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The X-Band All-Sky Survey (X-BASS) is a planned radio astronomy experiment aimed at mapping the sky in both temperature and polarisation. The main purpose of X-BASS is to aid observations of the cosmic microwave background (CMB) by removing various contaminating foreground emissions present in the galaxy, such as synchrotron and thermal dust emissions. When designing the antenna for such an instrument, one must rely on traditional figures of merit (FoMs) used to describe the performance of antenna radiation pattern characteristics. However, these FoMs often have a conflicting nature, the optimal trade-off of which may not be immediately apparent. What is more, traditional FoMs are usually developed from a telecommunications standpoint or determined heuristically from other radio astronomy projects with differing goals. Thus, the exact effect of antenna pattern characteristics on the end science goals of the experiment may be unclear. As such, this paper seeks to determine the limitations posed by antenna noise temperature, beamwidth and cross-polarisation on the end science goal of the experiment, which is the determination of various cosmological parameters through observation of the CMB. From this, a new FoM was developed which provides a single metric to be used in the optimisation of the antenna structure. This FoM was then used in the preliminary design of a corrugated horn antenna to feed the X-BASS reflector system.

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