3.3 Special Considerations for Spacecraft Antenna Design

3.3.1 Passive Intermodulation Effects

It is essential that all transmit antennas on board a spacecraft do not interfere with other receivers via the process of passive intermodulation (PIM). Here an interfering frequency component is generated by combining several of the transmit frequencies (e.g. E5, E6 and L1 for Galileo) seeing a nonlinear input–output characteristic and so generating a mixing effect [10]. Such nonlinearity can occur even at a junction between different metals that form the transmission line or antenna input terminals. Possible sources of PIM include:

• Non-homogeneous contacts between metals of different types.
• Presence of oxides, particularly across gaps and joints (often associated with RF connectors).