Radio Waves and Radio Engineering

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 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.

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 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.

Radio waves have many applications and many users. However, the radiofrequency spectrum is a limited natural resource. Harmful interference 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 primary and secondary services (regional limitations and secondary services are shown in parentheses). 

In addition to the frequency allocation, all radio and other electrical equipment must comply with the electromagnetic compatibility (EMC) requirements and standards to assure interference-free operation [2]. Standards set limits to the emission of equipment and give requirements for their immunity against interference.

The Scottish physicist and mathematician James Clerk Maxwell (1831–1879) predicted the existence of electromagnetic waves. He combined Gauss’ law for electric and magnetic fields, Ampe`re’s law for magnetic fields, and the Faraday-Henry law of electromagnetic induction, and added displacement current to Ampe`re’s law. He formulated a set of equations, which he published in 1864. These equations showed the interrelation of electric and magnetic fields. Maxwell proposed that visible light is formed of electromagnetic vibrations and that electromagnetic waves of other wavelengths propagating with the speed of light were possible.

The German physicist Heinrich Hertz (1857–1894) was the first to prove experimentally the existence of radio waves, thus verifying Maxwell’s equations [3]. In 1888, he released the results of his first experiments. The transmitter was an end-loaded dipole antenna with a spark gap. A current oscillating back and forth was produced as the charged antenna was discharged across the spark gap. The receiver consisted of a loop antenna and a spark gap. With this apparatus operating at about 50 MHz, Hertz was able to show that there are radio waves. Later he showed the reflection, diffraction, and polarization of radio waves, and he measured the wavelength from an interference pattern of radio waves.

The first person to use radio waves for communication was the Italian inventor Guglielmo Marconi (1874–1937). He made experiments in 1895 and submitted his patent application ‘‘Improvements in transmitting electrical impulses and signals and in apparatus therefor’’ in England in 1896. In 1901, Marconi, using his wireless telegraph, succeeded in sending the letter S in Morse code from Poldhu in Cornwall across the Atlantic to St. Johns in Newfoundland. Because the distance was over 3,000 km, this experiment demonstrated that radio waves could be sent beyond the horizon, contrary to the common belief of that time. The Russian physicist Alexander Popov (1859–1906) made experiments nearly simultaneously with Marconi. He demonstrated his apparatus in 1896 to a scientific audience in St. Petersburg.

Hertz used a spark gap between antenna terminals as a receiver. In 1891, the French physicist Edouard Branly (1846–1940) published a better detector, a coherer. It was based on the properties of small metal particles between two electrodes in an evacuated glass tube. Both Marconi and Popov used coherers in their early experiments. The invention of vacuum tubes was a great step forward toward better transmitters and receivers. In 1904, the British physicist John Ambrose Fleming (1849–1945) invented the rectifying vacuum tube, the diode. In 1906 the American inventor Lee De Forest (1873–1961) added a third electrode, called a grid, and thereby invented the triode. The grid controlled the current and made amplification possible. The efficiency of the electron tubes was greatly improved by using concentric cylinders as electrodes. One of the first inventors was the Finnish engineer Eric Tigerstedt (1886–1925), who filed his patent application for such a triode in 1914.

De Forest and the American engineer and inventor Edwin Armstrong (1890–1954) independently discovered regenerative feedback in 1912. This phenomenon was used to produce a continuous carrier wave, which could be modulated by a voice signal. Armstrong invented also the superheterodyne receiver. These techniques made broadcasting possible. AM stations began broadcasting in 1919 and 1920. Regular TV transmissions started in Germany in 1935. Armstrong’s third great broadcasting invention was FM radio, but FM broadcasting was accepted not until after World War II.

 Communication was not the only application of radio waves. Karl Jansky (1905–1950), while studying radio noise at Bell Labs in 1932, detected a steady hiss from our own galaxy, the Milky Way. This was the beginning of radio astronomy. The invention of microwave tubes, of klystron in 1939, and of magnetron in 1940 was essential for the development of microwave radar during World War II. The principle of radar had been introduced much earlier by the German engineer Christian Hu¨lsmeyer (1881–1957), who made experiments in 1903. Due to the lack of financing, the idea was abandoned until 1922, when Marconi proposed using radar for detecting ships in fog.

The Radiation Laboratory, which was established at the Massachusetts Institute of Technology during World War II, had a great impact on the development of radio engineering. Many leading American physicists were gathered there to develop radar, radionavigation, microwave components, microwave theory, electronics, and education in the field, and gave written 27 books on the research conducted there. The rectifying properties of semiconductors were noted in the late nineteenth century. However, the development of semiconductor devices was slow because vacuum tubes could do all the necessary operations, such as amplification and detection. A serious study of semiconductors began in the 1940s. The high-frequency capabilities of the point-contact semiconductor diode had already been observed. The invention of the transistor by Bardeen, Brattain, and Shockley started a new era in electronics. Their pointcontact transistor worked for the first time in 1947. The principle of the bipolar junction transistor was proposed the next year

The subsequent development of semiconductor devices is a prerequisite for the radio engineering of today. The continuous development of components and integrated circuits has made it possible to pack more complex functions to an ever-smaller space, which in turn has made possible many modern systems, such as mobile communication, satellite communication, and satellite navigation systems.