A wideband omnidirectional dipole antenna with parasitic loop based on multi-resonant modes

ABSTRACT A wideband omnidirectional dipole antenna with parasitic loop based on multi-resonant modes is proposed for WLAN, 5 G Sub-6 GHz, and RFID applications. The antenna is composed of a regular dipole with co-planar strips and a parasitic loop with four U-shaped branches. A regular dipole with co-planar strips is used to generate two resonant modes, and the parasitic loop around it generates another two resonant modes. By combining these four resonant modes, a wideband characteristic is achieved. In order to reduce the cross-polarization, four U-shaped branches are introduced, and then the cross-polarization levels are decreased from −2 dB to −13 dB. A prototype with a compact size of 0.6λ0 ×0.56λ0 ×0.012λ0 (λ0 is the wavelength in free space at center frequency) is fabricated and tested. Measurement shows that effective bandwidth is 1.8–5.6 GHz (102.7%), the measured gain variation is only 1.3 dBi, and omnidirectional radiation patterns are achieved over the operating band.