  Broadband antenna feeds are of particular interest to existing and future
radio telescopes for multi-frequency studies of astronomical sources. Although
a 1:15 range in frequency is difficult to achieve, the well-known Eleven feed
design offers a relatively uniform response over such a range, and reasonably
well-matched responses in E & H planes. However, given the severe Radio
Frequency Interference in several bands over such wide spectral range, one
desires to selectively reject the corresponding bands. With this view, we have
explored the possibilities of having a multi-band feed antenna spanning a wide
frequency range, but which would have good response only in a number of
pre-selected (relatively) RFI-free windows (for a particular telescope-site).
The designs we have investigated use the basic configuration of pairs of
dipoles as in the Eleven feed, but use simple wire dipoles instead of folded
dipoles used in the latter. From our study of the two designs we have
investigated, we find that the design with feed-lines constructed using
co-axial lines shows good rejection in the unwanted parts of the spectrum and
control over the locations of resonant bands.
