Low-profile antennas based on aperiodic and compact meta-surfaces

(eng) Artificial magnetic ground planes enable the placement of horizontal antennas close to the ground plane. However, their behaviour as AMCs (Artificial Magnetic Conductor) is valid only over a small frequency band. Hence, their use in wideband antenna applications remains a challenge. The antenna-AMC combination can be optimized to maximize the impedance bandwidth, thanks to the coupling between the antenna and the AMC. However, we have observed that the radiation patterns split at broadside when we work close to the AMC resonance frequency. This limits the useful operation bandwidth, i.e. the bandwidth at which good radiation and matching conditions are achieved at the same time. In this thesis, a new non-periodic structure acting as an AMC and as a leaky-wave surface at the same time has been studied and designed to recover the broadside directivity near and beyond the AMC resonance. An antenna with broadside directivity better than 9.2 dB over a 32.65% impedance bandwidth is finally presented. This thesis also focuses on the design of a low-profile RFID tag able to work on metallic objects. We have proved that including an AMC ground plane in the tag design avoids the tag antenna to be short-circuited when it is mounted on a metallic plane. But having a compact AMC is essential to obtain a competitive RFID tag. A new AMC surface of 42.6 mm x 42.6 mm size has been designed. It supports an inductively-fed meander dipole that provides the complex input impedance required to activate the microchip terminating the tag. A prototype has been fabricated in our laboratory to measure its input impedance. Very encouraging preliminary results have been obtained. The inductive coupling effect achieved with the feeding loop of the meander dipole is preserved in presence of the AMC ground plane.