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636 RICHARDSON'S BLUEBOOK OF PROJECTION
quency modulation. With such modulation the antenna current is all of the same volume (as shown at the left of Figure 225) while the sound current superimposed upon the carrier is represented by changes in the radio frequency.
(9) Radio frequencies of less than 100 kilocycles require great power to make them radiate over long distances. Radio frequencies from 100 kilocycles up to several thousand kilocycles require progressively less power to radiate to great distances. All these frequencies are reflected by the ionized layer of the upper air, called the ionosphere or Heaviside layer, and are thus enabled to follow the curvature of the earth. Beginning about 3,000 kilocycles, roughly speaking, radio frequencies are not evenly reflected by the ionosphere, and tend to be received only at "skip distances." Still higher frequencies, such as those used for television transmission, often are not reflected at all by the ionosphere but penetrate through it and continue on into outer space. Such frequencies cannot be transmitted beyond the horizon, since the fact that the ionosphere does not reflect them makes it impossible for them to follow the curvature of the earth. This is the reason why radio transmission of television signals, which utilizes carrier frequencies of 10,000 kilocycles or more, is limited to distances within the horizon. By placing the transmitting antenna on a tall hill, or at the top of a tall building, the horizon can of course be widened, to a radius of possibly fifty miles in practice. "Freak" transmissions of several hundred miles have taken place with these "ultra" frequencies but are not common and cannot be relied upon for any commercial purpose.
Television Transmission
(10) Television transmission is commonly transmitted by means of radio frequency carriers, essentially in the same way as sound broadcasting. However, the television signal is different in nature from the sound signal, requiring a higher carrier frequency.
(11) The sound signal, in radio broadcasting, is gen