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When radio waves from clean transmitters interact with dirty connections or corroded parts (which would then act as a diode) of a television, the rusty bolt effect, including the generation of harmonics and/or intermodulation in the signal, can occur. Rusty objects can re-radiate radio signals with harmonics and other unwanted signals; a television might then receive and attempt to interpret these signals.

If one experiences this problem, one should check both the transmitter and the television for dirty connections or corroded parts. One should also check for signs of corrosion in the cables which link the equipment to the aerials and for badly made joints. Beyond this, one might check any metal objects near the antenna for rust or corrosion. Any of these could be the source of the problem.

It is possible to cure this problem in several ways:

• Remove the corroded object. This is often the best cure because if you can eliminate the object then the interference it generates will cease entirely.
• Clean the object to allow proper electrical flow.
• Place an insulator between the two objects which are making the rust bolt. This will stop the RF current entirely.
• Lower the RF field strength, the maths below explains how this will greatly reduce the intensity of the effect.
• Get a better aerial which is more directional. It may be possible to point the aerial in such a direction that it does not pick up the unwanted signal coming from the rusty bolt.

Maths associated with the rusty bolt

The transfer characteristic of an object can be described by the following two equations.

E = E_{o K1 + (Eo)}

For an ideal perfect linear object K_{2, K3, K4, K5 etc are all zero.}

For a 'rusty bolt' or a frequency mixer stage K_{2, K3, K4 and/or K5 etc are not zero.}

Harmonic generation

If the incomming signal is a sine wave {E_{o sin(ωt)} then the output will be}

E = E_{o K1 sin(ωt) + (Eo)}

Hence by lowering the size of E the harmonics will be greatly reduced.

Mixing product generation

Second order

E_{f1+f2 = k Ef1 x Ef2}

E_{f1-f2 = k Ef1 x Ef2}

Third order

E_{f1+f2+f3 = k Ef1 x Ef2 x Ef3}

E_{f1-f2+f3 = k Ef1 x Ef2 x Ef3}

E_{f1+f2-f3 = k Ef1 x Ef2 x Ef3}

E_{f1-f2-f3 = k Ef1 x Ef2 x Ef3}

Hence the second order, third order and higher order mixing products can be greatly reduced by lowing the intensity of the original signals (f_{1, f2, f3, f4 ...... fn)}

• Radio electronics