Patent ID: 7848180

Claim:
An antenna apparatus comprising: a magnetic field detecting unit having an electric property which changes in accordance with a magnetic field change; a radio-frequency signal generating unit which applies a radio-frequency signal to the magnetic field detecting unit; a signal reducing unit which reduces a level of a first received signal obtained by the magnetic field detecting unit and the radio-frequency signal generating unit by an amount corresponding to a part of the radio-frequency signal, wherein the signal reducing unit includes: (i) an inverter which inverts the radio-frequency signal or a level-adjusted radio-frequency signal; and (ii) an adder which adds the first received signal and the inverted radio-frequency signal output from the inverter; and an envelope detecting unit which rectifies a second received signal that is obtained by reducing the level of the first received signal by the signal reducing unit, and which detects a signal indicating an envelope of a third received signal, that is obtained by rectifying the second received signal, by removing the radio-frequency signal from the third received signal.