This letter presents a high-performance filtering antenna with enhanced out-of-band suppression, high selectivity, and a simple structure. The antenna is composed of a radiating patch on the top surface of the dielectric substrate and parasitic stubs on the underside of the substrate. By utilizing a tripleresonance mode, bandwidth is significantly enhanced. The lowfrequency resonance mode is excited by etching a pair of symmetric L-shaped slots in the radiating patch. The midfrequency resonance mode corresponds to the fundamental mode of the main radiating patch. The high-frequency resonance mode is generated by introducing a quarter-wavelength open-circuit rectangular stub beneath the dielectric substrate. Additionally, these two structures introduce radiation nulls at the edges of the low- and high-frequency bands, respectively. Additional Tshaped and U-shaped stubs improve band selectivity with radiation nulls at specific edges. The proposed filtering antenna has been constructed and tested. Testing reveals a -10 dB impedance bandwidth of 17.7% (2.26–2.7 GHz), 8.14 dBi peak gain, the frequency selectivity exceeds 252 dB/GHz and an out-of-band gain below -10.5 dB.

A widely tunable bandpass filter adopting a novel fully tunable resonator is proposed in this letter. The fully tunable resonator is mainly consisted of an open ring, a frequency tuning structure, and a coupling tuning structure. Adopting the fully resonators, the center frequency, the input/output external Q value, and the coupling coefficients can be tuned at the same time. And the tuning range of a tunable filter adopting the novel fully tunable resonators can be expanded. The fully tunable resonators can be combined to achieve more excellent suppression performance. A widely tunable bandpass filter consisted of five fully tunable resonators with 86.5% frequency tuning range is designed, fabricated, and measured. The simulated results are in good agreement with the measured ones.

This letter proposes a multi-passband half-mode substrate integrated waveguide (HMSIW) filter based on the theory of odd and even mode analysis. The filter adopts a triangular HMSIW cavity cut along the diagonal of the rectangle. By etching two dual-mode resonators, the resonant mode of the HMSIW resonator is coupled with the odd-even mode of the dual-mode resonator to achieve multiple passbands. The defected ground structure (DGS) of the filter can reduce the resonance frequency of the HMSIW cavity without increasing the volume of the HMSIW cavity, making it easier to couple with the odd and even mode frequencies of the resonator. The input and output ports are directly coupled through a microstrip line. In this way, it adds an additional coupling path to the filter, which increases the out-of-band suppression without changing the performance in the passband, and improves the overall performance of the filter. To prove the feasibility of the above method, a multi-passband HMSIW filter was fabricated and tested. The center frequencies of the three passbands of the filter are 2.98 GHz, 4.78 GHz, and 6.62 GHz, respectively. The return loss in the passband is better than −15 dB, and the insertion loss is better than 2 dB. The measured results have a good agreement with the simulation results.

This letter presents a novel tunable coupling structure to simplify the design complexity of the miniaturized fully tunable filter by using open ring and varactors. Based on the proposed novel tunable coupling structure, a fully tunable bandpass filter is implemented with independently tunable operating frequency and bandwidth. The tunable resonator and tunable coupling structure can be easily combined to improve Out-of-band suppression performance. The design procedure of a fully tunable bandpass filter consists of five tunable cavities and tunable coupling rings is also proposed. A prototype of the proposed fully tunable filter is built to verify the design experimentally. Good agreement between simulated and measured results is obtained.