In this letter, a compact, low-profile, bandwidth-enhanced, dual-cavity substrate integrated waveguide (SIW) filtenna is demonstrated. Two SIW cavities are stacked vertically on top of each other. A complementary split-ring resonator slot is etched in the top surface of the uppermost cavity, causing the top surface to act as a patch antenna. The operational impedance bandwidth is significantly enhanced by merging the three resonances that arise from this configuration. One is introduced by the patch, and the other two are inherently generated by the two cavities. A metallized coupling post is introduced from the ground plane through both cavities to the upper surface to excite the fundamental resonant mode of the patch, as well as to electromagnetically couple the two cavities. The optimized filtenna was fabricated by a standard printed circuit board technology and tested. It has a low profile ${\text{0.03}}\lambda _{0}$ and a compact size ${\text{0.62}}\,\lambda _{0}\,\times {\text{0.62}}\,\lambda _{0}$ at its center frequency, $f_{0}= {\text{2.95}}\,{\text{ GHz}}$ . The measured results agree well with their simulated values. They demonstrate a 6.3% fractional bandwidth, a maximum realized gain of 6.73 dBi, a flat gain profile within its passband, and an excellent out-of-band selectivity.

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Compact, Low-Profile, Bandwidth-Enhanced Substrate Integrated Waveguide Filtenna

A novel dual-band microstrip bandstop filter with multiple poles using open/shorted coupled lines is proposed in this paper. Nine transmission poles near the two stopbands can be easily realized by conventional coupled lines and transmission lines. The two center frequencies of the dual-band bandstop filter can be adjusted flexibly by only changing the even/odd-mode of the coupled lines. The planar microstrip dual-band bandstop filter ( $\varepsilon r = 2.65$ and $h = 0.5$ mm) with nine transmission poles is designed and fabricated. High selectivity and good in-band performances can be achieved in the proposed filter.

Dual-Band Microstrip Bandstop Filter With Multiple Transmission Poles Using Coupled Lines

Quasi-lumped-element low-pass filter (LPF) and high-pass filter (HPF) based on substrate-integrated suspended line (SISL) technology are presented in this paper. Double interdigital capacitor, which has advantages of high capacitance density and high quality factor, is analyzed and designed to minimize the filter size. Based on the theoretical analysis, quasi-lumped-element SISL LPF and HPF prototypes are designed. The experimental results show that the SISL LPF achieves ultra-wide stopband up to 24 $\text{f}_{c}$ ( $\text{f}_{c}$ is the cut-off frequency) with compact size of $0.180~\lambda _{g} \times 0.082~\lambda _{g}$ ( $\lambda _{g}$ is the guide wavelength at $\text{f}_{c})$ . The SISL HPF features the low insertion loss of only 0.3 dB, while the size is only $0.108~\lambda _{g} \times 0.085~\lambda _{g}$ . Moreover, the methods of substrate excavation and packaging optimization are proposed to further enhance the performance of SISL LPF and SISL HPF. By doing so, the stopband of SISL LPF is further extended to 32 fc, and the insertion loss of SISL HPF is significantly reduced to 0.1 dB.

Quasi-Lumped-Element Filter Based on Substrate-Integrated Suspended Line Technology

A novel coupled-line stub is proposed for the design of a novel dual-band small-size bandstop filter (BSF). This kind of coupled-line dual-band BSF features a compact and simple structure, flexible frequency-ratio adjustment and high selectivity. To verify the proposed circuit structure and design approach, a microstrip example has been fabricated and measured. The measured results show a good agreement with the simulated responses.

Small-size high-selectivity bandstop filter with coupled-line stubs for dual-band applications

This article presents a simple design of a coaxial dual-band cavity filter and diplexer using stub-loaded resonators (SLRs). Based on the resonance characteristics of a coaxial SLR, the first two resonant frequencies can be tuned over a wide range by adjusting their structural parameters. Controllable external quality factors can be acquired by using the modified tapping structure for the two designated passbands. A coupling window and a coupling pin are proposed to obtain the expected coupling coefficients in the two bands. To demonstrate this concept, a third-order dual-band bandpass filter and an eight-pole diplexer are designed and fabricated. The measured results agree well with the full-wave simulation results.

Dual-Band Coaxial Filter and Diplexer Using Stub-Loaded Resonators

Compact microstrip low-pass filters (LPFs) with ultrawide stopband based on a defected ground structure (DGS) and their design procedures are presented in this paper. A key merit of the filter configuration is that the selectivity of the LPFs can be conveniently controlled, whereas the bandwidth is fixed. The LPFs are realized using open-circuited uniform impedance stubs and stepped-impedance stubs (SISs). The position of the transmission zeros can be freely controlled by tuning the impedance ratio of the SISs. To expand the stopband, the dumbbell DGS is used to suppress the spurious passbands of the LPFs. Two demonstration filters with a cutoff frequency at 1 GHz have been designed, fabricated, and measured. The measured results indicate good performance: broad stopband ( $>15 f_{c}$ or $20~f_{c}$ ), low passband insertion loss (<0.3 dB), compact size, and sharp skirt characteristic.

High-Selectivity Low-Pass Filters With Ultrawide Stopband Based on Defected Ground Structures

In this letter, stepped-impedance resonators are used in coaxial cavity filter to generate dual-band response. Two key merits of the proposed filter are the helical feeding structure and the introduction of mixed coupling for coupling control and generation of transmission zero. The helical feeding structure makes it possible to conveniently control the external quality factors of two bands. To illustrate the concept, a third-order dual-band coaxial cavity filter is designed, fabricated and measured. The measured results are in good agreement with the full-wave simulation results.

Dual-Band Coaxial Cavity Bandpass Filter With Helical Feeding Structure and Mixed Coupling

Dual-band bandstop filters using open-circuited stub-loaded resonators are presented. The first and second resonant frequencies of the open-circuited stub-loaded resonator can be flexibly controlled by tuning the length and position of the open-circuited stub. To illustrate the concept, a dual-band bandstop filter has been designed, fabricated and measured. The prototype of the dual-band bandstop filter achieved attenuation of 42 and 44 dB at 2.4 and 5.2 GHz.

Dual-band bandstop filter using stub-loaded resonators with sharp rejection characteristic

This letter proposes a lowpass-bandpass triplexer consisting of one lowpass channel and two bandpass channels. Stepped-impedance stubs are used in the lowpass filter (LPF) design, which bring a sharp selectivity. Defected ground structures are utilized to increase the impedance of one transmission line in the lowpass section, giving improved performance. High Isolation is achieved by careful consideration of the input signal splitting structure. To illustrate the concepts, a lowpass-bandpass triplexer is designed, fabricated and measured. The cut-off frequency of the LPF is chosen to be 1 GHz. The central passband frequencies are selected to be 2.4 and 5.8 GHz. Simulated and measured results are found to be in good agreement with each other. Isolations among the three bands are better than 40 dB.

Design of Microstrip Lowpass-Bandpass Triplexer With High Isolation

A novel wideband bandstop filter is proposed in this paper. Using one uniform impedance resonator and two stepped impedance resonators, five transmission zeros are introduced in the stopband. Appropriate positioning of these zeros ensures the high selectivity and flat rejection level of the stopband. The bandwidth of the filter can be conveniently controlled by the characteristic impedances of the connecting lines and resonators. A complete analytic analysis is proposed to fully control the positions of transmission zeros, and the design equations and design rules for the bandstop filter are given in detail by using the lossless transmission line model. To illustrate the concept, a wideband bandstop filter with center frequency at 2 GHz and rejection level below 20 dB is fabricated. Theoretical, simulated, and measured results are found in good agreement with each other, and the measured 20-dB rejection bandwidth is as wide as 150%.

Jie-Ming Qiu

Papers

High-Selectivity Low-Pass Filters With Ultrawide Stopband Based on Defected Ground Structures

Dual-Band Coaxial Cavity Bandpass Filter With Helical Feeding Structure and Mixed Coupling

Dual-band bandstop filter using stub-loaded resonators with sharp rejection characteristic

Design of Microstrip Lowpass-Bandpass Triplexer With High Isolation

Sharp-Rejection Wideband Bandstop Filter Using Stepped Impedance Resonators