# Metallic, 3D-Printed, K-Band-Stepped, Double-Ridged Square Horn Antennas
## 1 Introduction  
The 3D printing technology, invented in 1980s, is catching attention recently because of its eco-friendliness, short turn-around time and versatility with complex structures [1; 2; 3]. In terms of materials, 3D printing technology can be categorized as dielectric and metallic, both of which feature merits as well as drawbacks. Dielectric 3D printed parts usually bear low material and process costs. The low body mass is the other advantage which is highly appreciated in space engineering due to the tightly controlled payload. However, the weak mechanical robustness and the electrostatic discharge (ESD) from dielectric 3D printed parts are the major concerns under harsh circumstances. For the metallic 3D printed parts, the mechanical weakness and ESD might not be observed, while the heavy body mass and comparatively high material cost are not desirable.  
The horn antenna is widely used in microwave engineering. The traditional way to fabricate horn antennas is computer numerical control (CNC) machining [4], electrical discharge machining (EDM), deposition technology and stacked ring technology [5]. CNC is the most popular, especially for fabricating low-frequency horns. EDM and deposition are often used for terahertz (THz) antennafabrication because of the required tight dimensional tolerance. The stacked ring technology is limited to producing horns with small physical dimensions, due to the ring alignment problem. Recently, 3D printing technology has been attempted for horn antenna fabrication. For example, [PERSON] reported a dielectric 3D-printed, W-band (75-110 GHz) corrugated horn antenna, which was first printed in split pieces, then metal plated and finally assembled [6]. [PERSON] proposed a Ku-band (10-16 GHz) corrugated horn antenna by dielectric 3D printing [7], which went through the same process as [6]. [PERSON] managed to take advantage of this technology to print an H-band dielectric horn antenna [8]. The metallic 3D printing technology was first used for horn antenna fabrication in 2012 by [PERSON] [9], who printed Ku-band horn antennas using electron beam melting (EBM) turned up with 25.9 um surface roughness by an un-optimized post-process. The working principle of EBM (i.e., the high pre-heat temperature and the pre-sintering of the powder bed) is the main cause of the relatively poor surface finishing compared to laser-based powder processes. [PERSON] and [PERSON] pushed the upper limit of the metallic 3D printed horn antenna up to the H-band (220-325 GHz) [10; 11; 12]. They used selective laser melting (SLM) to print a series of millimeter-wave (mmWave) and terahertz (THz) passive devices of comparable performance with commercial ones. [PERSON] demonstrated a metallic 3D-printed corrugated horn antenna at the F-band (110-140 GHz) for CubeSat application [13]. [PERSON] presented a metallic 3D-printed K-band horn as the feeder for a Luneberg lens [14].  
The aforementioned dielectric, 3D-printed, corrugated horn antennas are usually printed in split pieces, then metal plated and assembled, which brings in complexities process-wise. The metallic, 3D-printed corrugated horn antenna was printed in a whole piece in one run [13], however, the surface finishing of roughness Ra = 125 um was far from satisfactory. In this paper, taking advantage of an optimized 3D printing technology, we propose two K-band-stepped, double-ridged horn antennas in aluminum alloy (89.5% aluminum, 10% silicon, and 0.5% magnesium) and copper (85% copper and 15% stannum). This paper is organized as follows. Section 2 explains the design and fabrication of the antennas. Section 3 introduces the characterization of the antennas. Section 4 concludes the paper.