The present invention provides an accurate and real-time acquisition and monitoring of three-dimensional location information about a target, in particular a target inside a subject's body during a medical procedure. This is achieved in accordance with the invention by marking the target location by a small-size location marker, which can be detected and located with high signal-to-noise ratio of the detection by a detection system located at a distance from the target. Provided by the invention is a location marker, a target location system utilizing such marker, and also a novel antenna system suitable to be used in the target location system. The marker of the present invention is a passive electronic device, which “responds” to an external high radio frequency electromagnetic field by a periodic time pattern of a single (certain fixed value) relatively low frequency, as compared to the known devices of the kind specified emitting a frequency coded signal.