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The Global Magneto-Ionic Medium Survey (GMIMS) was designed to study
polarized radio emission from our Galaxy and characterise the strength
and configuration of the Galactic magnetic field. The survey team
designed and built a feed and receiver covering the 300 to 900 MHz
band for the Parkes 64m radiotelescope in order to survey the sky
between declinations of -90 degrees and +20 degrees. The
specifications for the feed called for constant illumination of the
64-m reflector over the band 300 to 900 MHz, and detection of
left-hand and right-hand circular polarization. The Parkes Telescope
has a diameter of 64 m and a focal length of 26 m, with a focal ratio
f/D of 0.41, and an opening angle of the reflector as seen from the
focus of 126 degrees. The feed design was based on the Eleven Feed of
of Per-Simon Kildal, which consists of a pair of parallel half-wave
dipoles above a ground plane. Each dipole is expanded to become an
approximately log-periodic array of folded dipoles fed by a twin-wire
transmission line. The feed is shown above, supported above a ground
plane of dimensions 1.2×1.2 m. The survey is described in more detail
in
the paper by Wolleben et al. (2019)
in the Astronomical Journal.
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