A novel measurement technique has been developed to enhance the accuracy of insertion loss measurements using vector network analyzers (VNAs), addressing the challenge of data fluctuations — a critical factor in determining circuit bandwidth and ensuring high-frequency signal integrity. The proposed method employs a Point-wise Least Squares (PLS) approach, eliminating the need for manual selection of smoothing bandwidth and simplifying the measurement process with a dedicated flowchart. This technique accurately differentiates insertion loss variations in microstrip lines, even across the Ka-band frequency range, ensuring reliable and consistent data by reducing deviations, particularly at band edges. The improved measurement accuracy supports the design and optimization of high-performance communication systems.

The power divider with filtering characteristics is proposed. To enhance the selectivity at the two output ports, T-shaped grounded transmission lines are inserted into the series-connected parallel coupled lines of the bandpass filter. Moreover, by incorporating hairpin-type arrangement into the bandpass filter, additional transmission zeros are generated, resulting in sharp slopes and high attenuations around the passband skirts. The proposed filtering power divider is fabricated on Rogers RO4003C with a center frequency of 3.5 GHz and an overall circuit size of 20.82 mm x 14.93 mm. The performance of this circuit is verified by comparing the implementation and measurement results.

This paper presents a novel, compact, and cost-effective design for a planar microstrip triple diplexer. The design incorporates three high-selectivity bandpass filters to ensure excellent isolation between the three operating bands. Each filter is characterized by sharp attenuation skirts surrounding its passband, which further enhances isolation. Due to fabrication constraints, the maximum achievable fractional bandwidth is limited to 25%. As a demonstration, a triple diplexer with center frequencies of 1.8 GHz, 2.6 GHz, and 3.5 GHz was fabricated on a Rogers RO4003C substrate, with overall dimensions of 45 mm x 48 mm. This design is particularly well-suited for mobile communication systems ranging from 3G to 5G NR. The close alignment between theoretical predictions and measured results validates the performance of the proposed circuit.

A planar microstrip triple diplexer is proposed. By integrating the front ends of three mobile communication bands, the triple diplexer operates at frequencies of 1.8, 2.6, and 3.5 GHz. This compact device is developed by combining three highly selective bandpass filters. By inserting T -shaped grounded transmission lines between series-connected parallel coupled lines, the composite triple diplexer achieves sharp roll-off slopes and high attenuation. An exemplary triple diplexer is fabricated on a Rogers RO4003C substrate, with dimensions of 45 mm x 48 mm. The agreement between theoretical and measured results of the fabricated triple diplexer validates the proposed design.

This paper proposes a power divider with integrated filtering characteristics. To enhance selectivity at the two output ports, T-shaped grounded transmission lines are incorporated into the series-connected parallel-coupled lines of the bandpass filter. Additionally, by adopting a hairpin-type configuration within the filter, extra transmission zeros are introduced, resulting in sharper roll-offs and higher attenuation around the passband edges. The proposed filtering power divider is fabricated on a Rogers RO4003C substrate, with a center frequency of 3.5 GHz and an overall circuit size of 0.4 lambda(g) x 0.29 lambda(g). The performance of the circuit is validated through a comparison of simulated and measured results.

An efficient and precise method to distinguish the insertion-loss variance of the microstrip lines with different metal strips has been proposed. These microstrip lines with an impedance of $50~\Omega $ and a length of a unit inch have been adopted. Moreover, the test fixture with two protected adopters and short-open-load-through (SOLT) calibration is adopted. By connecting to the vector network analyzer (VNA) directly, the insertion loss of two adapters, to protect the test fixture, can be obtained. Moreover, the insertion loss of the test fixture can be derived by averaging the fluctuated 50- $\Omega $ microstrip lines. Therefore, carefully removing the influence from two adapters, the test fixture and the calibration method, consistent results of precise insertion losses of the 50- $\Omega $ microstrip lines with a unit length could be obtained.

A novel planar bandpass filter design achieving an extremely wide passband (1 GHz to 8 GHz) and a broad stopband is proposed. The design integrates highpass filter principles into the transmission line structure, enabling the wide passband. Moreover, modified short-circuited coupled-line pairs are employed to address the fabrication limitations of high-impedance microstrip lines on printed circuit boards. This approach results in a fractional bandwidth of 155.6%, exceeding conventional designs. Furthermore, the filter achieves significant harmonic suppression with 30 dB attenuation in the stopband. Overall, this design offers a promising solution for applications requiring extremely wideband filtering with superior out-of-band rejection.

A novel bandpass filter with four switchable configurations is presented in this paper. The proposed filter is composed of two parallel connected dual-band bandpass filters. In particular, each filter has two open stubs and one parallel-coupled line, series connected with the stepped transmission lines and switching diodes. With four switching diodes in the "on" state, the measured central frequencies of quadruple passbands are located at 2.65, 3.36, 4.65, and 5.36 GHz. The result matches well with the theoretical simulation.

The precise and efficient procedures to measure the fabricated microstrip lines with the impedance of 50W and unit-inch length has been proposed. The measurement fluctuation could be minimized by averaging lengths of 50-W microstrip lines. Moreover, taking test fixture and calibration method into careful consideration, consistent results of the insertion loss could be obtained because of accurate measurement on microstrip line with low loss.

A novel bandpass filter with four switchable configurations is presented in this brief. The proposed filter is composed of two parallel connected dual-band bandpass filters. In particular, each filter has two open stubs and one parallel-coupled line, series connected with the stepped transmission lines and switching diodes. With four switching diodes in the “on” state, the measured central frequencies of quadruple passbands are located at 2.65, 3.36, 4.65, and 5.36 GHz. The result matches well with the theoretical simulation.

A new method for the design of microstrip bandstop filter with a wide stopband and an extremely high attenuation is proposed. At the input and output ports of this bandstop filter, the tight coupling has been adopted for the coupled line with a single-ended ground. Then, a wide bandwidth and a steep slope will appear at the edge of the stopband. Moreover, two coupled bandstop-mode resonators added can result in an extremely high attenuation within the stopband. Two structures are then cascade connected. Eventually, this bandstop filter, with the attenuation of 56 dB and a fractional bandwidth of 68.4%, is developed.

The compact microstrip bandstop filters with high attenuation and wide stopband have been developed. In order to realize the bandstop filter with broad stopband and great attenuation within the stopband, the coupled lines with single-ended ground have been adopted at both input and output ports. Moreover, for a greater fractional bandwidth, a higher impedance ratio Ze/Zo of the adopted coupled lines is required. The bandstop filter, with the attenuation of 40 dB and a fractional bandwidth of 60%, has been developed for applied in Ku and K bands to verify the proposed method.

In this paper, a microstrip bandstop filter with dual stopbands and tunable function is provided. In order to realize dual stopbands, two shunt-connected coupled lines and T-type transmission line are adopted with parallel connection. Moreover, the bandwidths of dual stopbands can be controlled with opposite sizes by tuning the varactors' voltage. An example of dualband bandstop filter is designed at the central frequency of 2 GHz, and fabricated on the subtract Rogers RO4003C. Moreover, the measured results match well with the theoretical simulation.

The novel microstrip bandstop filters with extra-high attenuation and wide stopband have been proposed. In order to realize the bandstop filter with broad stopband and great attenuation within the stopband, the coupled lines with single-ended ground have been adopted at both input and output ports. Moreover, for a greater fractional bandwidth, a higher impedance ratio $Z_{ei}/Z_{oi}$ of two adopted coupled lines is required. Furthermore, by differentiating the even- and odd-mode impedances of coupled lines at the input and output ports, the attenuation within the stopband can be increased and the stopband's bandwidth would be slightly narrower. Two bandstop filters, with the attenuations of 46 dB and 55 dB and a fractional bandwidths of 58.1% and 50.3%, respectively, have been developed to verify the proposed method.

Abstract A switchable bandstop filter is proposed herein. In order to achieve switchability between a single ultra‐wide stopband and dual wide stopbands, a complex process of coupling synthesis is not necessarily required. By setting two diodes at the input and output port separately, this bandstop filter can be switched between single and dual stopbands. An exemplary bandstop filter with central frequency at 2 GHz is fabricated on Rogers RO4003C, a substrate with a thickness of 0.813 mm and an area occupying 29.3 × 29.7 mm. Agreement between the theoretical and measured results can validate the proposed circuit.

In this study, a switchable bandstop filter is proposed. In order to achieve the switchability between a single ultra-wide stopband and dual wide stopbands, the complex process of coupling synthesis is not required. By setting two diodes at the input and output port separately, this bandstop filter can be switched between single and dual stopbands. An exemplary bandstop filter with the central frequency at 2 GHz is fabricated on Rogers RO4003C, the substrate with thickness of 0.813 mm and with an area occupying 29.3 mm × 29.7 mm. The agreement between theoretical and measured results can validate our proposed circuit.

A planar bidirectional absorptive bandstop filter with a wide absorptive bandwidth and compact circuit size is proposed. The proposed absorptive bandstop filter is developed from the absorptive coupled line. Moreover, by adopting the symmetrical structure, both the input and output ports can be used to absorb the injected power. Furthermore, to broaden the stopband bandwidth, the π-shaped transmission lines are included. In addition, design equations are provided for synthesizing the proposed absorptive bandstop filter. In order to validate the analysis, a wide-absorptive bandstop filter has been fabricated, with 20 dB for the absorption and 18.7% for the fractional bandwidth.

The theoretical design of radial power divider with wide isolation bandwidths is proposed. This radial power divider can distribute a microwave signal into four directions without sacrificing the bandwidth performance. Moreover, by series connecting shorted stubs with four resistors which are shunted at four output ports, the isolation bandwidths among four output ports can be broaden. In order to validate the proposed circuit at 1 GHz, the compact radial power divider is fabricated. The wideband bandwidths of the fabricated radial power divider on -20 dB output matching and isolation are greater than 74.4% and 133.7%, respectively.

A compact microstrip bandstop filter is proposed with wider stopband and larger insertion loss. By adopting coupled lines added with open stubs connected at the input and output port of the bandstop filter, the selectivity and stopband rejection can be increased significantly. Moreover, the attenuation level can be controlled by choosing the resonant frequencies of four resonators. This bandstop filter is fabricated with 14.75% fractional bandwidth for measuring at the stopband of 50 dB.

A novel planar bandstop filter with high insertion loss is proposed. With the assistance of coupled lines with open stubs at the input and output ports, the newly proposed structure is different from the conventional bandstop filter. With the coupled lines mentioned above, selectivity and stopband rejection of the newly proposed planar bandstop filter can be effectively enhanced. Moreover, the attenuation level of the proposed structure can be controlled by adjusting the resonant frequency of each resonator. Design equations can be obtained with the assistance of the lossless transmission line model and parallel coupled-line model. In order to validate the analysis, a bandstop filter, with a fractional bandwidth of 14% for the measured stopband at 50 dB, is fabricated.