In exemplary implementations of this invention, a radio signal with a frequency of about 2.4
GHz is transmitted between a transmitter and a receiver. Either the transmitter, or receiver, or
both, have a directional antenna. When organic tissue passes between (or is stationary
between) the transmit ter and receiver, the tissue causes a reduction of the received signal
strength (RSS) of the signal, as compared to a baseline RSS. The larger the amount of
tissue, the greater is the reduc tion of the RSS. By analyzing the degradation of the signal …