Most mast radiators are built as guyed masts insulated against ground. Steel lattice masts, of triangular cross-section, are the most common type. Square lattice masts and tubular masts are also sometimes used.
Free-standing towers are also used as radiating structures. These towers can have a triangular or a square cross section. One of the best-known radiating towers is the Blosenbergturm in Beromünster.
Concrete towers are not suitable for insulation, because of their enormous weight, exceeding the weight of guyed masts and lattice masts in an order of at least 10.
However masts of glass fibre are sometimes used for some small constructions.
Mast height
The ideal height of a mast radiator depends on transmission frequency, demographics for the location, and terrain. For radio systems in the longwave and mediumwave range the value of the height should be in the range between one sixth and five eighths of the wavelength, with favourite values at the quarter or the half of the radiated wavelength. When this is not always possible, masts with a loading coil, ‘capacity hat’ or similar loading arrangement are used. The quarter-wave mast is a form of monopole antenna.
The height of the mast determines the radiation properties. For high power transmitters in the MW range, masts with heights around half of the radiated wavelength are preferred because they focus the radiated power better to the ground than structures with heights of quarter wavelengths, which are preferred of economical reasons for low power medium wave transmitters. A focus of radiated power towards the ground is much desired on frequencies below 3 megahertz, because groundwave propagation is very stable. Masts longer than five eighths of the wavelength are normally not used, because they show bad vertical radiation patterns, so masts for mediumwave transmitters do normally not exceed 300 metres. For longwave transmitters, however, the construction of masts with heights of half-wave wavelength is, because of economical reasons (and perhaps problems with the 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 Konstantynów, Poland. For frequencies below longwave masts are electrically enlarged by loading coils or capacity hats on the top, because masts of even quarter wave length would be too high.
Feed arrangements
There are three ways of feeding a mast radiator from a transmitter:
* Base feed: the mast is supported on an insulator, and the transmitter is connected to the mast just above it;
* Shunt feed: the mast is grounded and the transmitter feeds it via a wire connected to the mast part way up. (This is a similar approach, on a larger scale, to the ‘gamma match’ popular among amateur radio operators for VHF and UHF amateur radio antennas.)
* Multiple feed: the structure is divided into two (or more) sections with insulators between, each independently fed. This collinear arrangement enhances low-angle (ground wave) radiation and reduces high-angle (sky wave) radiation. This increases the distance to the mush zone where the ground wave and sky wave are at similar strength at night. This type of antenna is known as an anti-fading aerial (see below).
There is usually an antenna matching unit to match the impedance of the transmitter or feeder to the antenna. Depending on the power involved, this may be a small box or a hut or building. It will typically contain an L-network to transform the modulus of impedance, and a coil or capacitor in series with the mast connection to ‘tune out’ any reactive component.
Location on transmission facility
At some facilities, especially the older and higher-powered installations, the mast radiator may be located at some distance of the transmitter building, in order to reduce the field strength induced by the mast into the building, and to prevent the building from distorting the mast’s radiation pattern. Between the transmitter building and the antenna matching unit next to the mast radiator, there is a feeder: either an underground coaxial cable or an overhead wire ‘cage’ feeder.
At facilities with multiple masts, spacings are typically smaller, in order to fit them into the available space.
At modern transmitters or at low power transmitters situated in very small transmitter buildings the transmitter, matching unit and mast radiator can be very close together and even in the same building. This measure saves on feeders, land area and increases the efficiency of the transmitter, if only one mast radiator is in use.
At most facilities the mast radiator is on a separate base close to the antenna matching unit, but it can be sometimes set up the roof of it. This is for example the case at the main transmission mast of transmitter Muehlacker and the main transmission mast of transmitter Ismaning.
For a good groundwave propagation, mast radiators should be built always on a large flat area with good ground conductivty and if possible without inclination. The construction of a mast radiator on the top of a building or a tower, whose height is in the same magnitude range as the wavelengths beeing transmitted gives a bad groundwave propagation. So mast radiators are in opposite to FM broadcasting antennas normally not installed on the top of buildings or towers. Nevertheless in rare cases mast radiators for low power trnsmissions are installed on top of buildings. So carry some lighthouses like Reykjanesviti a mast radiator for a longwave radio beacon on its roof. Also low-power broadcasting stations uses and used such arrangements. The best-known is WGSO in New Orleans. In Europe the low-power broadcasting station at Campobasso uses a mast radiator on a castle.
Fencing
Mast radiators, as with all other equipment showing over 42 volts on exposed components within 4 metres of the ground, are required to be fenced in. Sometimes wooden fences are used in order to prevent signal interference, which could occur due to currents induced by radio signals in metallic fences. If the mast radiator is mounted on the top of the helix building, which must be over four meters in height; or the mast is grounded, with the feed being located not less than four meters above the ground, a fence is not required. Nevertheless it is always recommended to fence in any mast radiator in order to prevent unauthorized climbing.
Ancillary connections
A mast radiator may need various electrical connections other than the main RF one. Such connections include static drain chokes for lightning protection, power supplies for aircraft warning lamps, and coaxial feeders for ancillary antennas mounted on the mast. A variety of techniques are used to ‘isolate’ these connections at the main RF frequency, such as chokes, parallel tuned circuits and coupling loops, on a base-fed mast. On a shunt-fed mast, where the base is grounded, no such measures are necessary.
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