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