Low-Cost, Low-Loss, Ultra-Wideband Compact Feed for Interferometric Radio Telescopes

We have developed, manufactured, and tested a new feed design for interferometric radio telescopes with "large-N, small-D" designs. Such arrays require low-cost and low-complexity feeds for mass production on reasonable timescales and budgets, and also require those feeds to be compact to minimize obstruction of the dishes, along with having ultra-wide frequency bands of operation for most current and future science goals. The feed presented in this paper modifies the exponentially tapered slot antenna (Vivaldi) and quad-ridged flared horn antenna designs by having an oversized backshort, a novel method of maintaining a small size that is well-suited for deeper dishes (f/D <= 0.25). It is made of laser cut aluminum and printed circuit boards, such that it is inexpensive (less than or similar to 75 less than or similar to 75USD per feed in large-scale production) and quick to build; it has a 5:1 frequency ratio, and its size is approximately a third of its longest operating wavelength. We present the science and engineering constraints that went into design decisions, the development and optimization process, and the simulated performance. A version of this feed design was optimized and fabricated for the Canadian Hydrogen Observatory and Radio-transient Detector (CHORD) prototypes. When simulated on CHORD's very deep dishes (f/D=0.21) and with CHORD's custom first-stage amplifiers, the on-sky system temperature T(sys )of the complete receiving system from dish to digitizer remains below 30K over most of the 0.3-1.5GHz band, and maintains an aperture efficiency eta AA between 0.4 and 0.6. The entire receiving chain operates at ambient temperature. The feed is designed to slightly under-illuminate the CHORD dishes, in order to minimize coupling between array elements and spillover.