A technique and apparatus for obtaining automatic retro-directive performe from antennas and antenna arrays of general geometric configurations such as cones, ellipsoids, etc. The elements of the antennas or arrays are connected to multimodal feed networks. Conjugate phase circuits and gain adjustments are connected to some of the feed network terminals. The degree of retrodirectivity can be controlled by the gain adjustment and the number of conjugate phase circuits used.

A radar device (2) includes plural transmission antennas and plural reception antennas. The reception antennas constitute a reception-side antenna portion (20) and are arranged at an interval of d. The transmission antennas constitute a transmission-side antenna portion (18) and are arranged at an interval of d'=d.times.(n-1). The path length at which the electric wave is reflected from a target is identical between channels A9 and B1, and seventeen kinds of channels (A1 to A8, A9 or B1, B2 to B9) which are different in path length by every fixed distance are achieved. The data of the channels (A1 to A9 and B1 to B9) using different transmission antennas are respectively collected in different measuring cycles, and an error based on the time difference between the measuring cycles is corrected on the basis of a correction value calculated from the data of the channels A9, B1.

A signal processing unit detects the azimuthal direction of a target in accordance with the phase difference between signals received by two receiving antennas. Then, by sequentially switching plural transmitting antennas having mutually different beam directions with a switching device, received signals are obtained using each of the transmitting antennas.

An antenna system capable of receiving electromagnetic radiation having either right or left helical polarization, and either vertical or horizontal linear polarization includes an energy concentrator which concentrates energy at a focal point. A helical antenna is arranged in the proximity of the focal point. A first switch switches the helical antenna to a backfire mode of operation and a second switch switches the helical antenna to an end fire mode of operation. A control mechanism controls the switches to select the nature of the polarization.

An arrangement for feeding a centrally focused reflector antenna includes a waveguide (1), a dielectric field transformer (2) arranged on the waveguide (1) and a mounting platform (3) for a series of modules (8) which is embodied on one end of the waveguide (1). The waveguide (1) is provided with an arrangement for receiving the dielectric field transformer (2) which partially protrudes into the waveguide (1). Preferably, a dielectric support (9) may be provided in the vicinity of the dielectric field transformer (2). The middle of the support has a circular bore (12) whose diameter corresponds to the diameter of the dielectric field transformer (2). The centrally focused reflector antenna can be excited in the focal point thereof in a field-optimum broadband manner.