I noticed the exact same behavior while tuning the antenna. I have ~55 turns of 32 Ga magnet wire on a 120mm Diameter form made from laser-cut acrylic. I compensated for the probe capacitance with a 100pF ceramic capacitor in parallel with the antenna, and it reliably reads cards inside of 12-20cm from the antenna. I plan switch out for a better quality capacitor, and will report on any increase in range.
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