Abstract: Monopulse technique, also known as the simultaneous beam comparison method, is used mainly to measure the direction of arrival (DOA) (Sherman and Barton, 2011). Before the advent of monopulse technology, the method most widely used in radar direction finding was the lobe-switching technique (sequential lobing) (Lo, 1999). Compared with the sequential lobing method, the monopulse antenna can obtain information such as the pitch angle, azimuth angle, and distance of the target in a single pulse period (Vazquez-Roy et al., 2019). Due to the need for higher accuracy of angle measurement, the monopulse technique has been widely applied in SOTM missile guidance (Roy et al., 2019). When a monopulse system is working, multiple independent channels are adopted to receive signals reflected by targets at the same time. Then signals enter the comparator and finally the distance and angle information of targets can be obtained. There are three main types of monopulse antennas: lenses, reflectors, and arrays. A dielectric lens is generally a convex lens (Raman et al., 1998), which is difficult to manufacture due to its curved shape, high volume, and large mass. For common reflectors such as dish antennas, the feed and supporting structure are in front of the aperture, so a shielding effect will result in energy loss, sidelobe increase, and angle measurement error (Kou and Cheng, 2019). A more effective reflector is the Cassegrain antenna, which is more compact with dual reflectors (Zheng et al., 2016, 2017). However, the Cassegrain antenna has the problem of shielding effect caused by a secondary reflector and support rods. With growing demand for low cost and lightweight monopulse antennas in modern radar and telecommunications systems, array antennas are increasingly applied in monopulse antennas due to their characteristics of high gain and easy control of the beam direction. A typical example is the waveguide slot array, which has the advantages of low loss, low sidelobe level, and no aperture shielding problem (Vosoogh et al., 2018). The microstrip has attracted much attention in monopulse antennas because of its advantages of low profile, simple structure, and low cost (Yu et al., 2009; Kumar and Kumar, 2018). However, monopulse microstrip array antennas usually have a complex feed network, which often makes them large and expensive. In addition, substrate integrated waveguides (SIWs) (Cao et al., 2017; Zhu et al., 2018; Liu et al., 2019; Yang et al., 2019) and leaky wave antennas (LWAs) (Poveda-García et al., 2019) have been applied in monopulse antennas, with good results. Recently, reflectarrays have been proposed for use in monopulse antennas, and have shown good performance (Zhao et al., 2017, 2018). As an alternative to reflectarrays, transmitarrays not only have the advantages of reflectarrays, but also have no shielding effect from the feeds. Di Palma et al. (2016) proposed a 400-element reconfigurable transmitarray antenna to synthesize monopulse radiation patterns. However, the sum and difference patterns generated by the reconfigurable transmitarray antenna, which is controlled electronically by switches instead of by a SUM-DIFF comparator, are in time division.

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