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Introduction to Radio Waves and Radio Engineering
Electromagnetic waves propagate in a vacuum with the speed of light, c = 299,792,458 m/s or about 3 × 108 m/s. The electric and magnetic fields of a plane wave oscillate in phase and are perpendicular to each other and to the direction of propagation. The frequency of oscillation is f , and the wavelength is l = c /f . Electromagnetic waves also may be considered to behave like particles of zero rest mass. The radiation consists of quanta, photons that have an energy of W = hf where h = 6.6256 × 10−34 Js is Planck’s constant.
There are many sources of electromagnetic radiation. Accelerating charges produce electromagnetic radiation, as when charges decelerating in an electric field produce bremsstrahlung and charges orbiting in a magnetic field produce synchrotron radiation. The random thermal motion of charged particles in matter produces thermal radiation. Atoms and molecules emit spectral line radiation as their energy level changes. The radiation generated by oscillators and emitted by antennas is based on high-frequency alternating currents.
1.1Radio 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
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2 Radio Engineering for Wireless Communication and Sensor Applications
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 × 10−3 eV (1 eV = 1.6 × 10−19 Ws = 1.6 × 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
Name of Frequency Range and Abbreviation |
Frequencies |
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Very low frequency (VLF) |
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3–30 kHz |
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Low frequency (LF) |
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30–300 kHz |
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Medium frequency (MF) |
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300–3,000 kHz |
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High frequency (HF) |
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3–30 MHz |
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Very high frequency (VHF) |
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30–300 MHz |
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Ultrahigh frequency (UHF) |
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300–3,000 MHz |
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Superhigh frequency (SHF) |
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3–30 GHz |
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Extremely high frequency (EHF) |
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30–300 GHz |
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Table 1.2 |
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Frequency Bands of Microwaves |
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Band |
Frequencies |
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L |
1–2 GHz |
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S |
2–4 GHz |
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C |
4–8 GHz |
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X |
8–12 GHz |
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Ku |
12–18 GHz |
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K |
18–26 GHz |
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Ka |
26–40 GHz |
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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.
1.2 What Is Radio Engineering?
Radio engineering covers activities that use the possibilities offered by radio waves to serve the various goals of people. Some of these useful activities are:
•Broadcasting;
•Fixed communication (e.g., fixed radio links);
•Mobile communication;
•Radionavigation;
•Radiolocation (e.g., many radar applications);
•Amateur radio;
•Radio astronomy.
Radio engineering also covers the techniques needed to produce, process, investigate, measure, and use radio waves that make these services possible.
Electrical circuits and devices, in which the finite propagation speed of electric fields has to be taken into account or whose dimensions are of the same order as a wavelength, often are considered to belong to the field of radio engineering.
1.3 Allocation of Radio Frequencies
Radio waves have many applications and many users. However, the radiofrequency spectrum is a limited natural resource. Harmful interference
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between users would take place if everybody sent signals at will. Therefore, the use of radio frequencies for different applications has been coordinated internationally.
The International Telecommunication Union (ITU) was reorganized in 1993. The ITU Radiocommunication Sector (ITU-R) comprises the former Comite´ Consultatif International des Radiocommunications (CCIR) and International Frequency Registration Board (IFRB), and is responsible for all of the ITU’s work in the field of radiocommunication. The mission of ITU-R is to ensure rational, equitable, efficient, and economical use of the radio-frequency spectrum by all radiocommunication services, and to carry out studies and adopt recommendations on radiocommunication matters. Technical matters are drafted in ITU-R study groups and confirmed in
World Radiocommunication Conferences (WRCs) every second or third year. The use of the radio-frequency spectrum is regulated in the Radio Regulations [1], which are updated according to the decisions made by WRCs.
In most applications, the use of radio frequencies cannot cause interference worldwide. For example, microwaves cannot propagate far beyond the horizon. For the allocation of frequencies, the world has been divided into three regions, as shown in Figure 1.2. For example, Region 1 includes Europe, Russia, Africa, the Middle East, and parts of Asia.
The radio-frequency spectrum is allocated for about 40 radio services in the Radio Regulations. Table 1.3 is an extract of the table of frequency allocation [1] and shows the use of frequency band 10 to 10.7 GHz for
Figure 1.2 Division of world in three regions for frequency allocation. (After: [1].)