A radio mast is a guyed construction. It serves to accommodate antennas for transmission purposes and also for the direct use as a transmitting antenna (mast antenna). Opposite free standing transmitting towers of steel or concrete, radio masts possess smaller establishment costs, however they vary with wind usually more than free standing constructions of same height. In addition they have a higher space requirement than free standing constructions because of the basements of guys.
A problem with radio masts is the danger of wind-conditioned oscillations. This is particularly a concern with steel tube constructions. One can reduce this by building cylindrical shock-mounts into the construction. One finds such shock-mounts, which look like cylinders thicker than the mast, for example at the radio masts of DHO38 in Saterland. There are also constructions, which consists of a free standing tower (usually from reinforced concrete), onto which a guyed radio mast is installed. The most well-known such construction is the Gerbrandy Tower in Lopik (the Netherlands). Further towers of this building method can be found near Smilde (the Netherlands) and Waldenburg (Baden-Wuerttemberg, Germany).
Mast antenna
A mast antenna is a radio mast, at which the whole construction works as an antenna. It is used frequently as a transmitting antenna for long or medium wave. Depending upon requirements, a mast antenna can be isolated against earth or grounded. In the first case the feed takes place usually at ground level, just above the insulator the mast stands on. In the latter case the transmitting power must be fed for example over the guys in a certain height into the mast. Well-known representatives of the design that are feed over the guys are the radio masts in Aholming and Donebach for the long wave transmitters of the Deutschlandfunk . Insulated mast antennas are when the transmitter is working under high electrical voltages(up to 300 kV). They must become secured therefore against unauthorized approximation. Since such masts can be very high, frequently a flight safety beaconing is necessary. This is fed over a cable, which runs through a coil, which forms with a parallel switched capacitator an parallel resonant circuit for the transmitted frequency. By this it is also possible to feed transmitting antenna for UKW, if mounted, on the mast antenna.
Alternatively it is possible to use skybeamers for illuminating the top of the construction. This method was realized at the radio mast of the Deutschlandsender Herzberg/Elster in 1939.
In order to absorb wind-conditioned oscillations, one uses sometimes cylindrical shock-mounts, which look like cylinders thicker then the mast. Such shock-mounts are used for example at the radio masts by DHO38 in Saterland. They are also used at the mast antennas of the fading reducing transmitting antennas of the medium-wave transmission facilities Hamburg Billstedt, Wolfsheim and Ismaning, which are equipped with separation insulators for multiple feeding, directly above these insulators.
The guys of radiating radio masts may not turn out with the frequency in resonance, which is radiated. They are therefore usually partitioned with insulators. Alternatively the anchoring wires can sometimes be detuned over coils connected in series to the guys or their length is selected in such a way that no resonance arises with the used transmitter frequency. Latter construction has the advantage with itself that none maintenance on difficultly reachable insulators and surge arrestors in the guys is necessary. Also plastic guys for anchoring of radiating transmitting poles were already used. However there are problems with ageing with such guys, so that its deployment is not very common.
There are also masts which are divided into several sections by means of separation insulators. They can be fed in multiple modes and make possible flat radiation pattern (decrease-reducing antenna). Such constructions are used with the transmitters in Muehlacker, Ismaning, Hamburg Billstedt and Wolfsheim (Rheinsender). As insulators for the bracing, full core insulators made of steatite are used. As isolation for the mast construction against earth hollow body from steatite is used. Since they are very sensitive, like all ceramic(s), the mast must be put on them very carefully, usually by hydraulic devices after its completation. Because against earth isolated radiating transmitting poles are under high voltage when in use, the execution of maintenance work represents a larger problem, because either the antenna must be switched off in order to allow an entrance to the mast or a special isolated platform must be used in order to put workmen on the mast. In the first case a spare antenna must be present, if no interruption of the transmit mode is allowed.
On most radiating transmitting poles, people can be present during transmitter is running, because similarly as a bird on a power line, the current flows past the body. This does not apply however to some designs of decrease-reducing transmitting antennas in which the ladder is isolated against the mast construction and between this and the mast construction is a high voltage, if not for maintenance work by suitable circuit switching measures is provided that ladder and mast construction are on same potential. However in principle a disconnection must take place, if maintenance work at insulators or in their direct proximity must be done, because there is always a high voltage when the transmitter is running. As in normal radio masts alo in mast antennas there is a ladder for ascending the construction. This can be attached either within or outside of the mast construction. In particular with steel tube masts it is usually installed in the transmission mast. As already it mentions this ladder can serve also as conductor for the supply of mast parts lying above a separation insulator, for which however an isolated assembly at the transmitting pole is necessary.
In some radio masts, for the ease of maintenance work, a lift is sometimes installed, whereby this is usually implemented for reasons of the mast statics as self-propelled climbing elevator. Such a mechanism is particularly useful with very high constructions. Usually an electrical motor is used to drive such a lift but sometimes a combustion engine is used. The current supply of the elevator is provided by a cable, which runs together with the lines for the flight safety beaconing through the coil of an anti resonant circuit for the transmitter frequency, for which between earth and transmitting pole is appropriate. There are also free standing towers, which are isolated against earth. The most well-known of them is the radio tower Berlin, which however never was used as a mast antenna. Acquaintance against earth isolated free standing transmitting towers are the Blosenbergturm in Beromuenster and the antenna towers of radio Luxembourg in Junglinster.
See also