One-Port Reflectometry of a Skin-Mounted Passive UHF RFID Tag for Electrode-Free Heart-Rate Sensing at the Carotid Artery: A 15-Subject Multi-Posture Dataset

Abstract—This data descriptor presents one-port reflection (S11) measurements from a passive UHF RFID tag adhered over the carotid artery of 15 adults (ages 20–50). The tag is used as a resonant sensing element rather than a communicating transponder: S11 is measured directly through a UHF antenna connected to a vector network analyzer, with no reader interrogation or backscatter link. For each participant, S11 (dB) was sampled at 20 Hz over 30‑second windows, across five trials in up to three postures (sitting, standing, supine), with heart rate recorded by pulse oximetry at trial onset. Arterial pulsation perturbs the antenna–tissue impedance match, producing quasi‑periodic S11 oscillations from which heart rate can be recovered without electrodes or gel. Two estimators are evaluated across all trials: a threshold‑based peak counter and a spectral estimator using the dominant autocorrelation peak. On the highest‑quality cohort, both reproduce the reference closely (peak counting: bias –0.5 BPM, 95% limits of agreement –5.1 to +4.2 BPM; spectral: bias 0.0 BPM, –2.7 to +2.6 BPM). A second cohort recorded through the same front end exhibits no cardiac component, with oscillations dominated by respiration; this failure mode is documented so users can select appropriate subsets for their application.