The height of an antenna has a major impact on its performance. Aspects including the feed impedance, radiation diagram, radiation losses, distance from interference, reduction in possibility of exposure to RF radiation, etc.
In general the higher the antenna the better its performance will be, but sometimes there are some limits as there is a law of diminishing returns, but often this is outside the reach of amateur radio users but sometimes broadcasters will want particularly high antennas to gain the required coverage at VHF and UHF.
Broadcasters often invest in very high towers, especially for VHF and UHF broadcast transmissions. Gaining the greatest coverage area can often only be achieved by increasing the antenna height.
Antenna height at HF
Because of the wavelength of signals at HF, antennas tend to be mounted relatively close to the earth in terms of electrical wavelengths. This means that the ground interacts with the antenna, particularly a horizontal antenna in a variety of ways.
Two main factors come into play for HF antennas:
- Angle of radiation: For long distance communications at HF it is found that the lower the angle of radiation of the antenna, the better it is. Many authorities on antenna design and installation recommend the antenna should be at least half a wavelength high. This can be relatively easy for frequencies say, above 15 MHz or so, but for lower frequencies with longer wavelengths this is less likely to be the case.
It is possible to calculate the elevation of the lowest lobe for a horizontal antenna above a perfectly conducting ground. It can be determined from the formula below:Where:
θ = the wave or elevation angle for the lobe
h = antenna height above ground in wavelengths
In summary, the higher the horizontal antenna, the lower is the lowest lobe of the radiation pattern.
Of course a major issue is to determine exactly where the ground is. As the ground is not a perfectly conducting surface, the signal wave may penetrate the ground by a certain degree, dependent upon the type ground and its conductivity. It may be that the actual electrical ground is seen as being well below the physical earth level by the antenna. There is a degree of uncertainty as it is difficult to predict exactly how things will work out, and they may vary from day to day dependent upon the level of water in the soil at the time.
- Radiation losses: It is found that if a horizontal antenna gets closer to the ground, then the losses due to the ground itself become more important and, at very low heights, they can be the main factor determining antenna performance. For example for a signal at 2 MHz, the wavelength is around 150 metres. A typical radio amateur may have trouble getting a horizontal antenna for these frequencies as high as 3 or 4 metres at times. At these heights relative to a wavelength, the ground losses are most likely to be the dominant factor. It has been calculated that a 7MHz horizontal dipole antenna at a height of around 5 metres will only be around 50% efficient - half the available power will be lost as ground losses.
As a rough rule of thumb it is often said that doubling the height of an antenna will give a 6 dB increase in gain. Although this will depend upon the actual situation and a host of caveats, etc, studies have shown that it is generally not too far from the truth. At worst it gives a very good idea of the importance of raising the height of an antenna.
Antenna height for VHF & UHF
At VHF and UHF, radio propagation tends to be more line of sight, although not always.
One of the main advantages of increasing the height of an antenna is that it raises the antenna above the items that might obstruct the antenna. Trees house and the like will all absorb radio signals, especially at VHF and UHF.
For the VHF and UHF bands it is normally not difficult to erect the antenna so that it is well above the ground, and therefore the primary effect of a nearby ground will be less relevant.
Initially raising the antenna will raise it above objects like houses, trees etc that will serve to mask or shield the antenna in terms of radio signals. Raising the antenna above these obstructions will considerably improve the performance of the antenna.
A further advantage at VHF and UHF is that the higher the antenna, the more distant the radio horizon. Often for VHF / UHF broadcasting, the radio horizon is often taken to be 4/3 the visible horizon as a result of the bending effect caused by refractive index changes close to the ground. Raising the antenna will considerably increase this, and thereby extend the range of the transmission.
The height gain, however, needs to be balanced against the feeder loss. At VHF and more so at UHF, the losses in the feeder become considerable, and in some instances may be more than the gain resulting from increased height. It is necessary to make a judgment call on this and obtain the best balance between gain from increased height, and increased loss from increased feeder length.
Obviously using the best feeder possible will make a difference to this.
Antenna height and interference
By their very nature, antennas that are higher tend to be removed from other electronic and electrical items by a greater distance.
This has two effects. One is that as a receiver connected to the antenna is further away from any sources of interference on or near the ground. The second is that the transmitted signal from the antenna will be further away from anywhere that interference from the transmitted signal may cause an issue.
Antenna height and RF exposure
There is a growing awareness of exposure to RF. Where transmitters may carry relatively high levels of radiated RF power, this needs to be kept away as far as possible from any areas where people frequent.
Having the antenna at a high level by its very nature will keep the RF away from people and will reduce the possibility of people coming into areas where the RF is at a high level.
With any antenna, be it for HF, VHF or UHF, etc, high masts obviously add considerable cost to any antenna installation. They also create a much greater visual impact and may be subject to local planning laws. When determining the height of an antenna, many factors need to be balanced. Height provides gain, but at the cost of visual impact, additional cost of the higher mast, possibly increased feeder loss, and other factors.
In general, placing an antenna system higher in the air enhances its communication capabilities and also reduces the chances of RF exposure and electromagnetic interference.