Can only find this from RSGB TVI book


13. The rusty-bolt effect

High-pass filters (at the television receiver aerial terminals) and low-pass
filters (at the transmitter) do not always cure TVI problems. Substitution
of other TV sets can generally show if the cause is a faulty transmitter or
faulty television receiver, but if substitution shows the interference
effect to continue, then the cause becomes more difficult to establish. The
''rusty-bolt" effect is one of the hardest of all these TVI causes to
locate.
If a known clean transmitter is causing interference to a known good
television receiver, then an external cause can be suspected. Perhaps the
transmitter signal is being picked up by a local conductor such as a
clothes-line or fence-wire. A rusty or corroded joint in this conductor may
be acting as a diode. Harmonics of the transmitter signal could be produced
by this spurious diode detector and re-radiated. These harmonics can be
received by the television receiver and cause interference to the picture or
sound.
Such interference may vary with the weather. It may be intermittent and be
affected by wind as well as rain.
Typical offenders are metal-tile roofs, metal gutters and down pipes. A
heavy blow with a hammer may sometimes correct an offending joint. Applying
water from a hose can sometimes change or remove the interfering source and
help to identify the culprit.
Either bonding or insulating the offending joint may solve the problem. More
than one joint may be causing trouble. Bonding is generally impossible with
metal tiles. Shifting the television aerial away from the offending harmonic
source or sources is a more practical cure. A bonded wire mesh over the
offending joint may be considered. It is unlikely that a complete metal roof
will have to be bonded to effect a cure.
Bonding suspect joints can sometimes produce problems. With bonded
conductors, a better signal pick-up may result, larger radio frequency
currents may flow, and the problem may shift to another joint that was
hitherto not suspect. Insulating the suspect joints may sometimes be more
effective. A change to nylon guy-wires may sometimes eliminate problem
joints.
The accepted rule is that if the offending joints are on the amateur's
property, the problem is his. If the offending joints are on the property of
the television set's owner the problem is his, Unfortunately, few set-owners
understand this problem and so the radio amateur should offer technical
assistance and advice. Re-siting the television set aerial or the
transmitting aerial is often the only practicable cure.