An antenna apparatus for calibrating one or more of a plurality of antenna elements of an antenna array using one or more probes, where each of the one or more probes includes an element configured to wirelessly transmit and/or receive radio frequency, RF, signals, the antenna apparatus comprising: the one or more probes; a calibration arrangement comprising a switching circuit; and an upconverter and/or downconverter, UDC, circuit, wherein: the switching circuit is configured to enable operation of the UDC circuit in a first mode or in a second mode, in the first mode, the switching circuit is configured to disconnect the one or more probes from the UDC circuit, and in the second mode, the switching circuit is configured to connect at least one of the one or more probes to the UDC circuit, and the calibration arrangement is configured to use the UDC circuit to perform calibration of the antenna array; characterised in that: the switching circuit, the UDC circuit, and the one or more probes are provided on a die, wherein: the die includes a first face and a second face, the second face being opposite to the first face; the one or more probes are on the first face of the die; and the UDC circuit and the switching circuit are on the second face of the die. the one or more probes; a calibration arrangement comprising a switching circuit; and an upconverter and/or downconverter, UDC, circuit, wherein: the switching circuit is configured to enable operation of the UDC circuit in a first mode or in a second mode, in the first mode, the switching circuit is configured to disconnect the one or more probes from the UDC circuit, and in the second mode, the switching circuit is configured to connect at least one of the one or more probes to the UDC circuit, and the calibration arrangement is configured to use the UDC circuit to perform calibration of the antenna array; the switching circuit is configured to enable operation of the UDC circuit in a first mode or in a second mode, in the first mode, the switching circuit is configured to disconnect the one or more probes from the UDC circuit, and in the second mode, the switching circuit is configured to connect at least one of the one or more probes to the UDC circuit, and the calibration arrangement is configured to use the UDC circuit to perform calibration of the antenna array; characterised in that: the switching circuit, the UDC circuit, and the one or more probes are provided on a die, wherein: the die includes a first face and a second face, the second face being opposite to the first face; the one or more probes are on the first face of the die; and the UDC circuit and the switching circuit are on the second face of the die. the switching circuit, the UDC circuit, and the one or more probes are provided on a die, wherein: the die includes a first face and a second face, the second face being opposite to the first face; the one or more probes are on the first face of the die; and the UDC circuit and the switching circuit are on the second face of the die. the die includes a first face and a second face, the second face being opposite to the first face; the one or more probes are on the first face of the die; and the UDC circuit and the switching circuit are on the second face of the die.