Abstract: A rotor blade sensor for detecting a rotor blade (430) comprising an electrical oscillator arranged to generate an oscillating signal. An antenna (300) includes a coil (100) having one or two winding layers coupled to the electrical oscillator. The antenna (300) may instead or as well include a coil (100) comprising a plurality of winding layers, each layer being separated by al spacer for substantially reducing inter-layer capacitance, being coupled to the electrical oscillator. The antenna (300) is driven in use by the oscillating signal of the oscillator at substantially a resonant frequency of the antenna (300). The antenna generates an antenna electromagnetic field that interacts with a rotor blade (430) such that the electrical characteristics of the antenna (300) vary as the interaction between the antenna (300) and the rotor blade changes and a detector circuit is arranged to monitor these electrical characteristics.