## 1 Introduction  
The rapid development of electronic information technology has allowed electromagnetic technology to be widely used in various industries such as mobile phones, network security, radar systems, medical facilities, electronic household appliances, and even aerospace [1, 2]. While increasing convenience to human life, the pollution caused by electromagnetic radiation has also aroused widespread concern. This harmful pollution affects the regular operation of electronic equipment, interferes with people's daily lives, and directly threatens human health [3, 4]. Microwave-absorbing materials (MAMs) effectively prevent electromagnetic pollution and protect human health; therefore, they have attracted much attention. The development of lightweight, ultra-thin, high-efficiency, and broadband MAMs is needed [5, 6]. Carbon-based materials (such as carbon black, carbon nanotubes, graphene, etc.) have become the research focus of MAMs due to their advantages of low density, high specific surface area, good electrical conductivity, and high thermal stability [7, 8]. Carbon black (CB) is widely used because of its low cost, low density, and perfect dielectric and polarization characteristics [9]. [PERSON] et al. [10] used silicon rubber as the matrix and CB and T-ZnO as the absorbents to obtain composite absorbing patches by traditional preparation processes. When the mass fraction of CB and T-ZnO was 8% and 10%, respectively, its minimum reflection loss (RL) was only \(-\)14.68 dB, and the effective absorption bandwidth (EAB) was only 3.04 GHz. The absorbing properties of the composite materials need to be further improved. [PERSON] et al. [11] grew graphene oxide (RGO) in situ on CB by liquid phase methods to obtain a double composite dielectric material CB/RGO and studied its dielectric properties and microwave absorption energy. It was found that, when the thickness was 2.2 mm and the addition was 7%, the maximum absorption intensity reached \(-\)47.5 dB, and the EAB reached 5.92 GHz. However, this preparation process has many complicated procedures and long reaction times, which is not conducive to large-scale production. [PERSON] et al. [12] used NBR as the matrix and barium ferrite and CB as the absorbing agents to prepare single-layer, double-layer, and four-layer composites by mold pressing using a flat vulcanizing machine. The results showed that the absorption effect of double-layer and four-layer composite materials was better than that of single-layer, and the maximum absorption strength of four-layer composite materials only reached \(-\)11.38 dB. [PERSON] et al. [13] prepared a gradient multilayer microwave absorption patch (RAP) by molding with linear low-density polyethylene (LLDPE) as the matrix and CB as the absorber. When using CB with contents of 5%, 15%, 25%, and 35% as the fourth, third, second, and first layer of RAP (thickness of each layer, 2 mm; total thickness, 8 mm), in the frequency range of 4-18 GHz, the bandwidth of the RL value below \(-\)10 dB is 4.65 GHz. By structurally designing absorbing materials, the performance can be increased, but there is still room for improvement. At the same time, the traditional molding cost is high, and the mold depends on the tolerance, which is not conducive to wide applications, especially in manufacturing special structures; 3D printing technology can effectively solve this problem. Fused filament fabrication (FFF) process is the most widely used 3D printing process, and polylactic acid (PLA) is the most common polymer for preparing filament. PLA comes from a wide range of sources. It is a kind of green and renewable resource with good biodegradability, mechanical properties, and formability. In this paper, CB/PLA composite filaments were first prepared by a high-energy mixer and a twin-screw extruder, and then, the coaxial ring samples were printed via FFF technology to test the electromagnetic parameters.