TABLE OF CONTENTS One of the things that fascinated people about radio from its very earliest days is that signals arrive seemingly by magic through the air from long distances. It was very quickly discovered that radio signals are electromagnetic waves exactly like light and infrared, except for frequency and wavelength. Radiowaves have a much lower frequency than light; therefore, the wavelengths are much longer. The wavelengths of radio signals range from around 25,000 m at the low-VLF range down to millimeters in the upper microwave spectrum.
Consider three different forms of signals: continuous wave (CW), amplitude modulation (AM), and frequency modulation (FM). The CW signal is only considered very briefly because it is largely irrelevant to most readers of this book. The CW signal (Fig. 5-1) is made up of sinusoidal oscillations at the transmitter frequency. For example, a 500-kHz maritime signal oscillates 500,000 times per second. The critical point about the CW signal is that it has a constant amplitude while it is on. If the signal is turned on and off, to form dots and dashes of the Morse code, it is possible to send messages with the CW signal. This type of signal was transmitted by ships at sea during the early days of wireless and indeed maritime radiotelegraph CW is still heard on the air today (even though less and less The U.S. Coast Guard has stopped monitoring 500 kHz).
Modulation is the act of adding information to an unmodulated radio signal...
