TABLE OF CONTENTS The primary source of DF accuracy is the antenna. We will therefore study the various types of antennas used for microwave passive direction finding. There is almost an infinite variety of designs; however, the types to be covered here will be those used in currently deployed systems, as opposed to obscure designs and older types. The antenna, by our definition, is the elemental radiator (or receptor) that can be dispersed, arrayed, or used alone or in combination to achieve specific and predictable angular gain patterns that permit direction finding information to be obtained. A simple example of this definition is the use of a half-omniazimuthal (180 degree) antenna as a side-lobe inhibitor for an array of horn antennas, the gain pattern in this case being tailored to perform this specific function. More complex examples will follow.
Where applicable, each antenna will be studied in its principal mode of operation followed by a discussion of appropriate alternate modes. It is often possible to make a single antenna simultaneously operate in multiple ways to form useful beams for multiple-channel systems in order to improve accuracy or achieve special coverage, the attractiveness being the contiguous field of view (all modes are looking at the same space with the same antenna properties).
