Marker tag for a detection system

A receptor reradiator for use in a system for detecting the position of said receptor reradiator in a surveillance zone, the system transmitting a first frequency signal f.sub.1 and a second frequency signal f.sub.2, so that said first and second signals define a center frequency f.sub.c =(f.sub.1 +f.sub.2)/2, comprising:

• a dipole antenna having two metal conductive arms of a total length slightly less than half the wave length of the center frequency signal f.sub.c, said two arms coming together at a point defining the electrical center of the dipole;
• a non-linear semiconductor element disposed in one of said arms and offset from the electrical center of said dipole;
• and
• a parallel combination of a capacitance and inductance inserted in the arm containing the semiconductor element and tuned to receive said first and second frequency signals, so that said non-linear semiconductor element causes said dipole to reradiate an intermodulation signal generated by the first and second RF signals.