A Compact Dual-Polarized Filtering Antenna with Steep Cut-Off for Base-Station Applications

A dual-polarized filtering antenna with steep cut-off and compact size is developed for base station applications. In this design, four controllable radiation nulls are obtained by utilizing split rings, slotted T-shaped branches, a single-stub tuner, and a parasitic loop. Split rings are firstly used as the dipole arms to obtain the 1st radiation null at upper out-of-band. Four T-shaped branches working as DGS are printed under the crossed dipoles to achieve the 2nd radiation null. The connected outer conductors of the differential feed structure acting as a single-stub tuner can provide the 3rd radiation null to further enhance the upper-band rejection. Finally, a parasitic loop is incorporated around the split rings, and the out-of-band rejection of the lower-band is further enhanced by the 4th radiation null. More importantly, the impedance bandwidth of the antenna can be expended with two newly introduced resonant modes. As a result, a compact filtering antenna with a wide operational bandwidth of 1.7- 3.01 GHz (56%) is realized for |Sdd11| < -15 dB with the isolation higher than 38 dB. The out-of-band suppression is higher than 18.4 dB in 3.1-4.5 GHz and more than 47 dB in 0.8-1.1 GHz.