Nondestructive Quality Evaluation of Nakhon Phanom Lychee (NP.1) Using Parallel Coupled Line Microstrip via Isolation Loss Parameter Analysis

This study investigates the isolated-port response of a four-port parallel coupled-line microstrip structure as a nondestructive microwave indicator for monitoring storage-related changes in Nakhon Phanom Lychee (NP.1). The motivation is to explore the use of the nominally isolated port, represented by S31, as an additional sensing feature beyond conventional reflection or forward-transmission responses. Ten fruits were stored under refrigerated conditions and evaluated at 7-day intervals over three storage periods. Each fruit was measured ten times at room temperature using a two-port vector network analyzer over 0.3–0.8 GHz. In the adopted measurement configuration, the measured VNA S21 response corresponded to the isolated-port parameter S31 of the four-port sensor. The mean resonant frequency decreased from 487 MHz in Week 1 to 478 MHz in Weeks 2 and 3. The S31 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 magnitude 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 and provide a basis for future correlation with physicochemical and dielectric quality indicators.