An apparatus for duplexing signals to be transmitted and received wirelessly by an antenna, the apparatus comprising: a hybrid circuit comprising an antenna node for coupling to the antenna, an input node for receiving a signal to be transmitted wirelessly by the antenna, an output node for outputting a signal received wirelessly by the antenna, and a balancing node; a variable impedance operably coupled to the balancing node, the variable impedance comprising a control input to select an impedance setting of the variable impedance; a data store comprising a reference of impedance settings and corresponding calibration measurements in respect of a consistent impedance applied to the antenna node of the hybrid circuit, the impedance settings corresponding to impedances; and a controller arranged to: set the variable impedance to a first impedance setting; transmit a first transmit signal at the input node, the first transmit signal being at a first transmit frequency in a first frequency band; measure a first signal at the output node at a first receive frequency in a second frequency band as a result of the first signal transmitted at the first transmit frequency; select a second impedance setting; transmit a second transmit signal at the input node, the second transmit signal being at a second transmit frequency in the first frequency band; measure a second signal at the output node at a second receive frequency in the second frequency band as a result of the second signal transmitted at the second transmit frequency; employ a system of equations comprising variables associating the first measured signal, the second measured signal, a first calibration measurement from the reference in respect of the first impedance setting, and a second calibration measurement from the reference in respect of the second impedance setting; determine a balancing result using the system of equations; retrieve a third impedance setting from the reference by reference to the balancing result, the third impedance setting providing isolation of the input node from the output node of the hybrid circuit. a hybrid circuit comprising an antenna node for coupling to the antenna, an input node for receiving a signal to be transmitted wirelessly by the antenna, an output node for outputting a signal received wirelessly by the antenna, and a balancing node; a variable impedance operably coupled to the balancing node, the variable impedance comprising a control input to select an impedance setting of the variable impedance; a data store comprising a reference of impedance settings and corresponding calibration measurements in respect of a consistent impedance applied to the antenna node of the hybrid circuit, the impedance settings corresponding to impedances; and a controller arranged to: set the variable impedance to a first impedance setting; transmit a first transmit signal at the input node, the first transmit signal being at a first transmit frequency in a first frequency band; measure a first signal at the output node at a first receive frequency in a second frequency band as a result of the first signal transmitted at the first transmit frequency; select a second impedance setting; transmit a second transmit signal at the input node, the second transmit signal being at a second transmit frequency in the first frequency band; measure a second signal at the output node at a second receive frequency in the second frequency band as a result of the second signal transmitted at the second transmit frequency; employ a system of equations comprising variables associating the first measured signal, the second measured signal, a first calibration measurement from the reference in respect of the first impedance setting, and a second calibration measurement from the reference in respect of the second impedance setting; determine a balancing result using the system of equations; retrieve a third impedance setting from the reference by reference to the balancing result, the third impedance setting providing isolation of the input node from the output node of the hybrid circuit. set the variable impedance to a first impedance setting; transmit a first transmit signal at the input node, the first transmit signal being at a first transmit frequency in a first frequency band; measure a first signal at the output node at a first receive frequency in a second frequency band as a result of the first signal transmitted at the first transmit frequency; select a second impedance setting; transmit a second transmit signal at the input node, the second transmit signal being at a second transmit frequency in the first frequency band; measure a second signal at the output node at a second receive frequency in the second frequency band as a result of the second signal transmitted at the second transmit frequency; employ a system of equations comprising variables associating the first measured signal, the second measured signal, a first calibration measurement from the reference in respect of the first impedance setting, and a second calibration measurement from the reference in respect of the second impedance setting; determine a balancing result using the system of equations; retrieve a third impedance setting from the reference by reference to the balancing result, the third impedance setting providing isolation of the input node from the output node of the hybrid circuit.