In this paper, different coupling/decoupling circuits for powerline communications (PLC) in differential dc power buses onboard spacecrafts are theoretically compared in terms of transmission performance and immunity to the powerline (PL) noise. Particularly, the analysis investigates the possibility to connect the PLC modems in series with the PL via inductive couplers and to use capacitive decoupling networks to separate signal from power. Suitable design rules for dimensioning the involved circuit elements are derived, targeted at assuring channel performance comparable to those obtained by parallel connecting the PLC modems to the PL via capacitive coupling and inductive decoupling networks