Principles of low PIM hardware design

In space RF communications payloads, it is increasingly required that high-power multi-channel transmitters and broadband receivers have shared, or closely adjacent RF feeds. Because of the large power level difference between the transmit and receive signals and the limitation of frequency allocation, passive intermodulation (PIM) interference due to passive non-linearities in the high-power transmission path can be serious problem. One of the main sources of PIM in RF microwave systems is the metal-to-metal contact. A contact model regardless of the suspected non-linear mechanisms, i.e. semiconductor, electron tunnelling and microdischarge, has been used to explain the non-linear effect at metallic contacts. The model requires that the non-linear effect is current dependent and the PIM level is proportional to the current density at the contacts. Because metal-to-metal contact in an RF coaxial connector is unavoidable, an alternative has to be developed. The work reported represents some of our efforts toward developing a demountable RF coaxial connector which does not involve metal-to-metal contact in any electrically significant part, i.e. a "contactless connector". The novel design of the coaxial contactless connector and the theoretical analysis are described.