Chong-Hu Cheng

In this article, three ultracompact single-/dual-band folded substrate integrated waveguide (FSIW) filters using multiple embedded hybrid resonant modes are proposed. As a novel approach, multiple planar hybrid modes are embedded in the closed FSIW to construct multiple-hybrid-resonator units in the single cavity, which exhibit significant advantages, including ultrasmall size, high unloaded quality factor, and controllable modes. For the single-band fourth-order filter, two LC modes are embedded in dual-mode FSIW featuring uncoupled striplines (SLs) to achieve the ultraminiaturized hybrid-mode resonator. Moreover, embedded LC modes are cross-coupled with FSIW modes to generate a wide stopband and two transmission zeros near the passband. Furthermore, two SL resonators with magnetic coupling and three SL resonators with cross-coupling are integrated into the aforementioned four-hybrid-resonator unit to realize ultracompact, large frequency radio, and wide stopband dual-band fourth-/second-order and fourth-/third-order FSIW filters. Finally, one single-band and two dual-band filters are designed at 4.52 GHz (f₀), 4.51/8.18 GHz (f₁/f₂), and 4.56/8.89 GHz (f₁/f₂), and their upper stopband suppressions can reach 20 dB up to 2.82f₀, 18 dB up to 3.01f₁, and 20 dB up to 3.05f₁. They occupy only 0.095$\uplambda_{\mathrm{g}}^{{2}}$, 0.097$\uplambda _{\mathrm{g}}^{{2}}$, and 0.103$\uplambda _{\mathrm{g}}^{{2}}$, where $\uplambda_{\mathrm{g}}$ is the guided wavelength at f₀ or f₁.

This article introduces a new synthesis method for broadband continuously tunable phase shifters (PSs). In contrast to traditional reflective-type PSs (RTPSs) based on a 3-dB quadrature coupler, whose phase response is determined solely by the reflection load, we find that the even-and odd-mode impedances $Z_{{0e}}$ and $Z_{{0o}}$ and the electrical length $\Theta $ of the coupled line can adjust the phase properties of the tunable PS, including the phase shift and phase slope. Taking both the coupled line and reflection load into account, the proposed method can reduce the phase slope variation with respect to $C_{{v}}$ by more than 1.6 $\sim$ 4.6 times compared to conventional RTPSs. Without introducing extra circuits or designing complex reflection loads, the bandwidth and phase error performance of tunable PSs can be significantly improved. The transmission-line (TL) model of the coupled line with the prescribed fractional bandwidth (FBW) and return loss (RL) is developed to synthesize the proposed broadband continuously tunable PS with all circuit parameters, thus allowing for a quick design process. Finally, two PSs are designed, fabricated, and tested to verify the proposed design method and predicted frequency responses.

In this article, an analytical approach to diagnose coupling matrix, extract circuit model, and predict radiation for linearly polarized N th-order filtering patch antennas from the $S_{11}$ measurement is proposed and developed. In contrast to existing reported approaches, the proposed method does not require extra EM-simulation ports on the radiator or radiation measurement data. All coupling values and circuit parameters of an N th-order filtering patch antenna can be extracted with a single round of full-wave simulation or measurement. The proposed analytical approach can rigorously support lossless cases and is also effective for practical filtering patch antennas designed with commonly used copper conductors and low-loss substrates. It is suitable for both filtering patch antennas consisting of single/multiple single-mode radiators (SMRs) and the multimode radiator (MMR). An analytical method based on the diagnosed coupling matrix is proposed to predict the radiation performance of the filtering patch antenna, including the radiation pattern and the absolute value of the realized gain. In this context, the extraction procedure and closed-form formulas are presented in detail. To validate the proposed approach, three practical examples are provided: a stacked filtering patch antenna consisting of two SMRs, a filtering patch antenna with a dual-mode MMR, and a filtering patch antenna with a triple-mode MMR. The simulated and measured results demonstrate the effectiveness of the proposed approach.

In this brief, a new compact filtering power divider (FPD) with high isolation utilizing embedded capacitive isolation network (CIN) is proposed, which is based on the quadruple-mode folded substrate integrated waveguide (FSIW) resonator. The dual-mode stepped-impedance stub-loaded resonator (SISLR) is the first embedded in the closed dual-mode FSIW to achieve the quadruple-mode resonator with reduced size. In addition, the CIN containing two interdigital capacitors is embedded in two hybrid resonators, which greatly improves the isolation of output ports as well as further effectively reduces the size of the FPD. In final, a prototype with 3-dB fractional bandwidth of 15.2% is designed and fabricated at 4.98 GHz (f0), and its insertion loss is 1.47 dB. The isolation and upper-stopband rejection are better than 25.2 dB, and 20 dB extended to 1.97f0, respectively. It occupies only 0.26 λg2, where λg is the guided wavelength at f0.

In this brief, a compact diplexer on folded substrate integrated waveguide (FSIW)-LC resonator with high selectivity and narrow channel spacing is proposed. For Channel I/II, two LC modes are embedded in the dual-mode FSIW (TE101 and TE201) to achieve the miniaturized quadruple-mode resonator, and these two LC modes are cross-coupled with TE101 and TE201 to generate two transmission zeros near the passband. Therefore, the high selective fourth-order filtering response can be designed. In addition, the embedded stepped-impedance-resonator (SIR) isolation network is applied to improve input/output matching and output isolation, while further miniaturizing the diplexer. In final, the proposed diplexer with 3-dB fractional bandwidths of 14% (Channel I) and 12.6% (Channel II) is designed at 4.84 GHz (Channel I @ f1) and 5.86 GHz (Channel II @ f2). The shape factors, upper-stopband rejection and isolation are 1.49 / 1.28 (Channel I / Channel II), 24 dB extended to 2.49f1 and 33.4 dB, respectively. It occupies only 0.28 λg2, where λg is the guided wavelength at f1.

The wideband GaAs-based integrated passive device (IPD) filter with high selectivity and reduced size is proposed by utilizing single through via (TV) inductive coupling. One series and two parallel dual-mode π-type units containing two inductors and three capacitors are used to design the wideband fourth-order filter with two transmission zeros (TZs). Then, by combining eight TVs in three π-type units into only one TV, three different inductive coupling transmission paths can be generated between input and output ports, resulting in the proposed filter that can achieve five TZs near the passband, then, the higher selectivity and the lower / upper stopband suppression up to 23 dB / 32 dB can be achieved. Besides, size of the proposed filter layout with one TV is approximately 21% smaller than that of the conventional filter layout with eight TVs. In final, a prototype with 3-dB fractional bandwidth of 79.3% is designed and fabricated at 3.67 GHz ( f ₀ ). Its shape factor defined as 20-dB bandwidth / 3-dB bandwidth is only 1.25. This IPD filter occupies only 0.011 λ ₀ × 0.017 λ ₀ , where λ ₀ is the free-space wavelength at f ₀ .

In this letter, a new miniaturized wide-stopband filter with high-frequency selectivity based on the single-cavity quadruple-mode hybrid resonator is proposed. The modified patch with loading slot and metal via is first embedded in the folded substrate integrated waveguide (FSIW) to design the single-cavity quadruple-mode resonator (TM $_{100}$ , LC, TE $_{101}$ , and TE $_{201})$ . Meanwhile, this embedded patch reduces TE $_{101}$ and TE $_{201}$ , while other high-order FSIW modes keep unchanged, so miniaturized size and wide stopband can be realized easily. Additional transmission zeros near the passband can improve the frequency selectivity by the cross coupling between FSIW modes and two other modes (TM $_{100}$ and LC). In final, a prototype with 3-dB fractional bandwidth (FBW) of 11.2% is designed and fabricated at 5.94 GHz ( $f_{0})$ . The upper stopband of this filter is extended to 13.3 GHz (2.24 $f_{0})$ with a rejection level of 24 dB. It occupies only 0.165 $\lambda ^2_g$ , where $\lambda _{g}$ is the guided wavelength at $f_{0}$ .

In this article, a polarization rotation transmissive surface (PRTS) of arbitrary angle based on a three-dimensional (3-D) frequency selective structure is proposed. The PRTS unit cell is composed of three parallel-coupled slotline sections with open-short ends etched on vertically and horizontally inserted singlelayer printed circuit boards (PCBs). Under the incidence of vertically polarized waves, the spatial waves are converted into guided waves by the open-circuited end of one slotline section, and then the guided waves are coupled to two other slotline sections that can act as two independent coupling transmission paths in the orthogonal planes. This can be observed as a power divider, and the power division ratio can be fully controlled by virtue of the coupling strengths of the two coupling transmission paths. Thus, the polarization of incidence waves can be rotated at an arbitrary angle at the output port. To simplify the design procedure, a filtering power division theory and an equivalent circuit model are first used to obtain the theoretical response. Subsequently, a coupling matrix is employed to synthesize and design the proposed PRTS. To confirm the effectiveness of the proposed PRTS, two examples with different polarization rotation angles and fractional bandwidths at the same center frequency of 5 GHz and return loss of 20 dB are designed, fabricated, and measured. The measured results confirm our theoretical predictions and agree with the simulated results, thus demonstrating that the proposed PRTS exhibits stable frequency response under a large oblique incidence, in addition to their easiness in fabrication.

In this article, a second-order three-dimensional bandpass polarization-rotating surface (PRS) that can rotate vertically polarized incident waves by 90° and facilitate the output of horizontally polarized waves is presented. First, each PRS element is constructed using a pair of parallel-coupled slotline sections with open-short-ends, which are etched on vertically and horizontally inserted single-layer printed circuit boards, respectively. Then, under vertically polarized incident waves, one of the slotline sections can convert the incident spatial waves into guided waves through its open end. After which, the guided waves are coupled to the other slotline section and the electric field direction will be rotated by 90° through the coupling path, thus leading to a 90° polarization rotation. Subsequently, a direct coupling matrix is used to synthesize and design the proposed PRS based on the microwave filter theory. Investigations reveal that a desirable bandpass filtering response can be obtained by properly designing the internal coupling of the parallel-coupled slotline sections and the external coupling between the incident spatial waves and the PRS. To validate the design concept, two examples with different fractional bandwidths at the same center frequency of 5 GHz and the same return loss of 20 dB were designed, fabricated, and measured. Consequently, a good agreement was attained among the results from coupling matrices, full-wave electromagnetic simulations, and measurements. All the results demonstrate that the proposed PRSs feature simple topologies, easy fabrications, low insertion losses, high polarization-rotating ratios, and stable filtering performances under large oblique incidences. In addition, our proposed method can be applied to design higher-order PRSs with sharpened out-of-band roll-off skirts.