Abstract: A sensing unit for use in-body comprises a variable impedance circuit for connection to the distal end of a transmission line and reflecting a carrier signal received from the transmission line. The variable impedance circuit comprises a variable impedance component having an impedance which varies non-linearly with applied voltage, a sensor for generating a voltage in response to a stimulus and a voltage bias system for creating a voltage bias for the variable impedance component. The voltage bias sets the operation point of the variable impedance component, such that the voltage changes from the sensor change the impedance of the variable impedance component non-linearly.