nanoVNA resistance readings for 13.56MHz loop antenna never exceed 120 ohms

A project involving measurement of a small loop antenna at 13.56 MHz using a nanoVNA revealed that the resistance (R) readings at the antenna's self-resonant frequency never exceed 120 ohms, despite expectations from datasheets indicating values around 1.5 kΩ. Calibration issues were identified and resolved by properly resetting calibration data before performing SOL calibration at the 50-ohm reference plane, resulting in more accurate parallel resistance measurements up to approximately 30 kΩ. It was noted that small loop antennas typically do not exhibit true self-resonance and have highly reactive impedances (e.g., Za ≈ 0.01 + j400 Ω). Efficient operation requires tuning with a variable capacitor to reduce impedance magnitude. The nanoVNA's measurement range and accuracy for high resistance values were questioned, and users were directed to an active nanoVNA user support group for further assistance. Antenna modeling software was recommended for more precise analysis rather than relying solely on VNA measurements.
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