Antenna height
- overview, summary, tutorial about the basics or key elements of the effect
of height upon an RF antenna installation.
The installation of an antenna is crucial to its operation.
The choice of location and in particular its height can determine its
performance to a major degree..
It is generally found that the higher it is, the greater the
cost in installing it. A taller mast may be required, and longer feeders will be
needed. Long feeders may also reduce the effectiveness of any gain achieved in
increasing the height. However significant levels of gain can be achieved by
mounting antennas as high as reasonably possible.
For optimum performance the antenna should be mounted above
any local objects so that they do not screen it. A rule of thumb of 12 metres or
40 feet is a very good general guide because this tends to take the antenna
above the layer of electrical interference and also above signal variations
caused (at higher frequencies) by the heat layer above buildings. It may also
help reduce EMC problems. However the height to which an antenna can be raised
will be determined to some degree by its size. Larger antennas for lower
frequencies are not so easy to raise to great heights, although if used for
commercial applications there may be more funding for large towers.
If there is no screening by buildings and assuming the
antenna is over ground that is flat for several miles around then at low levels
the main lobe of the radiation will tend to be raised in the vertical plane. As
the antenna height increases the direction of the main lobe will become more
parallel to the ground. Although the performance will be affected by effects
such as minor lobes, usually there is an increase in gain as the height is
increased. As a general rule it is found that there is about a 6 dB gain for
every doubling in the height of the antenna. For example when using an antenna
on a 12 metre mast, assuming this clears all obstacles, then using the antenna
on a 24 metre mast would give a signal increase of 6 dB. This assumes that the
additional length of feeder required does not introduce any loss. In a real
installation this would need to be taken into consideration.
This is only a very general case. Should the antenna be
mounted on a hill, then increasing the height of the antenna is unlikely to give
any real improvement. Instead the effective height should relate to the bottom
of the hill. Conversely an antenna mounted in a valley will achieve much greater
levels of signal increase as a more favourable angle to the nearby hill top is
achieved.
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