Design and Analysis of Tri-Band Conformal Monopolar Antenna

A design approach of triband conformal monopolar antenna combining cavity mode and equivalent circuit (Ec) is proposed. First, based on mode analysis, two lower and closer resonant frequencies in the VHF-band are generated by introducing two slot rings and two circles of short vias to a circular parallel-plate structure. Second, its Ec is extracted and Idled. Then, the geometry structure is improved accordingly. An inverted truncate cone is introduced to modify the characteristic impedance of the monolithic radial transmission line to obtain an available band in the middle frequency (UHF). Finally, a high-frequency (L) antenna is embedded into the truncate cone and excited by a novel composite feeding structure. Furthermore, the conformal performance of the triband antenna is investigated. The conformal antenna prototype is fabricated and achieves a triband within 69.3-72.5 MHz, 317-348 MHz, and 1.38-1.71 GHz with a size of 0.42 lambda(0) x 0.42 lambda(0)x 0.014 lambda(0) (maximum wavelength). The realized gain in triband is -7.6 to -5.9, 4.7-5.4, and 4.7-5.9 dBi, respectively. For a 6 cm-thick FR-4 substrate combined with shorted metal columns, the triband conformal antenna has the potential on serving as a vehicle roof to bear the load, and it is attractive in vehicle communication too.