Svar till: Supporting Masts Ground Grid Connection

[AlexanderSAQ]

Dear Marcus,
Here are some details of the antenna:

The top lines
The antenna current is distributed over the antenna via eight top lines. These go from the tuning coil at the station building up to the cross arm of the first tower, then under the cross arms of the other towers, and finally down to the tuning coil at the last tower. The material in the top lines is phosphor bronze, which is sufficiently tensile to withstand spans as long as 380 meters. The top lines are suspended in 2.9 m long insulators.
(Originally there were twelve top lines. These were equipped with so-called break couplings, which had a dual purpose. They would break and let the lines fall down in the event of a large ice load or strong winds, partly to protect the lines from breakage, but also to protect the towers from extreme loads. However, the breakage couplings broke awkwardly often, and after a major cable race in the late 1960’s it was decided to remove four of the top lines, thus reducing the load on the towers so that the breakage couplings were not needed to protect them, and the breakage couplings could be replaced with fixed suspension. the problems with the top lines have been greatly reduced since then.)

Radiant parts of the antenna
Only vertical polarization works for long-distance connections. Therefore, useful radiation occurs only from elements that have a vertical component. From each of the intermediate towers, a conductor goes down to the ground almost vertically, and it is from these conductors that the radio radiation takes place. In addition, the ups and downs at the end towers are radiating. Between the towers, the top lines have no radiant function. On the other hand, the top lines act as capacitances in the oscillation circuits of the antenna.

The oscillation circuit
A capacitance and an inductance together form an oscillation circuit with a resonant frequency which is determined by the magnitude of the capacitance and the inductance. The capacitance in Grimeton’s antenna is formed by the top lines that make up one “plate” in the capacitance and the ground plane that make up the other “plate”. The capacitance of the antenna is about 47 nF. The inductance is mainly formed by the six tuning coils. The resonant frequency, i.e. the desired transmission frequency 17,200 Hz, is affected by adding on more or fewer turns on one or more of the tuning coils. Fine adjustment takes place with the variometer (variable inductance) inside the station building.

Tuning coils
The six tuning coils have a considerable format, about 1.8 m high and 2.7 m in diameter. They are set up on concrete foundations. The inductance of each coil is about 0.01 Henry.

Ground line network
An important part of the antenna is located below the earth’s surface. Along the entire length of the antenna, copper conductors are buried across the longitudinal direction of the antenna. These conductors reach about 250 m on each side of the antenna centerline. The distance between the copper conductors is normally about 6 m. Near the towers and the tuning coils they are denser. The copper conductors are interconnected about 100 meters from and on each side of the center line.

Balance network
To achieve good ground contact, each tuning coil is connected to the ground line network in 14 points. This is done via the so-called the balance network. This consists of conductors that are hung in wooden poles about 4 meters above the ground. The balance network branches out from the tuning coil to seven connection points on each side of resp. tower. The connection points are located 100 meters from the center line of the antenna and the distance between the connection points is approximately 54 meters. Thus, the ground current is evenly distributed to the ground line network along the entire length of the antenna.

I hope this will give you an idea of how the antenna works.

Fredrik/AlexanderSAQ