Velocity

Radio waves, like all other electromagnetic radiation, travel nearly 300,000 km (186,400 mi)

Table 21.1

The Electromagnetic Spectrum

Radiation Frequency Wavelength

X-ray 3 × 105 THz and higher 10 Å and shorter Ultraviolet 800 THz - 3 × 105 THz 4000 - 10 Å

Visible light 400 THz - 800 THz 8000 - 4000 Å

Infrared 300 GHz - 400 THz 1 mm - .0008 mm

Radio 10 kHz - 300 GHz 30,000 km - 1 mm

Fig 21.1—Electric and magnetic field components of the electromagnetic wave. The polarization of a radio wave is the same direction as the plane of its electric field.

per second in a vacuum. Radio waves travel more slowly through any other medium. The decrease in speed through the atmosphere is so slight that it is usually ignored, but sometimes even this small difference is significant. The speed of a radio wave in a piece of wire, by contrast, is about 95% that of free space, and the speed can be even slower in other media.

The speed of a radio wave is always the product of wavelength and frequency, whatever the me- dium. That relationship can be stated simply as:

c = f λ

where

c = speed in m/s

f = frequency in hertz

λ = wavelength in m

The wavelength (λ) of any radio frequency

can be determined from this simple formula. In free space, where the speed is 3 × 108 m/s, the wavelength of a 30-MHz radio signal is thus 10 m. Wavelength decreases in other media because the propagating speed is slower. In a piece of wire, the wavelength of a 30-MHz signal shortens to about 9.5 m. This factor must be taken into consideration in antenna designs and other applications.