tz invented the oscillator (an alternating- current generator) and was credited with the discovery of radio waves. Essentially, a radio wave is made of two fields: one electric and one magnetic. These two fields are perpendicular to each other, and the sum of the two fields is called the electro- magnetic field. Energy transfers back and forth from one field to the other, and this is known as oscillation. The invention of the first radio transmitter, in 1895, is cred- ited to Italian Guglielmo Marconi, who sent the first radio telegraph transmission across the English Channel in 1895 and across the Atlantic Ocean in 1901. In 1896, Marconi submitted an application for a patent for the world's first wireless telegraph using Hertzian waves. The use of public radio began in 1907. Since the beginning of radio communications, there have been two key issues to address, and they are still the main goals of the industry: expanding capacity and maintaining quality. Radio waves are classified by their frequency, which describes the number of times a signal cycles per second, commonly referred to as Hertz (Hz), in honor of Heinrich Hertz. The wavelength is the distance between repeating units of a wave pattern (see Figure 13.1). In a sine wave, the wavelength is the distance between any point on a wave and the corresponding point on the next wave in the wave train. As shown in Table 13.1, there is an inverse relationship between frequency and wavelength: As the frequency increases, the wavelength decreases. There are performance differences between radio frequencies. Low frequencies can travel much further without losing power (i.e., attenuating), but they carry much less information because the bandwidth (i.e., the difference between the high- est and lowest frequency carried in the band) is much lower. High frequencies (those in the HF band, from 3MHz to 30MHz) offer much greater bandwidth than lower frequencies, but they are greatly affected by interference from a variety of sources. Very high frequencies (those in the SH