It is well known that the bandwidth for a planar inverted-F antenna (PIFA) changes as the ground plane size changes. To gain insight into what causes bandwidth fluctuations, a process for applying characteristic mode theory to the finite ground plane and feed structure was developed. Four different PIFA designs are then evaluated to show how the modal significance of certain modes on the finite ground plane relate to the bandwidth minima and maxima for each PIFA. Next, finite ground planes are altered using the gained insight to enlarge the bandwidth for an antenna with a fixed maximum ground plane size. The goal of this work is to use the developed bandwidth analysis technique to inform the synthesis of PIFAs that require broader bandwidths.
Bounds on the frequency domain behavior of electrically small antennas are adapted to assess the time domain distortion or fidelity achievable by simple linear time-invariant (LTI) systems. Rigorous expressions for a TM01 spherical shell are used as a direct analog to the well known Chu limit on an antenna's bandwidth-efficiency product. These expressions are shown to agree with results obtained using narrowband single resonance approximations, allowing for the analysis of arbitrary small dipole-like structures through bounds on their single frequency Q-factor and efficiency. The resulting expressions are used as a basis for assessing the performance gains of electrically small non-LTI (e.g., direct antenna modulation) transmitters for which Q-factor and impedance bandwidth are not defined but which can be analyzed directly in the time domain via distortion.
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