveguide using the method of moments. Electric type dyadic Green's function has been used to compute the scattered field in a circular waveguide considering variation of current only in the axial direction of the post. Simulated data on return loss and power coupling in the dominant mode and higher order modes have been compared with the data computed using Ansoft's High Frequency Structure Simulator. The data on return loss and coupling for the incident TE11 mode have been compared with experimental data.

V. K. Singh

Papers

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

Multifrequency waveguide orthomode transducer

Design and development of a conjugate matched feed for an offset parabolic reflector antenna

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

Analysis of a Post Discontinuity in an Oversized Circular Waveguide