Small broadband receiving antenna using a Fabry-Perot etalon

Abstract

A novel concept for a broadband electric-field (E-field) receiving antenna is proposed. The reactive near field, induced by an impinging E-field, of a very short, short-circuited dipole is picked-up with the aid of an electro-optical modulator. The modulator is designed as a Fabry-Perot etalon. The laser light guided in the etalon is modulated proportionally to the near field of the dipole. The advantage of the etalon is to considerably increase the sensitivity of the antenna compared to known electro-optic probes. It is shown analytically for an ellipsoidal dipole and numerically for a cylindrical dipole, that only the quasi electrostatic terms of the dipole field are relevant. Important is the optimal choice of the material of the modulator. Since the linear electro-optical effect is used, best results are achieved if the index ellipsoid of the material parameters does not rotate under the influence of an external field but only varies its size. Such behaviour is found in materials as e.g. LiNbO/sub 3/ and poled polymers. Because of reflections, a second parameter of concern is the permittivity of the modulator base material. Polymers are the most promising materials known today for this type of antennas. The proposed antenna may either be used as broadband E-field probe or receiving antenna. It features low coupling because of the small size and the fibre optic feeding, low signal attenuation in the feeding and large bandwidth. This type of antennas is therefore optimally suited for distributed UWB antenna systems where only one way wireless communication is necessary.