This paper treats the theory and development of a fully passive field sensor for simultaneous electric and magnetic field measurements. The sensor head consists of three orthogonal double loaded loops and three orthogonal dipoles. The sensor outputs are connected to six passive, electro optic lithium niobate modulators. Summing the output voltages of each double loaded loop enables an accurate measurement of the magnetic field. The modulators provide an integrated electrode structure to form the vector sum or difference of two RF voltages in a frequency range from de up to 1 GHz. The loads of the loops are chosen to give a flat frequency response from 10 MHz to 1 GHz. Since the modulator input impedance is predominantly capacitive and the dipoles are electrically short, the frequency response of the latter are approximately flat up to 1 GHz. The measurement sensitivity limits of the electric and of the magnetic field depend on the pass band of the receiver and are 0.005 VIm/./Hzand 4·10-6 Nml.../H-i. respectively. This research work was funded by the Swiss PTT Telecom, R&D, FE117.