The height of a radio antenna or radio aerial can have a significant impact on its performance, especially at VHF and UHF. We often see many forms of radio antenna mounted on tall masts or towers or they may be mounted on the tops of buildings, fixed to chimneys fixed to the sides of the buildings and the like. We see many VHF and UHF television antennas mounted on the tops of houses, and mobile phone and other telecommunications antennas are often mounted on towers.
The antenna height is obviously very important. There are several reasons for this: It can be imagined that a clutter free signal path will enable a better signal to be received and this is true. An antenna mounted high up is more likely to be able to achieve this. Like there is a visible horizon, the same is true for radio signals. Mounting the antenna high up extends the radio horizon and hence the coverage. It is possible to calculate the distance to the radio horizon very easily.
A high antenna also helps when there are hills or even just undulations in the surface of the Earth. Additional benefits are gain from antenna height. It stands to reason that a high antenna will be further away from sources of interference. Similarly for a transmitting situation, if the radio antenna is mounted high up, the radiation levels will be reduced in any buildings.
However it is necessary to remember there will be other considerations when antennas are mounted high up. High antennas may be more difficult to maintain as they will be more difficult to access. They may also be more exposed to t e weather and storms, so a well installed antenna is essential.
Also high antennas will need a longer feeder or coaxial cable. It is essential to use low loss coax – use the lowest loss that is reasonable. This is particularly important for VHF and UHF antennas that are high up because feeder loss is higher at these frequencies than at lower frequencies.
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