Rusty bolt effect

[1] It is more properly known as passive intermodulation,[1] and can result from a variety of different causes such as ferromagnetic conduction metals,[2] or nonlinear microwave absorbers and loads.

[3] Corroded materials on antennas, waveguides, or even structural elements, can act as one or more diodes.

Galvanised fasteners and sheet roofing develop a coating of zinc oxide, a semiconductor commonly used for transient voltage suppression.

If the incoming signal is a sine wave {Ein sin(ωt)}, (and taking only first-order terms), then the output can be written: Clearly, the harmonic terms will be worse at high input signal amplitudes, as they increase exponentially with the amplitude of Ein.

Hence the second-order, third-order, and higher-order mixing products can be greatly reduced by lowering the intensity of the original signals (f1, f2, f3, f4, …, fn)