## 1 Introduction  
Monopulse systems have been widely used for target tracking in various military purposes such as missile seeker, a shipborne satellite communication (SATCOM) station, an airborne-based sense and avoid (ABSAA) and satellite tracking [1; 2; 3; 4; 5]. In such applications, the monopulse systems typically detect the direction of targets using the amplitude ratio of the sum and difference channels of the received signals [6; 7]. Therefore, these systems should consist of multiple antenna elements and a feeding network with a channel comparator. Horn antennas connected by metallic waveguide feeding networks are traditionally used for monopulse radiators and comparators to achieve stable system characteristics and a durable structure [8; 9]. However, horn-based monopulse systems have significant disadvantages as a result of their bulky structure sizes, complicated feeding networks, and high manufacturing costs. To overcome these problems, bulky radiators and waveguide feeding networks can be replaced by planar microstrip patches and substrate integrated waveguide (SIW) feeding networks, which can provide lightweight and low-profile structures [10; 11; 12; 13; 14]. For radiators, various antenna types have been investigated, including microstrip patches, Yagi-Uda geometry, and slotted antennas [15; 16; 17; 18; 19; 20]. Comparators have also been extensively studied by employing microstrip couplers and magic tee comparators in monopulse systems [21; 22; 23; 24; 25]. While these techniques have allowed us to reduce the size of the monopulse system to some extent, the monopulse array antenna still requires a large aperture size due to the use of multiple radiators and the sophisticated design of the monopulse comparators.  
In this paper, we propose a novel design structure of the monopulse antenna system that can operate with only a single radiator to reduce the aperture size, where the complicated waveguide comparator is also replaced by the simple planar SIW multi-mode feeding network that can obtain the ratio of sum and difference patterns. The proposed antenna consists of a single rectangular patch printed on a dielectric substrate, which is connected to a two-port SIW feeding network. The two ports in the SIW feeding network transmit transverse electric (TE)\({}_{10}\) and TE\({}_{20}\) modes to the single patch antenna, respectively, resulting in the sum and difference radiation patterns. The direction of arrival (DOA) is then estimated by the amplitude ratio of the sum to difference patterns. In this design procedure, the proposed monopulse array antenna is designed at the target frequency of 4.85 GHz in order to demonstrate the capability of the commercial Industry-Science-Medical (ISM) band applications such as internet of things (IoT), unmanned aerial vehicles (UAVs), and mobile devices [26; 27], although the conventional monopulse system is usually utilized in the military application. To verify the feasibility, the single patch antenna with the dual-mode SIW feeding system is fabricated, and its antenna characteristics such as the reflection coefficient, mutual coupling, and radiation patterns are measured in a full anechoic chamber. To better understand the operating principle of the antenna, we observe near-field distributions, the sum and difference radiation patterns, and impedance matching characteristics in accordance with some important design parameters. The accuracy of the DOA estimation is further improved by applying the derived calibration factor to the resulting sum and difference ratio. The results demonstrate that the proposed antenna, with its single radiator and simple multi-mode feeding network, is suitable for compact and lightweight monopulse systems.