A Frequency-Reconfigurable Dual-Band Low-Profile Monopolar Antenna

Abstract

A concept of a low-profile monopolar antenna operating in two independently reconfigurable frequency bands is introduced in this paper. For the lower band, the design utilizes a center-fed patch with shorting rods at its edges, forcing the patch to radiate as an equivalent magnetic-current loop. For the upper band, another magnetic current loop is created by adding four symmetrical resonant slots on the patch, which radiates almost independently of the shorted patch. To allow coverage of a larger bandwidth, two sets of varactor diodes are used to independently control the resonance frequencies of the two bands. As a validation of the proposed concept, two antenna prototypes, with and without reconfigurabilities, have been optimized and fabricated. Measurement results show that the low-profile reconfigurable monopolar antenna achieves two independent tunable bands, with −10-dB-tuning ranges of 31% and 22% centered at about 0.9 and 1.7 GHz, respectively. The antenna height is only $0.020\lambda _{\text {min}}$ , where $\lambda _{\text {min}}$ is the free-space wavelength at the minimum operating frequency. Importantly, stable omnidirectional patterns and vertical polarization are consistently achieved across both tuning ranges.