Compact K/Ka-band antenna feed for multi-beam satellite communications

Objective(s): The goal of this project has been the design, manufacturing and testing, a compact K/Ka-band two-port feed breadboard for optimum electrical performance with a simple and compact geometry. A novel feed has been explored, composed of four elements: diplexer, single-mode waveguide, bi-phase polarizer and horn antenna. It presents the advantages of wide-band, low axial ratio, high isolation, compactness and modular design.

Background and justification: Next satellite systems operate in the 18.2-20.2 GHz and 28-30 GHz bands, with very stringent mechanical constraints, especially the footprint dimension. In these systems, the prior solution for user-type feeds was based on a dual-band ortho-mode transducer plus polarizer, which requires better bandwidth and axial ratio. Another solution, typically bulky and cumbersome, is to use a feed chain based on six-port junctions with matched loads at the two non-used ports. This project has addressed a different alternative based on bi-phase polarizers. Achievements and status: a) The design of the antenna feed has involved three components shown in the figure: •Diplexer with a well-known configuration based on a low-pass and a high-pass filter, for low insertion loss. •Bi-phase polarizer: integrated by a mode-launcher and bi-phase shifter with folded stubs for maximum compactness. •Horn antenna with optimized profile. Depending on its performance, it can be also integrated on the design process b) The design has led to a breadboard feed manufactured in Aluminum in two parts for the bi-phase polarizer and the diplexer. c) Finally, the breadboard feed has been tested in both S-parameters and radiation. Return loss and isolation was close to the simulations. More discrepancies were obtained for the cross-polar, which is much more sensitive to geometrical tolerances. Benefits: This configuration provides great compactness with enhanced electrical characteristics: bandwidth, return loss, axial ratio, and isolation. The proposed antenna feed, unlike other alternatives, does not need a careful alignment between some of its components, thereby providing great versatility to the antenna structure, which can be used to reduce their size and complexity. Nevertheless, in this activity a more classic inline configuration has been used, minimizing the length and the transversal envelope.