A Method to Design Trapped-Mode Free Quadruple-Ridged Flared Horn Antennas

A quadruple-ridged flared horn (QRFH) antenna is typically fed asymmetrically by the centre conductor of a coaxial transmission line. Both the desired odd and undesired even modes are excited at the feeding junction. The coupling to the even mode is normally much lower than to the odd mode, but this is a function of the asymmetry of the structure, exacerbated by mechanical manufacturing and assembly tolerances. If the even mode is trapped along the length of the horn, resonances may be observed in the operating frequency band. Given that these trapped mode resonances occur over a very narrow frequency band, determining them with a computational electromagnetic solver requires a fine frequency sweep over the entire frequency band. Computationally this requires an enormous effort and thus cannot be done for each iteration during optimization. Trapped modes are, however, easily predictable given the cut-off frequencies of the horn’s waveguide structure. With a little initial effort, a surrogate model can be constructed of the cut-off frequencies for all possible waveguide geometries. A geometric feasibility parameter is then included in the optimization, indicating the possibility of trapped modes (and possibly other geometric constraints) before any computational electromagnetic solver is invoked. The infeasible geometries are removed from the computation domain. The technique is illustrated by an all-metal QRFH design for the ngVLA optics.