## 1 Introduction  
Monopulse antennas are commonly employed in radar applications for target identification and tracking [1, 2]. Recently, monopulse antennas have also been suggested for 5G and millimeter wave applications in the millimeter-wave band to enhance the identification of user signals [3, 4]. Many antenna structures have been proposed for monopulse systems during the past decades [5, 6, 7, 8, 9, 10]. They include a radiating part and a feeding part, the latter of which is a specific feeder circuit in charge of properly exciting in the radiating part the following independent radiation diagrams: sum pattern \(\Sigma\) (for receiving and transmitting data) and the difference pattern \(\Delta\) (just for receiving data). By amplitude comparison of both patterns, the user direction of arrival can be calculated with very high precision [11]. Depending on the topology and technology, they can be classified as planar two-dimensional (2D) or three-dimensional (3D) structures, as is the one proposed in this work. In the context of planar 2D structures, microstrip circuitry in the form of arrays are widely employed for a number of applications due to their small weight, low cost, and ease of fabrication [12, 13, 14, 15, 16, 17, 18, 19]. In these cases, the radiating part is based on array topologies divided into at least two or more sections connected to the feeding part, which is generally composed of a two-input port network with couplers and phase shifters to generate an in-phase feeding of the array for the sum pattern while an opposite-phase is generated for the difference pattern. However, planar microstrip structures present high losses, which make them unsuitable especially for the millimeter-wave range. Substrate integrated waveguide technology (SIW) is used to minimize the loss problems associated with microstrip array antennas while the compactness of a 2D structure is maintained. The same feeding network topology based on coupler circuits is also used in SIW monopulse designs from the Ka to W bands [20; 21; 22]. Nevertheless, as SIW circuits are based on waveguide topologies, this kind of monopulse feeding part takes up considerable space compared to the radiating part, which is mainly formed by slotted waveguide arrays printed on the same substrate layer. In order to avoid this space problem and reduce the complexity, monopulse feeding networks based on the excitation of two [23; 24] or more [25] high-order modes on the printed waveguide have been proposed, which on occasion can be implemented even in multilayer disposition [26]. On the other hand, these multimode feeding networks present a narrower bandwidth response. In satellite communications, high-gain antennas are necessary due to the long link distance. For this reason, the required bigger size of 2D planar array configurations leads to an efficiency deterioration, especially in the millimeter band, due to the associated losses of the printed monopulse feeding network, even with the use of SIW technology. In recent years, 2.5D intricated multilayer planar structures implemented in full metallic gap waveguide technology have been proposed in the millimeter band (up to W band [27; 28]) as a solution to minimize feeding network losses in high-gain array monopulse systems, mainly applied to terrestrial terminals but with a significant cost increase. On-board satellite antenna systems, in which a broader frequency band is required, are mostly implemented with 3D systems, mainly composed of a single directive horn antenna [12; 13] in the radiating part (adding a reflector structure if higher gain is required [29; 30; 31]) and metallic waveguide devices in the feeding part.