A nautical navigation aid comprising a photoluminescent element and a light wave re-directing means for improved photoluminescent qualities used to enhance the safety of nautical travelers on waterways in conditions where visibility is limited. The relative indexes, the refractive index profiles and the radii of the segments are chosen to provide low total dispersion and dispersion slope. The medical sensor can provide space between the skin of the applied part and the emitter and detector. When a sensor is in interrogate mode, it measures distance from a neighboring sensor using radar measurements. A transmitter generates a transmission signal from the baseband signal, and transmits the transmission signal to another apparatus via an antenna. The openings are separately arranged in the longitudinal direction of the antenna.