Ultrathin Multimode Microstrip Antenna Using Differential Feed for Smartphone Applications

This paper proposes an ultrathin multimode microstrip antenna with a stable broadside radiation pattern across the operating band. Different from previous works using gridded patches with a relatively high profile, the proposed antenna has a much lower thickness of only 0.4 mm (0.008 $\boldsymbol{\lambda}_{\mathbf{0}},\ \boldsymbol{\lambda}_{\mathbf{0}}$ is the free-space wavelength at 5.75 GHz). In this case, an undesirable monopole mode will be excited in-band using the conventional L-shaped probe, destroying the broadside pattern. Therefore, a $\boldsymbol{\pi}-\mathbf{shaped}$ probe with differential feed is used instead. The original antiphase- $\mathbf{TM}_{\mathbf{20}}$ and antiphase- $\mathbf{TM}_{\mathbf{02}}$ modes are preserved to enhance the bandwidth to 4.4%. This method can be also extended to the cases of dual-linear or circular polarization, showing potential applications in mobile terminals.