TABLE OF CONTENTS A single horn antenna can be made to exhibit properties of multiple horns to develop monopulse patterns by making use of various mode characteristics of waveguides. An interesting example of this can be found in a single aperture multimode direction-finding system described in Ref. 22. Consider the phase monopulse system described in Chapter 2: Two parallel antennas, closely spaced, receive signals on axis equally; hybrid subtraction is used to develop a null on boresight. A signal displaced off axis in either direction will develop a phase (or amplitude depending on the antenna orientation) difference signal that is proportional to the angle on either side of bore sight. In radar, the antennas for angular determination are physically constructed and mounted in a cluster of four to provide simultaneous azimuth, elevation, and sum patterns. Howard and Lewis of NRL (ref. 18, 19, and 22) recognized that the same outputs from the cluster could be obtained by exciting multiple waveguide modes simultaneously in a single aperture.
In Figure 3-41, the TE 10 mode in a rectangular wavelength is shown as a sine wave or symmetrical field with a maximum at b/2 and 0 at each equipotential boundary (per our convention). The TE 20 mode of this guide when excited by an off-bore-sight axis signal will develop a reversal of the E field at b/2 and will exhibit sinusoidal field properties. In effect, the TE 20
