Antenna height considerations
From Biocrawler, the free encyclopedia.
The Aspects for Antenna heights considerations are depending upon the wave range and economical reasons.
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Longwave/Low frequency antennae
At VLF, LF, MF the aerial mast or tower is often directly used as aerial. Its height determines the vertical radiation pattern. Masts and towers with heights around a quarter wave or shorter radiate much power towards the sky. This allows at nighttime only a small area of fading-free reception, because the distance at which groundwave and skywave are from same strength and can interfere with each other is not so big (approximately 40 kilometres to 200 kilometres from transmission site, depending from frequency and ground conductivity).
For high power transmitters, masts with lengths of around half of the radiated wavelength are preferred because they focus the radiated power to the ground. This enlarges the distance at which selective fading occurs. However masts of lengths around half wave length are much more expensive than shorter ones and therefore too expensive for low-power mediumwave transmitters in most cases.
For longwave transmitters, however, the construction of masts with heights of half-wave wavelength is, because of economical reasons (and perhaps problems with flight safety), in most cases impossible. The only so far realized radio mast for longwave with a height of the half length of the radiated wavelength was the Warszawa Radio Mast at Konstantynow, Poland. For frequencies below longwave the masts have to be electrically enlarged by coils or structures on the top, because the required heights for masts of even quarter wave length are too large.
The usage of masts longer than fifth eights of the wavelength is normally not done, because these masts show (except of some special constructions for high power mediumwave broadcasting) bad vertical radiation diagrams. So the heights of masts for mediumwave transmitters normally do not exceed the 300 metre level.
Sometimes cage aerials or long wire aerials are used for LF and MF transmission. In this case the height of the used tower may be higher than in case of poor LF and MF transmission. Because towers or masts used for cage or long wire aerials are grounded, they are especially suitable for also carrying aerials for UHF-/VHF-transmitters.
Shortwave/High frequency antennae
For transmissions in the shortwave range, the mast height has no influence on the efficiency when the masts are used for carrying the aerial. Normally shortwave transmitters do not use masts higher than 100 metres.
Antennae for UHF/VHF
For transmissions in the VHF and UHF range the value can vary depending on the area to be served. The cost of a tower must be re-couped primarily through advertising on the broadcasts, if there are no broadcasting charges as e.g. in Germany. Things like population density, how far a signal can travel (affected by terrain for line of sight for some kinds of transmission), cost of a higher tower, maintenance cost, etc. - all must be balanced for an ideal tower size. According to this there are often restrictions from the flight safety authority concerning the maximum tower height. Two shorter towers may be a better option the one taller one, for example, or a higher tower might not be useful if the signal is blocked by terrain nor if all the listeners are in a concentrated area and a higher tower cannot pay for itself.
Antennae for directional radio services
In most applications a line of sight is required between the transmitting and reception aerial for directional services, so the antennas have to be mounted in a certain height about ground. For directional radio systems with large bandwidth, it is not possible to use long cables between transmitter and aerial, so towers with equipment rooms in the height of the aerials are required. Such towers are especially common in Germany.
