Abstract: With the growing demand for artificial intelligence (AI) data centers, 800 Gb·s⁻¹ copper cables are becoming the preferred solution for short-distance transmission. However, key performance metrics such as signal attenuation still limit their transmission distance. To reduce signal attenuation (|S_DD21|), a double-layer insulation structure and foamed insulation materials were used. Compared with traditional insulation structure, the 30 AWG, 100 Ω copper cables with double-layer insulation structure exhibited an |S_DD21| reduction of 1.83 dB·(3 m)⁻¹ at 26.56 GHz. Furthermore, incorporating foamed insulation materials could further reduce |S_DD21| in copper cables with double-layer insulation structure. By combining foamed and solid insulation materials in various configurations, four types of double-layer insulation structures were investigated. Among them, "foam-inner and foam-outer" configuration exhibited the lowest |S_DD21|. However, reliability testing with a 90° bend revealed a significant impedance discontinuity up to 11.61 Ω, indicating poor mechanical reliability. Taking reliability into account, "foam-inner and solid-outer" configuration demonstrated an optimal balance between |S_DD21| and mechanical performance, achieving an |S_DD21| reduction of 2.92 dB·(3 m)⁻¹ at 26.56 GHz compared to traditional insulation structures. Finally, key manufacturing techniques of 800 Gb·s⁻¹ copper cables were also discussed. In particular, extrusion process of the outer insulation layer could play a crucial role in maintaining structural symmetry, directly influencing differential signal balance and S_CD21 performance.