This paper presents a technique to improve the spurious suppression performance at the second harmonic (2f0) of a Wilkinson power divider. The technique is achieved by the insertion of bandstop filters realizing with parallel-coupled lines with the characteristic impedance of 2√Z0Ω and the electrical length of λ/8 in a conventional structure. The simulated and measured results of the conventional and the proposed divider are compared at 0.9 GHz. The insertion losses (S21, S31) are less than 3.1 dB, while the return losses (S11) are less than 34.6 dB across 100% fractional bandwidth. Moreover, the proposed Wilkinson power divider achieves more than 25.5 dB suppression at 2f0.

This paper presents the resonator bandpass filter design using the proposed stepped-impedance resonator technique. The proposed resonator is consisted of a quarter wavelength coupler with the stepped-impedance connected at the end of the lines. Each ports have a step impedance feed line to adjust the impedance of filter closed to the characteristic impedance Z0. This resonator can reduce the spurious response at 2f0 and 3f0. The experimental results and designs of all types are 0.9 GHz to implement bandpass filters. This filter can suppress the second and third spurious response that both the theoretical and experimental performance is presented.

A planar 3-dB quadrature branch-line coupler (BLC) employing simultaneous inductive and capacitive compensation of parallel coupled microstrip lines is proposed for size-constrained RF front-ends. Closed-form synthesis equations derived from even/odd-mode analysis enable a first-pass design without electromagnetic optimization. Fabricated on Rogers AD260, the 3.0 GHz prototype occupies only 0.54 λ²_g , corresponding to a 46% size reduction compared to the conventional λ/4 implementation, while achieving |S₁₁| < −23 dB, isolation > 25 dB, insertion loss of 3.25 ± 0.25 dB, amplitude imbalance ≤ 0.4 dB, and phase error < 1.2° over a 27% fractional bandwidth. Sensitivity analysis confirms stable operation under ±10% component tolerances, and the measured results deviate from theory by less than 3%. Compared with recent compact BLC designs, this work achieves the highest bandwidth-to-area ratio using only two chip inductors and two planar capacitors, while remaining fully compatible with single-layer PCB fabrication and extensible to multiband or reconfigurable hybrid circuits.

A compact resonator with inductively compensated coupled lines is proposed. The resonator is achieved by applying an inductive compensation technique to miniaturize quarter-wave coupled lines. The theory and equivalent circuit of the resonator are developed, and design closed-form equations are derived. To demonstrate the application of the resonator, a 1.57 GHz bandpass filter with a 5% fractional bandwidth is designed and fabricated. The experimental results show good agreement with the electromagnetic simulation results. The measured in-band insertion (S21) and return losses (S11) are less than 1.20 dB and 23.20 dB, respectively.

This paper presents a compact coupled-line Marchand balun with symmetric inductive compensation for high-impedance transformation. The proposed circuit transforms a 50-Ω unbalanced input into a 300-Ω differential balanced output for S-band RF applications. The structure consists of two coupled-line sections, an input reactive matching network, and two identical compensation inductors placed symmetrically at the balanced output paths. The input matching network is used to support the impedance transformation at the unbalanced port, while the symmetric inductive compensation is introduced to adjust the effective modal response of the coupled-line paths and improve the output balance. The design is analyzed using even- and odd-mode circuit representation and optimized using ADS circuit-level simulation. A prototype was fabricated and measured, and its performance was compared with that of a conventional coupled-line Marchand balun under the same port-reference condition. At the design frequency of 2.25 GHz, the proposed balun achieved a measured input reflection coefficient of |S₁₁|=−28.05 dB. The measured transmission magnitudes were |S₂₁|=−3.11 dB and |S₃₁|=−3.05 dB, corresponding to an amplitude imbalance of 0.052 dB. The measured output phase difference was 180.52^∘, giving a phase error of 0.52^∘. Compared with the conventional prototype, the proposed balun provides improved input matching, smaller amplitude imbalance, and an output phase response closer to the ideal 180^∘ balanced condition. The measured results indicate that symmetric inductive compensation is a practical approach for improving compact high-impedance coupled-line Marchand baluns.

This study investigates the isolated-port response of a four-port parallel coupled-line microstrip structure as a nondestructive microwave indicator for storage-related changes in Nakhon Phanom Lychee (NP.1). Unlike conventional one-port or two-port microwave sensors that mainly rely on reflection, forward transmission, resonant-frequency shift, or quality factor, the present approach monitors the nominally isolated port, where the measured signal is affected by perturbation of the coupled-line field balance. Ten fruits were stored under refrigerated conditions and evaluated at 7-day intervals over three storage periods. Each fruit was measured ten times during each period at room temperature using a two-port vector network analyzer over 0.3–0.8 GHz. In the adopted connection, the measured VNA $S_{21}$ corresponded to the sensor isolated-port parameter $S_{31}$. The mean resonant frequency decreased from 487 MHz in Week 1 to 478 MHz in Weeks 2 and 3. The $S_{31}$ magnitude at the resonant dip became progressively less negative, changing from $-59.59$ dB to $-53.95$ dB and $-48.27$ dB, corresponding to a total change of 11.32 dB. Although the resonant frequency did not distinguish Weeks 2 and 3, the isolation magnitude remained different between these periods. The measured response is consistent with storage-related changes in the effective electromagnetic properties of the fruit. These preliminary results support the feasibility of using isolated-port magnitude as an additional microwave feature for nondestructive lychee monitoring. Further validation using direct physicochemical, dielectric, and metrological measurements is required before quantitative quality assessment can be established.