## 3 Materials and Methods  
In this study, the crosstalk levels at various frequency ranges were measured to evaluate the mutual interactions between signals. The conducted measurements aimed to quantify the extent of contamination caused by electromagnetic coupling. Tests were performed using a two-layer circuit board with two different transmission line lengths. The choice of varying line lengths was deliberate, enabling the observation of how line length influences the effectiveness of capacitors placed between ground planes. The methodology of this study integrates multiple techniques, including the arrangement of signal paths, strategic component placement, shielding, and electromagnetic interference-based crosstalk measurement. These techniques specifically target the reduction of near-end crosstalk (S\({}_{31}\)). The circuit board designs were created using Altium Designer 21.5.1 software. The scenario of routing UCPW lines side by side was tested to simulate realistic high-frequency transmission environments. High-quality components were selected for assembly to ensure measurement accuracy. For crosstalk testing, a Siglent SSA3032X Plus model VNA (SIGLENT Technologies, Shenzhen, China) was employed. The VNA was calibrated using F503 ME series calibration kits prior to testing, ensuring measurement precision. Capacitors from Johanson Technology's designer kits (Johanson Technology, Camarillo, CA, USA) were utilized during the tests, specifically the S603 DS series, which features a self-resonant frequency (SRF) of approximately 20 GHz. The SRF of these capacitors is critical, as it determines their effectiveness in mitigating high-frequency interference. Termination resistors compatible with the operational frequency range were used on the signal-applied lines to maintain impedance matching and minimize reflections. For this study, Eastsheep 2W-SMA-JR-12G resistors (Eastsheep, Beijing, China), compatible up to 12 GHz, were chosen. To eliminate environmental factors that could affect test accuracy, all measurements were conducted under controlled conditions. Both circuit boards were housed inside grounded metal enclosures to shield them from external electromagnetic interference, ensuring consistent and reproducible test results. Prior to each measurement session, the VNA was calibrated using the F503 ME series calibration kit with Short, Open, Load, and Through (SOLT) standards. This calibration method corrects systematic errors, ensuring accurate impedance matching and reliable S-parameter measurements across the tested frequency range. The procedure followed the manufacturer's recommended guidelines to minimize measurement uncertainties. The test environment is depicted in Figure 3.  
A capacitor is placed between the ground planes flanking the transmission line. The positions of these capacitors are strategically selected as the starting point, midpoint, and endpoint of the segment where CPW traces run in parallel. These positions were chosen to evaluate the effect of capacitors on electromagnetic coupling at different points along the transmission line. Additionally, the lengths of the transmission lines are set to progress together at 100 mm and 200 mm, allowing for a comparison of crosstalk behavior across different line lengths. The physical structure of the test card and the placement of passive components are shown in Figure 4.  
To systematically evaluate the influence of capacitors, repeated tests were conducted by fixing the values of the capacitors added to the UCPW traces on the circuit board. The selection of capacitor values (1 pF, 15 pF, and 82 pF) was determined based on practical applicability in high-frequency PCB designs and compatibility with industry-standard component kits. While numerous capacitance values exist, high-frequency performance is highly dependent on the SRF rather than the absolute capacitance alone.