Quad-mode antenna for sparse L-band tiles

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Since 2013 the MIDPREP staff exchange programme has strengthened collaborations between ASTRON and the South African Universities of Cape Town, Rhodes and Stellenbosch. As part of the MIDPREP staff exchange scheme fellowship, David Prinsloo from Stellenbosch University spent six weeks at ASTRON this summer working toward a sparse demonstrator array for the mid-frequency band of the Square Kilometre Array telescope.

This work presents an antenna design for sparse arrays that utilises multiple orthogonal port excitation modes propagating in multi-conductor transmission lines in order to extend the field-of-view coverage of the array. The antenna illustrated in the figure - referred to as a quad-mode antenna - achieves hemispherical field-of-view coverage by integrating and co-locating two perpendicular bow-tie dipoles, four tapered slot antenna elements, and a conical monopole section into a single antenna excited through a quadraxial transmission line.

Indicated alongside the quad-mode antenna, are the four orthogonal port excitation modes supported by the quadraxial transmission line - shown together with the respective far-field radiation pattern excited by each mode. Implemented in an array environment these four excitation modes can be used, collectively, to maximise the performance in terms of gain, sensitivity, or polarisation discrimination at each scan angle over a hemispherical field-of-view.

In the present phase of the work the design of a tile of quad-mode antennas is underway. Once completed, this tile will serve as the base element in a sparse, irregular station proposed for the mid-frequency band of the Square Kilometre Array telescope.