Radio Broadcast (Nov 1926-Apr 1927)

Record Details:

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FEBRUARY, 1927 LINKING CONTINENTS WITH TWENTY KILOWATTS 353 system, so as to insure that the phase of the oscillations in all the wires is the same. It has been proved by calculations, and confirmed by experiments, that the directional effect of such an arrangement is a function of its dimensions relative to the wavelengths utilized. A similar system of antennas and reflecting wires is used at the receiving stations. THE BEAM ANTENNA SYSTEM IN A typical short-wave beam station, the Bodmin station, for example, there are five steel lattice masts, each 277 feet high, erected in a straight line at right angles to a line passing through both sending and receiving station. These five masts provide four spans of wire (one between each pair of masts) the sole purpose of which is to support the vertical antenna and reflecting wires. Except as supports, the horizontal spans serve no useful purpose in the actual transmission. At each station there are two distinct transmitters for each point it is intended to communicate with, operating on different frequencies, but whether the second one is to be used as a standby or as a supplementary channel has not as yet been definitely decided. Of the two transmitters, one operates on 11,500 kc. (26.09 meters), while the second will operate on a slightly higher frequency. Cross-arms at the top of the masts extend for forty-five feet on either side of the vertical, forming a support for the horizontal sustaining wires. For the 11,500-kc. band, there are thirty-two vertical antenna wires, grouped in fours, in a parallel row with which are the reflector wires. There are twice as many reflector wires as antenna wires. The horizontal wire spans between the first second and third masts (two spans) support the antenna-reflector system for the 11,500-kc. transmitter. Each of the reflector wires is divided into five complete sections by means of insulators. The reflector wires are placed on that side of the actual antenna wires which is remote from the distant receiving station. Counterweights are attached to the lower ends of both antenna and reflector wires, the object of these being to keep an equal tension on each wire irrespective of changes in wind pressure. The system of wires which constitutes the connecting link between the transmitter and the antenna is known as the "feeder system." This system consists of two concentric copper tubes, air insulated from each other to avoid loss. The outer tube is grounded and carried on metal standards a short distance above the ground, while the inner tube carries the current to the antenna. In order to insure an equal amount of current for each of the separate antenna wires, the feeder system is arranged so that the distance which the current has to travel through the feeders is exactly the same for each individual wire in the entire antenna system. In order to prevent the presence of reflected waves in the feeder system, which would cause trouble, equalization may be obtained by means of coupling transformers located at each junction box. A check against any reflected wave in the feeder system is provided by three highfrequency thermo-ammeters wired at three different points 32 feet apart. Actually the three meters are located at one point for ease in reading. With no reflected waves in the system, the meters will all register alike. Each antenna coupling box is grounded by means of metal plates three feet square, arranged in a circle of 50 feet in diameter. This is for the short-wave antenna. For the long-wave antenna the diameter of the circle is 100 feet. Each transmitter is grounded near the building with a galvanized iron plate six feet by three feet connected to a copper tray placed under the transmitter proper by means of heavy copper bars. All masts and guy wires are grounded, as is the support for the feeder system. The transmitting apparatus follows closely that used in standard practice.