Causes of Passive Intermodulation (PIM)

Ø Coaxial connectors - these connectors have joints between dissimilar metals and areas where oxidation can occur. Connectors that are assembled well using high quality components and water proofed well if they are to be used externally should last, but any connector will deteriorate with time. Connections made with poor connectors, or of the connectors have been assembled poorly will generate higher levels of PIM faster.

Ø Feeder lines - feeders such as coaxial feeders generate levels of passive intermodulation. The braid in the outer conductor provides many metallic interfaces in which passive intermodulation, PIM can be generated. Even if foil is used, there are interfaces between the layered construction of the foil wound round the dielectric. For applications where passive intermodulation is important coax with a good PIM performance can be bought. Some manufacturers may guarantee levels to below a given level, e.g. -140dBc, although at a cost.

Ø Joints where dissimilar metals meet and oxidation, etc. occurs converting the joint into an area that exhibits some diode effects.

Ø Dirty connections

Ø Loose connections and irregular contact areas, on a macroscopic or even a microscopic scale, can cause an inconsistent flow of current and generate inhomogeneous electromagnetic fields.

Ø General anodic effects

Ø The use of ferromagnetic metals, like iron, nickel and steel can give rise to passive intermodulation. These metals show magnetic hysteresis effects when energy is applied. This means that the resulting signal level is dependent, even to a small degree on input level. Hence this is a non-linear effect.

Ø Spark discharges can give rise to passive intermodulation. The spark will cause craters to be formed and some oxidation that will generate a diode effect. Sparks can result from poor connections or even the hot connection or disconnection of a connector.