Title:
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Study of ground plane architectures for wideband spiral antennas
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The key objective of this study was to investigate and develop various types of ground plane
architectures for wideband planar spiral antennas. A two-arm, four-turn Archimedean spiral, which is
designed to operate between 3- 10 GHz, and five new and innovative cavity arrangements have
been studied using CST Microwave Studio® software. Each of the structures is used as means to
provide transformation from bidirectional to unidirectional radiation and simultaneously increase the
gain of the antenna, whilst exhibiting a good impedance match at the operating frequencies.
A simple flat metal plate is modified to reduce modal contamination by inserting slots carefully
arranged in a radial pattern to disrupt the current that flows on the surface of the reflector. More
complex stepped ground plane and FSS cavity arrangements are designed to operate in conjunction
with the spiral over the frequency range 3-10 GHz without the need to mechanically reposition the
reflector to optimize the performance at each frequency. The stepped cavity is composed of eight
metal rings each positioned λ/4 below the corresponding active region of the spiral, whilst the latter
arrangement consists of a two-layer FSS and a metal plate which reflect signals in the upper (7- 10
GHz) and lower (3-6 GHz) frequency band, respectively. Two High Impedance Surfaces (HIS)
designs are studied and shown to provide a more compact cavity compared to the previous
arrangements; 1) selective loading by placing dissimilar HIS below the 3 and 6 GHz active region of
the spiral, and 2) a uniformly distributed multi-resonant HIS for a tri-band WLAN antenna. Simulated
and measured radiation patterns and key performance metrics are shown to be in good agreement
for all five ground plane architectures. Following this, the results of a pilot study are presented to
demonstrate that a reconfigurable HIS ground plane based on electronically tunable liquid crystals,
can provide either a monopulse Sum (∑) or a Difference (∆) shaped beam by dynamically switching
the permittivity of the tunable substrate between two states
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