Development of the Eleven Antenna - A Decade Bandwidth Feed for Reflector Antennas

Licentiate thesis, 2005

Recently there has been a growing interest in reducing the number of antennas on different platforms by combining several functions in the same antenna. To achieve this, the multifunction antenna needs to be either very broadband or multi band to cover the frequencies of all functions. In some proposed future wideband communication systems there is also need for very wideband antennas. The term ultra wide band (UWB) is often used in connection with new broadband technology. Furthermore, there is increasing interest in antennas with extremely large bandwidth in connection with proposed scientific systems such as e.g. the so-called Square Kilometer Array (SKA), a radio telescope covering frequencies from 100 MHz to 25 GHz. The present thesis relates mainly to the development of a novel dual polarized UWB feed with more than a decade bandwidth for use in single or dual reflector antennas in the US proposal for SKA. The new feed is referred to as the Eleven antenna because its basic geometry is two parallel dipoles (11 configuration) separated by 0.5 wavelengths and located above a ground plane, and because it has nearly constant beam width and 11 dBi directivity over more than a decade bandwidth (and 11 > 10). The feed gives an aperture efficiency of the reflector of 66% or better over the same bandwidth when the subtended angle towards the sub or main reflector is about 53 deg, and an overall efficiency better than 47% including mismatch. The return loss is better than 6 dB over the same bandwidth and the phase center of the feed does not change with frequency. The calculated results have been verified with measurements on several lab models, both linearly and dually polarized. Furthermore, a dual polarized feed covering 150 MHz to 1700 MHz has been designed, manufactured and delivered to MIT Lincoln Labs for use in the 43 m Radio Telescope in Green Bank. The radiation efficiency (due to ohmic losses) of that feed has been both computed and measured and it is better than -0.4 dB across most of the band. The feed has no balun as it is intended to be integrated with an active 180 deg balun and receiver or to be used with a wideband 180 degree hybrid. We also believe that 11 dB return loss is achievable in the near future. The Eleven feed represents a major breakthrough in UWB antenna technology in terms of both performance and size. Competing feed solutions are about 10 times larger and has worse performance.