Antenna installation materials

- Overview, summary, tutorial about key points or basics about the materials used for RF antenna installations.

The choice of materials and particularly metals used in an RF antenna installation can have a great effect on the way in which the atmosphere affects the system. The use of dissimilar metals will cause considerable trouble to the RF antenna installation as a result of electrolytic action. This arises because each metal has its own electro-potential and unless metals with similar potentials are used then the difference in potential will mean that corrosion results, even when they are dry. If moisture is present then the effect is greatly increased, and this is further worsened when atmospheric pollution is present. If it is absolutely necessary that dissimilar metals are used then great care must be made to reduce the effect to a minimum by ensuring that all moisture is excluded.

Electrochemical series

Metals can be arranged in the electrochemical series to determine the extent of the effect. If metals are close together then the corrosion effects will be less whereas if metals are further apart then the effect will be much greater. It is found that metals in the lower part of the series will corrode those in the upper section. For example brass or copper screws that are in the lower half will corrode an aluminium tube as aluminium is in the upper half. However cadmium plated screws would cause less corrosion.

Anodic

Magnesium
Aluminium
Duralumin
Zinc
Cadmium
Iron
Chromium iron alloys
Chromium nickel iron alloys
Soft solder tin lead alloys
Tin

Lead
Nickel
Brasses
Bronzes
Nickel copper alloys
Copper
Silver solders
Silver
Gold
Platinum

Cathodic

Corrosion not only causes a reduction in the strength of a mechanical structure, but it will also make it less easy to disassemble when the need arises. It can also increase contact resistance between elements and feeders. This occurs because many antenna elements are aluminium and feeders are generally copper. This can result in joints becoming corroded and presenting a high resistance where power is dissipated, thereby reducing the efficiency of the antenna.

Semiconductor effects

Under some instances the corroded materials can acts as semiconductor devices providing a non-linear component that can generate harmonics and intermodulation products. This is often known as the "rusty bolt" effect. Signals generated in this way can affect other users both in and out of the amateur band. As a result it is necessary to select materials to minimise corrosion, keep moisture away from any areas where dissimilar materials need to be used, and inspect and refurbish antennas and their weather protection measures at least every two years.

Summary

Although it is possible to install an antenna using many general materials, the rigours of the weather mean that the correct choice of materials is required if the antenna is to provide long service and also be less likely t fail mechanically. If an antenna fails mechanically there is always the risk of damage, or worse injury. Bearing this in mind it makes the choice of the correct materials particularly important.