Normal-Mode Helical Antennas With Near-Field Coupling: Improving Design Flexibility [Antenna Applications Corner]

摘要

The near-field coupling between closely placed metallic structures has been utilized to cope with the dilemma of a normal-mode helical antenna (NMHA) design. For small NMHAs, the impedance-matching difficulty is mitigated by designing appropriated coupling feeds, either electrically or magnetically. For long NMHAs, the radiation pattern distortion is rectified by dividing the complete helix into properly designed small sections. With mutual section coupling, the currents on all of the sections can be nearly in-phase, dictating an omnidirectional radiation pattern. The design principle has been described as twoelement models and a corresponding equivalent circuit analysis. The simulated and measured results show that, compared to conventional NMHAs, it is possible to design an NMHA of simultaneous good matching and radiation properties but with a much more flexible helix length and size.