Design considerations for monopulse tracking spacecraft antennas

An analysis of amplitude and phase sensing sum and difference monopulse antenna systems is first made to show various parameter effects on system performance and to introduce basic differences between the two sensing techniques. Parameters such as pre-comparator voltage unbalance and phase shifts, and post-comparator phase shifts are considered. Next, unique designs and techniques used for the Apollo single channel monopulse antenna which are considered applicable for future spacecraft antenna designs are discussed and presented by simplified formula. The design considerations are primarily related to the microwave performance in the areas of gain and beamwidth, target acquisition regions, spacecraft structure effects, and overall system compatibility. Methods of obtaining theoretical antenna gain and beamwidth values for circular and square apertures are presented and used to show the advantages of a circular aperture. The target acquisition regions are expressed analytically for both amplitude and phase sensing systems with a discussion of the technique used on the Apollo antenna for yielding large acquisition regions. The effects of reflected energy from spacecraft structure on both amplitude and phase sensing systems are analyzed and applied to the Apollo antenna configuration as a specific example to show how these effects can be reduced. Finally, the approach used to analyze overall system compatibility for the Apollo single channel monopulse antenna is discussed. Also, the techniques used to measure system compatibility are illustrated. The favorable comparison between analytical and measured values relative to system compatibility are illustrated.