A monitoring tag adapted to be secured around the appendage of an individual, the monitoring tag comprising: a primary antenna for transmitting and/or receiving a main electromagnetic signal; and a proximity sensor comprising: an oscillator; a secondary antenna adapted to transmit and receive electromagnetic signals for proximity detection, the secondary antenna being located adjacent to the primary antenna; a detector (20, 22); and a directional coupler having a first port and a second port connected together by a main line and a third port and a fourth port connected together by a coupled line, the first port being connected to the oscillator, the second port being connected to the secondary antenna, and the third port being connected to the detector (20, 22) and acting as an isolated port of the directional coupler when a transmit signal is supplied from the oscillator to the secondary antenna along the main line and as a coupled port when a receive signal is supplied from the secondary antenna to the oscillator along the main line; wherein the oscillator is controlled to provide a transmit signal to the secondary antenna along the main line of the directional coupler on detection that the main electromagnetic signal is not being transmitted and/or received by the primary antenna; and wherein the detector (20, 22) is adapted to provide an alert indicating the presence of a blocking object in close proximity to the secondary antenna if the magnitude of the power of the transmit signal that is reflected back to the secondary antenna in the form of a receive signal exceeds a threshold. a primary antenna for transmitting and/or receiving a main electromagnetic signal; and a proximity sensor comprising: an oscillator; a secondary antenna adapted to transmit and receive electromagnetic signals for proximity detection, the secondary antenna being located adjacent to the primary antenna; a detector (20, 22); and a directional coupler having a first port and a second port connected together by a main line and a third port and a fourth port connected together by a coupled line, the first port being connected to the oscillator, the second port being connected to the secondary antenna, and the third port being connected to the detector (20, 22) and acting as an isolated port of the directional coupler when a transmit signal is supplied from the oscillator to the secondary antenna along the main line and as a coupled port when a receive signal is supplied from the secondary antenna to the oscillator along the main line; an oscillator; a secondary antenna adapted to transmit and receive electromagnetic signals for proximity detection, the secondary antenna being located adjacent to the primary antenna; a detector (20, 22); and a directional coupler having a first port and a second port connected together by a main line and a third port and a fourth port connected together by a coupled line, the first port being connected to the oscillator, the second port being connected to the secondary antenna, and the third port being connected to the detector (20, 22) and acting as an isolated port of the directional coupler when a transmit signal is supplied from the oscillator to the secondary antenna along the main line and as a coupled port when a receive signal is supplied from the secondary antenna to the oscillator along the main line; wherein the oscillator is controlled to provide a transmit signal to the secondary antenna along the main line of the directional coupler on detection that the main electromagnetic signal is not being transmitted and/or received by the primary antenna; and wherein the detector (20, 22) is adapted to provide an alert indicating the presence of a blocking object in close proximity to the secondary antenna if the magnitude of the power of the transmit signal that is reflected back to the secondary antenna in the form of a receive signal exceeds a threshold.