Wideband focal-plane array with improved scanning capabilities

April 17, 2023

Aleksei Dubok defended his PhD thesis at the department of Electrical Engineering on April 13th.

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In the past decades, focal-plane arrays (FPAs) have become an interesting alternative to conventional horn-fed reflector antennas in a number of applications, e.g. in radio astronomy and in Ka-band satellite communications. For his PhD thesis Aleksei investigated novel multi-beam wideband antenna systems with optical beamforming as part of the NWO FREEBEAM project: fiber to RF-FREEspace multiBEAM converter. The aim of the project has been to develop a hybrid system using an FPA that combines the benefits of phased-arrays and traditional reflector-based solutions.

In addition, a novel optical interface to the FPA was used, which provides an ultra-wideband interface and a compact versatile solution enabling fast beam steering. The main applications are radio astronomy, satellite communications, satellite TV, broadband internet via satellite, 5G/6G point-to-point wireless communications, and low-cost Ka-band (30-40 GHz) multi-function radar. For the next generation of radio astronomy, FPAs offer a high sensitivity in combination with a large field of view (FoV) at a relatively low price.

Increasement simultaneously active elements

One of the fundamental limitations of FPAs are the small number of simultaneously active array elements limiting the achievable effective isotropic radiated power (EIRP). A low EIRP level limits the use of low-cost silicon integrated circuits. In addition, it is an issue to realize wide-angle scanning, which is related to the challenge to provide a proper exposure of the FPA even for relatively small scan angles of the main beam, due to the significant beam deviation in the focal plane. Moreover, there are bandwidth limitations related to mutual coupling between array elements. Therefore, in this work various reflector configurations have been investigated in order to improve the FPA illumination and increase the number of simultaneously active elements, during beam scanning over a wide instantaneous bandwidth. At the same time, array designs with improved active impedance matching performance and reflector illumination have been investigated and realized including the integration with a low noise amplifier (LNA).

New concept for optimal array illumination

A new concept for optimal array illumination by the reflector has been proposed. The idea is based on increasing the number of active elements of the FPA in order to overcome the main limitation of conventional FPA systems in which only a very small region in the focal plane is illuminated and the necessity to achieve a uniform distribution of power and phase along the array elements, which is required for optical beamforming. This allows to increase the achievable EIRP and the system sensitivity.

We have developed reflector optimization software based on geometrical optics which was used to achieve one of the main project challenges: wide angle scanning with a reflector up to ± 20 deg in the azimuth plane. Different systems of single and double reflector configurations have been optimized for this purpose and verified with commercial software. The final double-reflector configuration achieves the required scanning range with a compact array of which almost half of the array elements are active during scanning. The scanning capabilities have been significantly improved as compared to classical prime focus and double-parabolic reflectors. The phase linearity between the array elements has been sufficiently improved in order to use the optical beamforming.

Full capability of the wideband antenna and reflector

Consequently, co-design of arrays and reflectors has been investigated in the frame of this project in order to use the full capability of the wideband antenna and wide-scan range reflector and to avoid integration problems that could result in a limited active frequency band during FPA scanning. As a result, the proposed wideband array of modified bow-tie antennas with the complex double-reflector provides high radiation efficiency and shows an active frequency bandwidth of more than one octave (20 - 40 GHz) within ±20 deg azimuth scan. Two types of high-efficient array configurations have been implemented in a 3D EM simulation software tool, namely the connected array and a conventional one. Those results have been successfully confirmed by several experimental validations in the near-field test facility of Eindhoven University of Technology.

Title of PhD thesis: Wideband focal-plane array with improved scanning capabilities

Supervisors: Bart Smolders and Giampiero Gerini.

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