UBC Theses and Dissertations
Ultra-wideband nested coaxial waveguide feed antenna for radio astronomy Du, Xuan
Radio astronomy is becoming increasingly demanding in bandwidth. This thesis presents a new type of ultra-wideband feed antenna, the nested coaxial waveguide feed, for radio telescopes. The goal is to achieve a continuous wideband coverage by nesting octave-bandwidth coaxial feeds. The challenges of coaxial feeds, including aperture mismatch to free space and the narrowing of the radiation pattern with frequency, were studied using both an analytical model and numerical simulation. It was concluded that conventional matching and beam control techniques cannot be used for wideband applications. Aperture mismatch was solved by extending the use of an iris matching network from a 1.3:1 bandwidth to a 2:1 bandwidth. A new design method that combines electromagnetic simulation and a simpliﬁed circuit model was developed. The matching network also displays properties like a bandpass ﬁlter, which is very beneﬁcial to the nested feed design. Beam narrowing was solved by a novel multi-cavity choke at the waveguide opening. It expands the radiation aperture at low frequency and maintains its electric size across the band. A 2.2–4.4 GHz feed antenna was designed, built, and tested to experimentally verify the matching network and choke designs. Measurements agree with simulation extremely well. A feed antenna was designed for the Dominion Radio Astrophysical Observatory Synthesis Telescope and covers two closely spaced bands, 400–800 MHz and 900-1800 MHz, by using two waveguides. Iris matching networks were employed to improve aperture matching and to keep signals in the two waveguides apart to reduce coupling. A corrugated choke ﬂange was designed for the 4.5:1 bandwidth and was integrated with the outer waveguide. A novel PCB (printed circuit board) waveguide excitation structure was developed. The concept of the nested coaxial waveguide feed is presented from principle to prototypes. The presented feed design has low ohmic loss, low cross-polarization, a nearly constant beam shape at diﬀerent frequencies, and a compact and lightweight structure that is easy to build. It is competitive with other modern designs.
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