A frequency reconfigurable antenna with dual-band operation is presented. The antenna has a circular radiating patch loaded with an annular slot, and the slotted patch is shorted to the ground plane with four conducting posts. The antenna has three feed ports. Two of the ports are used to excite the slot mode resonant at a lower frequency, and broadside radiation with dual orthogonal linear polarizations can be obtained. The other port is used to excite the monopolar-patch mode resonant at a higher frequency, and conical radiation with vertical polarization can be yielded. To reconfigure the operation frequencies, four varactors are symmetrically placed across the annular slot. The simulated results indicate that the resonant frequency of the slot mode can be tuned from 1.62 to 1.17 GHz when the capacitance of the varactors is varied from 0.6 to 1.8 pF; besides, for each capacitance value, the impedance bandwidth of the antenna operating in the monopolar-patch mode can cover the frequencies from 2.4 to 2.5 GHz. Experiments are also carried out to validate the simulated data.

A compact antenna with tri-polarization switching is presented. The antenna is mainly composed of a monopole slot and a shorted loop, and the two radiating elements are connected in series and placed in a metallic box filled with dielectrics. To realize polarization switching, four PIN diodes are introduced into the antenna structure, and they cannot only suppress the excitation of the loop antenna but also vary the direction of the current flowing on the loop. Both simulated and measured results indicate that the proposed antenna can be operated in one linear polarization and dual circular polarization modes within an impedance bandwidth of around 15%; moreover, the aperture size of the antenna is about 0.09 λ 0 2 . Based on the polarization reconfigurable antenna, a 1 Â 8 phased array with an aperture size of 0.3 λ 0 Â 2.8 λ 0 is then developed. Simulated results show that the phased array operating in each polarization mode can give a gain fluctuation of smaller than 1.5 dB and a side lobe level of less than À10 dB when the beam is scanned from À60 to 60 .Experiments are also carried out, and their results have good agreements with the simulated data.