The lower part of the mast is also surrounded by a cage of six ropes extending from the bottom of the mast to the feeding point at 90 m. At a certain height of the mast the cage rope and the mast are connected by horizontal ropes forming a short circuit for the "coaxial line" with the mast as inner conductor and the rope cage as outer conductor. The length of this line determines the reactance which occurs at the "input" of the line at the foot of the mast.
By adjusting the height of the short circuit it is possible to modify the height of the null of the current distribution on the radiator. In order to realize this for two frequencies it was necessary to put an additional tuning network in parallel to the reactance of the rope cage. This network allows to tune every reactance necessary for the frequencies.
The guy ropes of a mast are usually galvanized steel ropes which are interrupted by insulators. The spacing between two adjacent insulators is much shorter than the length of resonance. Hence the current on the guy is small and the RF voltages across the insulators can be insulated without difficulties. But during a thunder storm an electrostatic field strength occurs and the guy insulators have to insulate additional electrostatic voltages. The electrostatic field during thunder storms which is homogeneous when no conducting elements are present, could lead to field strengths greater than 10 kV/m. In the vicinity of the mast the field strength is very inhomogeneous with high field strength close to the mast.
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A flash over an insulator, which is caused by high differencesof electrostatic potentials along the guy may result in a lasting arc, which is fed by the RF energy of the transmitter. This effect can be avoided by grounding the guy ropes electrostatically. Only two insulators, one at the top close to the mast, and one at the bottom close to the guy foundation are used.
The lower insulator is bridged by a coil, which grounds the guy statically. Since the mast is also electrostatically grounded, no electrostatic voltages can occur across the insulators.
The value of the reactance, which bridges the bottom insulator, is chosen to obtain small currents on the guy rope. Small currents on the guys do not influence the horizontal and vertical patterns.
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As two frequencies are radiated from the antenna, it is necessary to have a tuning network, which provides reactances which can not be obtained by one coil only for two frequencies. At the same time the network must ground the guy rope statically.
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