An investigation into the passive intermodulation properties of space qualified materials

Intermodulation products are spurious frequency components which are produced when two or more signals mix in nonlinear components and devices. Particularly in multi-signal environments like those encountered on satellites, the spurious frequency components may severely degrade system performance through interference.

Nonlinearities are commonly known to exist in the active devices associated with components in transmitters and receivers. However, nonlinearities are also found in passive components and give rise to a phenomenon known as passive intermodulation interference.

This thesis describes the work undertaken during a study of passive intermodulation (PIM). The project was directed towards the characterisation of PIM in materials which are commonly used in spaceborne RF hardware. Satellite systems are particularly vulnerable to interference from passive non-linearities and little information is available to help engineers avoid the problem.

The work described includes the development of a highly sensitive PIM detection system. This involved the design and development of several custom components including connectors, band-stop filters and two different 3 dB couplers. The system was used to perform an investigation into the parameters which affect the generation of PIM signals in aerospace materials. Results are presented for measurements carried out at L-band frequencies.

An extensive review of previous studies on passive intermodulation is presented including descriptions of the nonlinear mechanisms which are most likely to occur in passive structures. The experimental results are then used to propose the mechanisms which are likely to dominate the levels of passive intermodulation in the samples tested.

The measurement system was also used to carry out measurements for the European Space Agency at the European Space Research and Technology Centre. The tests involved the detection and measurement of PIM sources in a test chamber used for characterising satellite payloads.