Solving the Efficiency-Dip Problem of Adding Parasitic Elements for Ultrathin Microstrip Antennas

As a common technique to broaden the bandwidth of ultrathin microstrip antennas, adding the parasitic elements often encounters the problem of efficiency dip. This phenomenon is due to the radiation cancellation caused by the out-of-phase currents between the parasitic element and the main radiator. This article proposes a solution by introducing another parasitic element near the initial one and enabling them to resonate in differential mode (DM), and the radiation cancellation at the efficiency dip can be significantly mitigated without attenuating bandwidth expansion. To illustrate this method, an ultrathin dual-polarized microstrip antenna is presented as an example. By designing a pair of L-shaped parasitic strips and converting the parasitic mode from common mode (CM) to DM, the in-band gain variation is reduced from 5.8 to 1.9 dB. The measured results show that the antenna operates with an impedance bandwidth of 2.3% in an extremely low profile of $0.008~\lambda _{L}$ ( $\lambda _{L}$ is the free-space wavelength at 7.9 GHz). The proposed method mitigates efficiency dips caused by the parasitic elements, exhibiting a feasible solution for bandwidth expansion of ultrathin microstrip antennas while holding the high efficiency.