To solve the small frequency ratio of the frequency-reconfigurable antenna operating at the millimeter-wave band, a millimeter-wave dual-band frequency-reconfigurable filtering antenna is proposed. The proposed filtering antenna consists of a reconfigurable bandpass filter and an ultra-wideband double-layer Vivaldi antenna. The reconfigurable bandpass filter comprises of several components, including parallel coupled lines, stepped impedance resonator (SIR), PIN diodes, U-shaped branches, and bias network. The reconfigurable filter is integrated in the feedline of the Vivaldi antenna, which provides a simple structure and offers flexibility for further expansion. The reconfigurable characteristic is realized by controlling the electrical length of the open circuit stepped impedance resonator through two PIN diodes, which not only acts as a switch but also affects the impedance matching within the millimeter wave band. Firstly, the equivalent circuit model of the SIR loaded with PIN diode and bias network is analyzed and simulated to achieve dual-band reconfigurability in the Ka-band. The bias network consists of fan-shaped branches and high-impedance microstrip lines, which suppresses the flow of RF signals. Two notches are introduced by the two U-shaped branches, which are placed beside the parallel coupling line without affecting the performance of the reconfigurable filter. The realized two notches are located in the non-operating frequency band between the two reconfigurable bands, which enhances the out-of-band performances of the reconfigurable filter. Then, to suppress the unnecessary coupling effect and concentrate the energy on the feedline of the Vivaldi antenna, some metallized vias are loaded at the two sides of the feedline. Finally, a metal cavity is introduced to isolate the radiation and filtering components, and some metal columns are loaded inside the cavity for improving the self-resonance of the metal cavity, which effectively improves the cross-polarization level of the filtering antenna. The measured results show that the proposed antenna operates in the range from 25.9 to 28.6 GHz with a maximum gain of 8.83 dBi when the PIN diodes are in the ON state, and in the range from 32.6 to 35.9 GHz with a maximum gain of 9.97 dBi when the PIN diodes are in the OFF state. The center frequency ratio of two reconfigurable frequency bands reaches 1:1.26, and the cross-polarization levels are all less than -20 dB in both operating bands.
