Low-Loss Wideband Multimodal Interferometric Antenna for DOA in Azimuth and Elevation

A novel direction-finding (DF) antenna is presented. The antenna uses interferometry with the three low-order circular phase-modes (CPMs) (-1, 0, +1) to estimate the direction-of-ar- rival (DOA) of a received microwave signal. The antenna system consists of two antennas positioned one above the other. The omnidirectional n = 0 mode created by a top biconical dipole antenna serves as the phase reference for the other plusmn1 modes. The second antenna consists of a novel two-port waveguide feed creating left- and right-hand circularly polarized (CP) fields inside a thick coax that opens gradually to a radial waveguide and a biconical horn antenna. Each CP-field propagates between the circular parallel plates in a spiral path (a TEio radial line mode) creating a linear phase field in the aperture. The amplitude of the resulting radiation field is uniform in azimuth, with a continuous linear phase in azimuth. Thus, in transmit mode, one port creates a positive phase slope (+1 port) and the other creates a negative phase slope (-1 port). In the operating receive mode, the linearly polarized TE₁₁ mode of the thick coax is resolved into right- and left-hand CP TE₁₁ modes. The DOA information is obtained by measuring the phase differences between the CP modes and the reference n = 0 mode. The two phase differences, when processed, yield azimuth and elevation DO As of the received signal. The suggested antenna is simpler in its structure, smaller in size, and light compared with a previous multiport biconical antenna. It is also more wideband and less lossy, and it is capable of detecting the elevation DOA of the received signal.