Multiwavelength SEDs of the MW versus NGC 3521 across UV to radio decameter ranges, shown as log₁₀[λL_λ(λ)/L_⊙]. Black curve: Best theoretical SED for NGC 3521 (Sect. 5.1) converted to intrinsic luminosity assuming D = 10.7 Mpc, and including 28 MHz UTR-2 measurement as a constraint. Red curve: Intrinsic SED of MW obtained by Natale et al. (2022) (originally provided as L_ν) and recast to λL_λ/L_⊙ without any distance scaling. Yellow star: MW 28 MHz UTR-2 flux density scaled to D = 10.7 Mpc and converted to intrinsic luminosity; plotted as λL_λ/L_⊙.