fluorescent screen in the receiver. The resulting light and dark areas in the screen are integrated into a complete
picture by the eye. This integration is made possible by the characteristic known as persistence of vision. As in
fax transmission, synchronization of the receiving system by the transmitting system is essential for accurate
reproduction of the received image.
4-3. ELEMENTS OF RADIO TRANSMISSION
The radiating effect caused by a pebble dropped into a quiet pool of water illustrates the action of radio
waves. The effect of the pebble's impact is distributed to all parts of the pool by waves which radiate in circles
from the spot where the pebble entered. These waves crest and fall at the same rate throughout their travel time.
Radio waves from a vertical antenna act in much the same manner. Most of the energy is radiated along the
ground (groundwaves), but some of it is radiated at varying angles skyward (skywaves). However, virtually none
is radiated directly above the axis of the antenna because of the nature of the radiation pattern. The characteristics
of radio waves are as follows:
a. Speed, or velocity, is approximately 300,000,000 meters (186,000 miles) per second, the speed of light.
b. The wavelengths of radio waves are expressed in meters or feet. The distance that the leading part of one
wave has traveled when the next wave starts is the wavelength. In figure 4-2, this is indicated as the distance
between the crest of the first wave and the crest of the next wave.
Figure 4-2. Relationship of frequency, wavelength, speed,
and amplitude of- a radio wave.
c. Frequency of radio waves is expressed in Hz, kilohertz (kHz), or megahertz (MHz), and is the actual
number of waves transmitted or received in 1 second. A hertz is equal to one wave in 1 second, a kHz is equal to
1,000 Hz, and a MHz is equal to 1,000,000 Hz.
d. The important relationship of speed, frequency, and wavelength is shown by the formula: