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Capacitive sensors have been designed for use as user proximity sensors in mobile-phone antennas. Sensor-induced losses were measured in 900- and 1800-MHz capacitive coupling element antennas and a 1900-MHz planar inverted-F antenna (PIFA) and compared to the RF losses caused by an impedance-matching sensor. The lengths of the capacitance sensors varied from 1 to 43 mm, which induced antenna losses from 0.05 to 1.35 dB at 1 GHz. A pair of pad sensors induced 0.1-0.25-dB losses at 0.8-1.85 GHz. The loss of capacitive sensors in sizes below 5 mm2 was comparable to that of antenna-matching sensors. Total RF insertion losses due to the compared proximity sensors and loss compensation systems were 0.4 and 0.55 dB, respectively, across the 900- and 1900-MHz frequency bands.