Electrically Small Unidirectional Mixed-Multipole Antennas: Recent Advances and Their Applications

This paper reviews recent advances in and applications of electrically small unidirectional mixed-multipole antennas (ES-UMMAs), with a focus on notable improvements in bandwidth and directivity. The bandwidth enhancement methodologies applied to ES Huygens dipole antennas (HDAs), which are the lowest order UMMAs, are based on near-field resonant parasitic (NFRP) grid-shaped elements that act as electric dipoles and interdigitated capacitor (IDC) loaded loops that act as magnetic dipoles. Directivity enhancements include the realization of a superdirective ES Huygens quadrupole antenna (ES-HQA). These innovative dipole and quadrupole designs have been demonstrated experimentally; they achieve significantly improved performance characteristics over previously reported ES-HDAs. Their recent applications include integration with and direct impedance matching to complex circuits to realize rectennas for wireless power transfer (WPT) systems that exhibit superior AC-to-DC power conversion efficiencies. Examples include battery-less sensors such as an RFID epidermal skin temperature monitoring tag whose reading range is significantly beyond that of commercially available products. These developments enable ES-UMMAs as essential technologies for future generation (XG) compact and high-performance wireless systems, particularly for pioneering IoT and healthcare applications.