A bi-conical multibeam antenna for space division multiple access

Bi-conical horn antennas are commonly used as high bandwidth omni-directional antennas for vertical polarisation. They are excited by a TEM-mode in the coaxial feed of the bi-conical antenna, which results in a field distribution in the aperture surface that is independent of the azimuth angle. In a new approach the bi-conical horn antenna is excited by a mixture of TEM- and TE/sub n1/-modes. Single directional beams can be radiated, and the use of multiple feed connectors allows simple and inexpensive implementation of space division multiple access (SDMA). Also, adaptive antenna pattern synthesis, that was not available up to now, is realised by this set-up. For the excitation of the additional higher modes the coaxial waveguide feeding the bi-conical antenna is overmoded. In the special case shown the cut-off frequencies for some of the higher modes are given, with an outer radius chosen as 20 mm. Linear superposition of the modes in the coaxial waveguide results in angle dependent electromagnetic fields in the radiating aperture as well. As the field distribution in this surface determines the far-field radiation pattern of the antenna, pattern synthesis can be realised by adequate mode mixing.