Robust, electrically small, circular inverted-F antenna for energy-efficient wireless microsensors

The size of wireless microsensors is fundamentally limited by the bulk of the antenna and the energy pool; total energy efficiency and battery size are determined by total antenna efficiency. A printed circular inverted-F antenna (CIFA) is mirrored against a small, crescent-like ground plane (GNDP). The CIFA is printed on a discoid substrate that conforms to the element, the GNDP and the circumscribing sphere. It is an electrically small element designed with only three parameters: two angles and a radius. The CIFA-plus-GNDP radiator is resonant and matched at 2.5 GHz, where numerical results indicate that it achieves a 3.4% fractional bandwidth (FBW) combined with over 90% of efficiency, exhibits a near-isotropic gain pattern (8 dB deviation), and approaches Thal´s limit on minimum Q-factor within 43%. The prototype CIFA achieves the required half-radian size while scoring FBW = 3.1% (55% deviation from minimum Q) and 89.5-91.5% efficiency. Two problems pertinent to the design of small antennas for miniature portable devices are also discussed. First, an upper bound on achievable directivity is proposed, which facilitates quick assessment of antenna feasibility as a function of specified electrical size. Then, a forecast model for the prediction of minimum required operating frequency is developed based on the properties of the equivalent circuit at antenna input.