Overall Layout The AN/SPG-55B antenna assembly consists of two separate antennas: the main antenna, which provides the track beam, guidance beam, and
cwi beam, and the capture antenna, which provides the capture beam. The capture-guidance mode is employed for control of beam-riding missiles, and the track cwi mode is used for semi-active radar homing (SARH) missiles. The SPG-55 has two antennas: the main antenna, and the capture antenna. The main antenna handles tracking, guidance, and CW illumination. The capture antenna is used to capture and guide the beam-riding Terriers into the guidance beam. The CW feed horn facing the main dish and the four horns of the C-band feed horn assembly are cross-polarized, thereby reducing incident X-band radiation on the C-band, four-horn cluster. In addition, low-pass filters inserted in all the microwave channels leading to the receiving system effectively filter out any X-band signals superimposed on C-band channels.
Capture Antenna The capture antenna subassembly, like the main antenna, is a Cassegrainian type. It consists of a circular waveguide feed horn, a polarized parabolic reflector, and a polarization converter. The circular waveguide feed horn operates in the transverse electric mode. The feed horn is skewed at a preset angle from the boresight to produce a circular symmetrical beam pattern. The RF energy emitted from the feed horn is vertically polarized. The rotation pattern is such that the phase front of radiated energy is not affected during rotation. Therefore, the radiated energy from the feed horn, which is incident on the parabolic reflector, is still vertically polarized. The parabolic reflector serves as a vertically polarized focusing radome. Therefore, horizontally polarized energy is reflected and only vertically polarized energy is transmitted. Since the RF energy from the feed horn is horizontally polarized, the radome reflects and focuses this incident energy into the polarization converter. The polarization converter consists of metal plate polarization grids, which are one-quarter wavelength (C-band frequency) in thickness and bonded to the surface of the plate. The polarization grids are oriented at an angle of 45 degrees to the incident energy. The operation of the polarization converter is identical to the polarization converter of the main antenna assembly. Therefore, the horizontally polarized incident energy is reflected as vertically polarized energy and transmitted through the radome as the capture beam. == On board ships ==