A Modular Approach for the Design of Quadruple Ridged Flared Horn Antenna Feeds

This paper presents the methodology followed for the design of a quad-ridged flared horn (QRFH) feed for the VLBI Global Observing System (VGOS) antennas. The study demonstrates the potential undesired effects caused by the presence of non-fundamental modes at the throat of the horn antenna, and how they can be avoided following some design considerations. The approach consists of a separate design of the feeding transition and the flared section of the horn, and it can be extrapolated to other implementations. Firstly, the geometry of the coaxial to quad-ridged waveguide (QRWG) transition is defined. The proposed design technique ensures negligible propagation of higher order modes at the waveguide interface. Secondly, the ridges and sidewall profiles of the flared part are optimized to achieve the desired performance in terms of input reflection and aperture efficiency on the VGOS 13.2-meter displaced-axis two-reflector radio telescope. Following the optimized design, five QRFH antenna units have been manufactured and tested, all showing reflection coefficients better than -10 dB and predicted aperture efficiencies better than 55 % in the band from 2.2 to 14 GHz. The excellent agreement between simulations and experimental results demonstrates the validity of the proposed methodology.