Proximity of a user body part can be detected by measuring the effects
such proximity has on antenna impedance mismatches. The amount of
mismatch affects the amount of RF signal energy reflected back into a
transmission line connecting the antenna to a RF signal source. A
directional coupler has a main line electrically connected to the
transmission line, as well as a coupled line. The directional coupler
produces a signal on its coupled line in relation to the magnitude of
reflected energy on the transmission line; the amount of reflected energy
varies in response to how well the antenna impedance matches the
transmission line impedance. A signal detector is electrically connected
to the coupled line, and responds to signals produced in the coupled line
by the main line. The signal detector output is then used to determine
whether a body part is in proximity. Other aspects the invention include
an adaptive algorithm to adjust a threshold for proximity determination.