Upper bounds on fixed-geometry wheeler cap efficiency measurements - Part I: System model and rectangular cavities

Antenna efficiency is a critical performance metric, but also difficult to determine accurately. Wheeler´s elegant idea is a simple, fast, repeatable and accurate technique of measuring small antenna efficiency inside shielded cavities. Natural cavity resonances violate the Wheeler cap principle, produce negative efficiency values, and reduce the measurable bandwidth (BW) and/or size of the antenna. Resonances can be avoided by using variable-geometry cavities; however, these require precise manufacturing to avoid energy leakage. Fixed-geometry cavities are easier to build and far more cost-efficient. This two-part study estimates the limits of dip-free efficiency measurements for planar antennas inside fixed-geometry caps. Part I firstly describes the joint antenna-cavity system model enabling the analyses. This is followed by a treatment of rectangular cavities, which display coherent measurable BWs in the range 15-33% for antenna sizes 0.5-0.78 times the wavelength. Finally, part I shows that the methodology can be readily extended to arbitrary antennas.