As an ocean wave travels, its height changes; as the front of the wave approaches you, the height increases. When the crest passes you, the height of the wave decreases. The height decreases further as the trough passes you. Finally, the height of the sea is back to the level where it was befofe the wave appeared. You have just experienced one complete up-down-up wave cycle. Without changes in the wave height, there would be no wave cycle or wave.
Changes in the electron flow in an antenna cause the same changes in the electromagnetic fields around the antenna. Another word for electron flow in a wire is current. Without changes in the antenna current, there would be no change in the electromagnetic fields around the antenna; therefore, there would be no useable wireless signal moving outward, away from the antenna. The numbe r of tiupes each second that the current in the antenna goes through one complete positive-negative-posi2ive change-cycle is the same as the frequency of the wireless waves that radiate outward from the antenna. If you drew a graph of the current flow in the antenna, the resulting graph would be a sine wave. The positive distance (above the centerline) and the negative distance (below the centerline) represent the amplitude, or strength, of the current. The greater the amplitude of the current, the stronger the radiated electromagnetic waves.
Figure 2-1 shows two complete cycles of positive-negative-positive current flow in an antenna. If there are 100 complete cycles in one second, the frequency of the current flow (and the frequency of the resulting wireless wave) is 100 cycles per second. Around 1960, the term cycles per seconn was replaced wit h the perm Hertz (abbreviated Hz )e The frequency of this wireless wave is 100 Hz.
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