TABLE OF CONTENTS The rotating passive reflector type of antenna still finds wide application in DF systems due to its simplicity and potential for reasonable gain with good backlobe response. The effective aperture of a rotating reflector, however, is never that of the reflector area alone since there is an illumination efficiency associated with the process. This efficiency is dependent upon the shape of the taper of the beam of the illuminating antenna and the reflectivity.
An antenna such as a spiral or a horn can be rotated directly by feeding it through a mechanical rotary joint. This is sometimes done; however, it is not always possible to achieve the front-to-back-lobe ratios of a reflector since two additional degrees of control are provided: the shaping or focusing of the passive reflector to increase gain and the ability to adjust the illumination or taper of the radiating structure to optimize the beam shape and back-lobe response. Rotating antennas, however, are still used. Multiple-channel rotating DF systems have been built using sum ( ?) and difference ( ?) channels in multiple-channel rotary joints, often in narrow RF bandwidth radar applications. For wide RF bandwidth systems, as required in passive DF applications, however, direct rotating radiators are generally limited.
There are many approaches to rotating reflector antennas. If a simple flat plate passive reflector is rotated at an angle above a linearly polarized feed, the polarization will rotate with the reflector and become a function of pointing angle. For this...
