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The Antenna System

Sixty-two rigid tests were made to find a location which would be adaptable to favorable broadcasting before Saxonony was definitely selected. These tests were made to determine the proper altitude with no interference from metal construction, free from trees so as to prevent undue interference and ample area for erection of the necessary structures and antennas.

The site at the new transmitter occupies 130 acres. The long-wave antenna over which KDKA broadcasts consists of a circular arrangement of poles surrounding about eight acres. The poles are 100 feet above the ground and painted, according to law, yellow and black. Ariel hazard beacons also have been erected to serve as a warning to aviators.

The short-wave antenna system at the Saxonburg site. Through station W8XK programs are regularly sent to listeners in foreign countries

A much smaller space is required for the short-wave transmitter of W8XK. It is through the short-wave station that programs are sent great distances for reception in far away lands.

The functioning of this antenna is based on the fact that local reception is due to radiation horizontally outward from the antenna that is, the ground wave; whereas distant reception depends considerably on radiation initially at an angle of elevation, due to the wave front descending as it moves outward from the Heaviside layer. The descent of the wave front as it travels outward is due to the fact that the waves travel slower at the surfaces of the earth than in the upper atmosphere; and the reflections from the Heaviside layer are caused by the ionized condition of the layer which makes it conduction, i.e., like a short-circuit on the end of a transmission line, except that the lines of force are not guided by wires, and hence follow the ordinary law of equal angle or incidence and reflection. There is really no line of demarcation between the ground wave and the upward radiation, conveniently called the sky wave. The part of the wave which reaches the receiving antenna by moving out horizontally from the antenna or by the descent of the wave front, is what is usually meant by the ground wave, and the part which reaches the receiving antenna by reflection downward from the ionized layer, the sky wave. Since the ground wave is attenuated with comparative rapidity, the distant signals are carried largely by the sky wave, and if the ratio of horizontal radiation to sky-wave radiation can be decreased, the desired ratio of local or distant signal strength should be attained.

This decreased ratio of ground to sky wave radiation is obtained in the new KDKA antenna by using an arrangement and spacing of vertical antennas fed with currents in time phase, such that interference between waves from the various antenna reduces the horizontal field strength in comparison to the field strength at an angle of elevation. The interference of the arrangement of vertical antennas will make the horizontal radiation much less than for a single antenna with the same total current; it will make radiation at an upward angle decreasingly less as the angle is increased, so that at high angles the radiation is only slightly less than for a single pole. This great decrease in horizontal radiation will decrease the radiation resistance of the antenna system, and so will cause the current to increase until the total power radiated is the same. This will make the upward radiation greater, and the horizontal radiation less than for a single antenna with the same power radiation.

In its physical arrangement the new KDKA antenna differs considerably from usual transmitting antennas. The usual antenna has two or more fabricated steel tower several hundred feet high, with a concentrated flat top suspended between them, and a single vertical down lead usually near the center. To cut down absorption losses, the steel towers are usually detuned and kept as far as possible from the flat top and down lead, and are sometimes insulated from the ground or even broken into insulated sections. In contrast to this the new KDKA antenna has eight wood poles about 100 feet high, spaced on a circuit more than 700 feet in diameter and having a vertical down lead at each of the poles, the cage top being suspended between adjacent poles to form a complete circle around the poles. The proper time phase of the currents in the eight vertical leads is obtained by running the transmission line to the center of the system and tuning individual feeder lines to each pole.

Since the factors governing radio transmission are so many and variable that satisfactory formulas, applicable winter and summer have not been developed even for the simplest antenna, it cannot be expected that reception will be exactly as calculated. It is possible that in certain localities absorption and minor reflections will entirely over-balance the effect produced by the antenna. design. Also while transmission by means of the sky-wave works well at night, it is usually not satisfactory in the daytime. Therefore, for daytime transmission it would be preferable to get the opposite of the effect described above, and to broadcast to a more limited area by means of the ground wave. The opposite effect, suppression of sky-wave and reinforcement of ground wave, can be produced by making currents in opposite poles at 180 degrees out of phase, so that adjacent poles are 45 degrees out of phase. The difference in phase can be obtained by making the feeder lines of different lengths or by artificial lines. Thus, when conditions are not favorable for transmission by reflections of the sky-wave, it is possible to change over by simply changing the feeder line connections, to an antenna arrangement suitable for ground-wave transmission.

 

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