Hem SAQ Forum SAQ Technical Discussions Supporting Masts Ground Grid Connection

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  • #9739 Svara

    Do the 127 m tall antenna support masts connect to the grid of copper wires in the ground?

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    • #10065 Svara
      Karl-Arne Markström

      I can say for sure that the towers are indeed connected to the ground or counterpoise wires.

      The reason is that I in the late-90s designed the 518 kHz NAVTEX antenna which uses the third tower from the transmitter as a 1/4 wave vertical radiator, creating
      a folded monopole antenna.

      The design and implementation was made in cooperation with Bengt Dagås, the station manager and chief engineer, and worked exactly as predicted.
      In a small hut at the tower base, which originally housed a mid-50s FM broadcast transmitter, two Nautel 1 kW solid-state MFtransmitters are located.

    • #9770 Svara
      Stefan Johansson

      The answer is in fact aviable at The Alexander association website however only in swedish.

      After having selected using the swedish language website then in the “Grimeton Radiostation” menu under
      the entry “Så fungerar Alexanderson-sändaren” and at the end there is an excerpt from the DVD-movie
      “SAQ, Varbergs Radiostation – Grimeton”. This excerpt contains many details that are very useful
      for anyone that is interested in the Alexanderson antenna!

      At about 11:40 is the following sentense: “längs symetriaxelns löper en 6,5 mm koppartåd som förbinder
      trådarna med varandra och tornens stålkonstruktioner”

      This can be translated as that the steel towers are electricaly connected to the ground network.

      After having selected using the english language website then in the “Grimeton Radiostation” menu under
      the entry “Make a virtual visit to SAQ – Grimeton” and at the end there is “– Antenna tower no 1 from below.”
      Zooming in on the tower leg that is closest to the transformer building and power distribution substation
      there is visible a metall ribbon bolted to the tower leg and that likely extends into the ground.

      From https://de.wikipedia.org/wiki/Alexanderson-Antenne
      “Sie besteht normalerweise aus geerdeten Türmen”

      This can be translated as that the towers are grounded.

      From https://sv.wikipedia.org/wiki/Radiostationen_i_Grimeton
      “Denna antennkombination är i praktiken rundstrålande, eftersom den trots sin längd på 2,2 km är kort i
      förhållande till våglängden. Till någon del gynnas ändå den nordvästliga – sydostliga riktning som raden
      av torn vänder sin bredsida mot i och med att strålningsdiagrammet är svagt elliptiskt.”

      However the above text is missing in the english language wikipedia article.
      The the text can be translated as that the antenna radiation pattern is weakly elleptical.

      A few of the many other useful references aviable:
      Patent US1360167 efw alexanderson. – antenna. – application filed sept. 13, 1917. (Use Google!)

      “The Electrical Plant of Transocean Radio Telegraphy”

      “The alexanderson 200 kw high-frequency alternator transmitter”

      Obviously antenna modeling and simulation software was not aviable
      to Alexanderson when he filed the patent in 1917. However the design
      with the grounded towers apears to have been invented after 1917.

      From https://en.wikipedia.org/wiki/T-antenna
      “The antenna was used in the powerful radio stations of the wireless telegraphy era
      but has fallen out of favor due to the expense of multiple loading coils.”

      Umbrella antenna:

      References possibly useful for modeling and simulation:

      “ELF/VLF/LF Radio Propagation and Systems Aspects” (Use Google to find it!)

      Best Regards,
      Stefan Johansson

    • #9769 Svara

      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.


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