Non-Contact Reflection-Coefficient Measurement of Automotive Transmission Fluid Degradation with a Hexagonal Patch Array Antenna: Measurement Model, Repeatability and Uncertainty Considerations.

Authors

Mehmet Duman

Published in

Sensors (Basel, Switzerland). Volume 26. Issue 20. Oct 13, 2026. Epub Oct 13, 2026.

Abstract

This study formulates and experimentally characterizes a non-contact reflection-coefficient (S₁₁) measurement method for assessing the degradation state of automotive transmission fluid (ATF). The measurand is the reflection coefficient, at the SMA (SubMiniature version A) reference plane, of a 4 × 4 hexagonal patch array antenna whose near field is loaded by a steel-backed measurement cell containing a fluid layer of approximately 9-10 mm; fluid condition is read from the frequency and level of the fluid-loaded resonance notch. The antenna was characterized by full-wave simulation (resonance at 4.852 GHz) and by SOLT (short-open-load-thru)-calibrated bench measurements (4.725 GHz, -24.32 dB), with the far-field pattern measured in an anechoic chamber. Fluid spectra were acquired using a SOLT-calibrated LibreVNA (2-6 GHz, 4 MHz step) within a single session under one calibration in a bracketed sequence in which the fresh fluid was measured both before and after the degraded fluid. Repeated acquisitions (n = 6 fresh; n = 4 degraded, ~200,000 km of service) yield a fresh-fluid notch of -28.6 ± 0.6 dB at 4.755 GHz and a degraded-fluid notch of -43.0 ± 4.3 dB at 4.775 GHz. Decomposition of the measured complex reflection coefficient shows that the resistive part of the loaded impedance at the notch is essentially unchanged (49.9 → 50.5 Ω) whereas the reactive part collapses (+3.7 → +0.4 Ω); the upward frequency shift and the deepening are therefore two consequences of a single reactive mechanism rather than two independent signatures. The two specimens were separated at p = 0.0095 (level) and p = 0.0080 (frequency) by the Mann-Whitney U test against a bracketed session drift of -1.3 MHz and +0.31 dB. Type A repeatability results and Type B considerations are reported following the GUM/VIM. These results establish a metrologically characterized, low-cost, non-invasive proof of concept for ATF condition monitoring, validated here on a single fresh/degraded sample pair.

PMID:
42944327
Bibliographic data and abstract were imported from PubMed on 27 Oct 2026.

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