Abstract
An iterative alternate projection-based algorithm is developed to design structured surface reflectors to operate as beam splitters at GHz and THz frequencies. To validate the method, a surface profile is determined to achieve a reflector at 610 GHz that generates four equal-intensity beams towards desired directions of ±12.6° with respect to the specular reflection axis. A prototype is fabricated and the beam splitter behavior is experimentally demonstrated. Measurements confirm a good agreement (within 1%) with computer simulations using Feko, validating the method. The beam splitter at 610 GHz has a measured efficiency of 78% under oblique incidence illumination that ensures a similar intensity between the four reflected beams (variation of about 1%).
Highlights
Heterodyne receivers [1] are an essential part of radio-astronomy
The observed sky signal, called radio frequency (RF) signal, is mixed with an artificial monochromatic signal created by a local oscillator (LO) in order to shift the frequency of the sky signal to a much lower frequency without losing any amplitude or frequency information (Fig. 1)
Phase gratings [3] are the perfect tool for achieving this goal and are already used in some receivers (e.g. CHAMP, SMART, upGREAT [4,5,6])
Summary
Heterodyne receivers [1] are an essential part of radio-astronomy. They can achieve very high spectral resolution and are well suited to observe molecular and atomic transition lines, from which physical and chemical conditions of the interstellar medium can be determined, as well as its kinematics. Arrays of heterodyne receivers are being developed to simultaneously measure spectra at several positions in the sky In this configuration, each pixel includes a mixer. Stepped phase gratings, such as Dammann gratings [7,8,9,10], exhibit discrete level step profiles, while Fourier gratings are generated by Fourier series and exhibit a continuous geometry [11,12,13] Both gratings are periodic and have a constrained geometry, which limits the beam patterns that can be efficiently achieved. An iterative design algorithm that takes into account the dipolar behavior of the equivalent magnetic current radiation was developed This ensures the four generated beams to have identical power density under oblique incidence beam illumination while reaching a high overall efficiency in the THz domain. The concept has been proved with a reflector that is designed and fabricated, and for which both simulation and measurement results confirm its operation at 610 GHz
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