# Fully Metallic Additively Manufactured Monopulse Horn Array Antenna in Ka-Band
## 1 Introduction
Similarly, in wireless communication systems employing beamforming or MIMO techniques, difference pattern antennas improve signal reception and transmission by directing beams toward the desired directions while minimizing interference [25]. In radio astronomy, they facilitate the capture and analysis of celestial signals, assisting astronomers in deciphering the properties of celestial objects through meticulous measurements of signal disparities across the antenna array [26]. Moreover, in direction-finding systems, these antennas are vital for navigation, search and rescue operations, and military endeavors, as they enable the determination of signal source directions by comparing signals received across the array [27]. Difference pattern antennas can be designed using reflective antennas, which integrate four horns as a feed [28]. As an alternative, they have also been designed using either substrate-integrated waveguides (SIWs) [29; 30; 31] or metasurfaces [32]. Although SIW-based arrays offer a compact and low-cost solution, their operational bandwidth is limited. Additionally, if electrically large arrays are required, they exhibit high losses in the mm-Wave band due to the necessary substrate. As an alternative, arrays with a corporate feeding network based on waveguide technology and an array of radiating elements, typically slot or horn elements, are often used. By using waveguides in the feeding network, not only is bandwidth increased, but losses are also reduced, as no substrate is required, thereby improving efficiency. Furthermore, this solution is easily scalable. These antennas are typically fabricated by dividing the feeding network into different blocks, each associated with a different RF component (filter, waveguide, OMT, etc.) and radiating elements, using CNC for each of them. Considering the simplest element, a waveguide section, the manufacturing process is relatively complex in mm-Wave frequencies, as it is usually produced using machined and soldered or brazed techniques, which are not trivial due to the dimensions of the waveguide hole at these frequencies. As the complexity of the feeding network element increases, the fabrication process also becomes more complex. In addition, even when CNC is used, the device must be manufactured in at least two pieces, which requires subsequent assembly with good electrical contact, a non-trivial task. This is where AM, particularly LPBF, through direct metal printing, can help reduce manufacturing complexities and thus facilitate the design and prototyping of more complex and compact structures. This work demonstrates the potential of the LPBF AM technique in the design, manufacturing, and experimental validation of critical and precise microwave components in the \(\text{K}_{\text{a}}\)-band, such as the presented monolithic monopulse antenna with curved surfaces in different directions and small elements in the inner cavities. A difference pattern horn array antenna composed of sixteen elements, which are integrated with 1:4 \(H\)-plane power dividers and twisted sections in waveguide technology, is manufactured in a fully metallic monolithic piece. The twists are designed in pairs, each with a 180-degree phase difference, which enables counterclockwise phase excitation in the elements, producing a difference pattern. These four twisted components are highly complex to manufacture, as they require curved and rotated surfaces within a relatively small waveguide section. This configuration results in a highly complex yet compact structure that is only feasible through AM, particularly when produced as a single piece. The prototype was manufactured and experimentally validated, demonstrating very stable in-band performance and achieving an operating bandwidth of 10% with almost non-beam squint. These results showcase the potential of LPBF AM in designing antenna systems based on waveguide-fed arrays, allowing for relatively complex and compact designs suitable for monopulse antennas.