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In this paper, immunity of differential dc power lines, recently considered as the physical layer for power line communications (PLC) onboard spacecraft, is assessed from the theoretical viewpoint. The analysis focuses on conducted immunity of the PLC channel to noise generated by the terminal units of the power line (PL). Particularly, multiconductor transmission line theory and modal decomposition are used to characterize the susceptibility of the PLC link to common mode and differential mode (DM) components of the conducted noise. Closed-form expressions for signal and noise contributions at the receiving modem are derived and used to develop design rules for the involved coupling/decoupling networks. By combining the spectral properties of the PL noise with the propagation properties of the link, it is shown that DM signaling, besides assuring a nearly flat channel frequency response, results to be more immune to the PL noise. Finally, the role that possible unbalance of the power units may play on the system conducted immunity and radiation properties is discussed by deriving analytical expressions of the longitudinal and transversal conversion loss associated with the PL terminal networks.