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This brief presents a wideband differential-fed filtering antenna with end-fire radiation characteristics. The proposed differential-fed end-fire filtering antenna consists of a $\lambda $ /4 stepped impedance slot, a dual-mode resonator, a pair of rectangular slots, a microstrip line, and a folded microstrip line. The $\lambda $ /4 stepped impedance slot resonates at its first two odd modes and exhibits end-fire radiation under differential ports excitation. Then, the dual-mode resonator is utilized to simultaneously broaden the impedance bandwidth and improve the out-of-band frequency selectivity of the gain curve. In order to improve the front-to-back ratio of the antenna, the rectangular slots with one open end are etched at the edges of the metallic ground. Finally, the multiple coupling paths are realized from the dual-mode resonator to the $\lambda $ /4 stepped impedance slot by adding the microstrip line and the folded microstrip line to generate two radiation nulls at the passband edges, and thus the frequency selectivity can be further enhanced. For the purpose of verification, a prototype of the proposed antenna is designed, fabricated and measured. The measured results are in good agreement with the simulated results, exhibiting an impedance bandwidth of 49.1% from 3.2 to 5.28 GHz, a flat in-band gain response with a maximum realized gain of 6.7 dBi, a front-to-back ratio over 11.5 dB within the entire operating passband. Besides, the suppression levels are higher than 18 dB and 29 dB for low and high frequency, respectively.

A low-profile single-layer half-mode substrate integrated waveguide (HMSIW) filtering antenna with compact circuit size is presented in this brief. The antenna features simple structure and consists of a half-mode substrate integrated waveguide cavity (HMSIWC), a shorted parasitic patch, and a folded defected ground structure (DGS) etched into the metallized ground plane. The half-mode technology is employed to reduce the overall dimension in the design. The shorted parasitic patch and the HMSIWC, both operating at half-mode states, are coupled with each other to form a second-order filtering response. And also a radiation null at higher stopband is introduced owing to the effects of the cross coupling. Furtherly, by etching a folded DGS into the metallized ground plane, an additional resonance in the passband and a controllable radiation null at lower stopband are simultaneously generated to broaden the impedance bandwidth and improve the selectivity of lower stopband respectively. For verification, a prototype of the developed HMSIW filtering antenna is designed, fabricated, and measured. The measured and simulated results are in good agreement, showing a wide impedance bandwidth of 9.2% at center frequency of 2.84 GHz, a flat in-band gain response with two controllable radiation nulls at both edges of the operating band, and a maximum boresight realized gain of 5.75 dBi. The proposed HMSIW filtering antenna demonstrates good in-band and out-of-band performance even with a very low profile of $0.015~\mathbf {\mathrm {\lambda }} _{0}$ .