Capacitive Sensing of Antenna Loading With an R–C Voltage Divider in a Tunable Antenna

This paper presents a capacitive sensor for antenna applications. The sensor is based on measuring voltage in a voltage divider made of a resistor and a capacitor formed in an antenna structure. The purpose of the device is to detect when the user of a mobile device holds a finger near the antenna element and allow triggering of compensation measures to regain antenna performance. A planar monopole antenna with an impedance tuning element was constructed for this purpose, and measurements showed that 10% of lost total efficiency can be regained by applying retuning. The sensor can measure capacitance changes of 5 fF and detect a human finger at up to a distance of 1 cm. Some design issues and caveats are also discussed.