Radio Waves as a Part of the Electromagnetic Spectrum

Electromagnetic waves cover a wide range of frequencies or wavelengths, as shown in Figure 1.1. The classification is based mainly on the sources of

Gamma rays

Xrays f/Hz

10"

Ultraviolet

Visible light <

Infrared

Submm waves 10 ~

Millimeter waves

Microwaves

Radio waves <

RF waves

io3H

10"

10"

Figure 1.1 Electromagnetic spectrum.

radiation. Boundaries of the ranges are not sharp, since different sources may produce waves in overlapping ranges of frequencies. The wavelengths of radio waves range from thousands of kilometers down to 0.1 mm. The frequency range is from a few hertz up to 3 THz. The waves having shorter wavelengths or higher frequencies than radio waves are classified as infrared, visible light, ultraviolet, x-rays, and gamma rays. Infrared waves are produced by molecules and hot bodies, light and ultraviolet waves by atoms and molecules, and x-rays by the inner electrons in atoms. Commercial x-ray tubes emit bremsstrahlung. Gamma rays originate in the nuclei of atoms and overlap the upper part of the x-ray spectrum.

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The spectrum of radio waves is divided into ranges having a width of one decade, as indicated in Table 1.1 and Figure 1.1. Waves below 300 MHz are often called radio frequency (RF) waves. Ultrahigh frequency (UHF) and superhigh frequency (SHF) waves (300 MHz to 30 GHz) are called microwaves. Often the boundary between RF waves and microwaves is set to 1 GHz. The microwave range is further subdivided into bands according to waveguide bands, as shown in Table 1.2. Extremely high frequency (EHF) range is called the millimeter-wave range and the frequency range from 300 GHz to 3,000 GHz the submillimeter-wave range.

The interaction of electromagnetic waves with matter depends on the energy of photons. In general, shorter waves corresponding to energetic photons interact more strongly than longer waves. The photons of radio waves have low energies; for example, at 1,000 GHz the energy is only 4 X 10"3 eV (1 eV = 1.6 X 10"19 Ws = 1.6 X 10"19 J). The energy needed to ionize molecules in biological tissue is at least 12 eV. Thus, ultraviolet

Table 1.1

Ranges of Radio Waves

Table 1.1

Ranges of Radio Waves

Name of Frequency Range and Abbreviation

Frequencies

Very low frequency (VLF)

3-30 kHz

Low frequency (LF)

30-300 kHz

Medium frequency (MF)

300-3,000 kHz

High frequency (HF)

3-30 MHz

Very high frequency (VHF)

30-300 MHz

Ultrahigh frequency (UHF)

300-3,000 MHz

Superhigh frequency (SHF)

3-30 GHz

Extremely high frequency (EHF)

Frequency Bands of Microwaves

Table 1.2

Frequency Bands of Microwaves

Band

Frequencies

L

1-2 GHz

S

2-4 GHz

C

4-8 GHz

X

8-12 GHz

Ku

12-18 GHz

K

18-26 GHz

Ka

26-40 GHz

and radiation having even shorter wavelengths can ionize and dissociate molecules of biological tissues. Radio waves can only heat these materials. For example, water molecules are polar, and an electric field turns them back and forth, thus warming the food in a microwave oven.

Human beings gather a lot of information through electromagnetic waves. The retina of our eyes is sensitive to visible light, that is, wavelengths from 380 nm to 780 nm. The human skin can sense infrared or thermal radiation. Other parts of the spectrum cannot be sensed directly; they require their own specialized techniques to make the information carried by electromagnetic waves detectable. This book deals with the basic physics of radio waves and the techniques, which are needed to generate, transmit, and detect radio waves.

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