Broadening the Bandwidth of Electrically Small Antennas using Multi-Mode Synthesis

Traditional broadband antenna designs with compact radiating elements often explores the multiple resonant modes existing in the designed physical structure to obtain more bandwidth, or say to decrease the quality factor. It is well-known that antenna arrays could support multiple radiation modes. Recently, the modes in a coupled antenna system have been explored for MIMO and for beam-forming applications in a compact radiation platform. The underlying physical principle is taking the advantage of the collocated and orthogonal radiation pattern modes. Those modes in a compact radiation space could be isolated with each other after the mode-decomposition network. To maximize the bandwidth of the mode-decoupled antenna array, a systematic approach is proposed to enlarge the bandwidth electrically small antennas of the broadband array. Different from traditional antenna array design whose objective is usually to combine the antenna gain, the approach proposed here is to synthesize a broader bandwidth in the desired frequency range either in a few discrete band or one continuous band. The approach can be directly applied to electrically small antennas to achieve the maximum bandwidth available out of a small form factor platform. Electrically small antennas often utilize wire structures to explore lower quality factor and hence higher bandwidth. It has also been discussed that folded-arm spherical helical antennas exhibit an omni-directional, linearly polarized radiation pattern consistent with that of an electrical dipole.