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.
|
