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KDKA Builds for Tomorrow

Anticipating the continuing and increasing rapid growth of broadcasting service, KDKA has recently built a new 400,000-watt transmitter at Saxonburg, PA. The new station has been operating experimentally between 1 a.m. and 6 a.m. For normal operations, however, the power will be held at 50,000-watts as required by the Federal Radio Commission.

The new AW-220 tube is 72 inches in height, has a diameter of 8 inches and weighs 60 pounds. It has a capacity of 200 kilowatts and is water-cooled.

One of the most important innovations is the new tube with a capacity of 200 kilowatts. This tube not mearly and enlarged edition of a smaller tube, but is thoroughly engineered as tube of larger size and of a distinctly novel design. The mere building of a larger tube following the style of the smaller type would not produce the results desired. The quality of output and the life of such a tube would not be up to the desired standard.

The new tube, called the AW-220, is 72 inches in height, has a diameter of eight inches and weighs 60 pounds. Two of the new tubes are incorporated in the transmitter.

In its design engineers found one of their greatest problems to be that of cooling the grid. This difficulty has been solved by I.E. Mourmomtseff, company research engineer, who has produced a water cooled tube of great mechanical strength and sturdiness embodying a double end construction.

A modern water softening System has been installed for use in connection with the cooling system for these gigantic vacuum tubes. There is a practical reason for this installation because these giant tubes are expensive and unless a properly protected their useful life period is limited. Therefore anything that can be done to increase their life constitutes a saving in costly replacements.

The power stage equipment of the Saxonburg transmitter and from this panel energy goes directly to the huge spray type antenna designed by Dr. Frank Conrad. Two of the new tubes are shown set up in their tube racks. The large plates at the right and the coil at the top handle the enormous radio frequency energy generated at this point. They compose the tank condenser of this transmitter and are made of aluminum.

According to E.B. Landon, chef operator of the transmitting station, the water must be tested frequently to protect the tubes and the cooling system. Ordinary city water cannot be used because it contains lime and other minerals which form harmful scale in the coils. At the KDKA station this ultra-soft water is pumped to an outdoor cooling pool before going to the cooling coils and the tubes. Hot water coming from the station circulatory system enters the pool through a fountain-like spray. After cooling in the pool, it is pumped through the same circuit again.

An idea of the capacity of the cooling problem may be obtained from the statement that approximately five tons of cooling water must be passed through the water jacket of the cube each hour it is in operation. This water cools the tube in the same manner as water in an automobile cools the motor. One hour's operation of the tube would heat enough water to supply the domestic requirements of the average home for several weeks.

While of course these AW-200 tubes will only be used to generate high frequency power for radio stations, an appreciation of the power capacity of one of these tubes can best be gained by a comparing familiar household devices. For example, a similar amount of power of the kind distributed commercially would operate simultaneously four hundred toasters or flat irons. This would also be the equivalent of power required to light one thousand average homes of five to six rooms or the energy to operate two modern street cars.

The antenna system of the new KDKA radio transmitting station, conceived by Dr. Conrad, is designed to make the local signal strength less than it would be for the conventional type of antenna, and to make the distant signal strength greater than for the conventional antenna.

This is desirable from the standpoint of both listener and broadcaster; it makes the signal less objectionably loud to local listeners, and yet gives better reception to distant listeners; and it allows the broadcaster to use the radiated energy to better advantage in reaching distant points, since less energy is absorbed by objects in the local area.

The Antenna System


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