6 Radio Engineering for Wireless Communication and Sensor Applications

Table 1.3

Frequency Allocation for the Frequency Band 10–10.7 GHz

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.

1.4History of Radio Engineering from Maxwell to the Present

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.

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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.

References

[1]Radio Regulations, Vol. I, Geneva, Switzerland: International Telecommunication Union, 2001.

[2]Paul, C. R., Introduction to Electromagnetic Compatibility, New York: John Wiley & Sons, 1992.

[3]Levy, R., (ed.), ‘‘Special Issue Commemorating the Centennial of Heinrich Hertz,’’

IEEE Trans. on Microwave Theory and Techniques, Vol. 36, No. 5, 1988, pp. 801–858.

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The electric flux density D and the magnetic field strength H take into account the presence of materials. The electric and magnetic properties of media bind the field strengths and flux densities; the constitutive relations are

where e is the permittivity [F/m = As/Vm] and m is the permeability [H/ m = Vs/Am] of the medium.

Maxwell’s equations in differential form are

As also mentioned in the above equations, a lot of the knowledge of electromagnetic theory was already developed before Maxwell by Gauss, Faraday, Ampe`re, and others. Maxwell’s contribution was to put the existing knowledge together and to add the hypothetical displacement current, which then led to Hertz and Marconi’s discoveries and to modern radio engineering.

How did Maxwell discover the displacement current? We may speculate and simplify this process of invention as follows (see [1], Chapter 18): Maxwell studied the known laws and expressed them as differential equations for each vector component, because the nabla notation (curl and divergence of a vector quantity) was not yet known. Nevertheless, we use the nabla notation here. He found that while Gauss’ and Faraday’s laws are true in general, there is a problem in Ampe`re’s law:

If one takes the divergence of this equation, the left-hand side is zero, because the divergence of a curl is always zero. However, if the divergence of J is zero, then the total flux of current through any closed surface is zero. Maxwell

correctly understood the law of charge conservation: The flux of current through a closed surface must be equal to the change of charge inside the surface (in the volume), that is,

In order to avoid the controversy, Maxwell added the displacement current term, ∂D/∂t , to the right-hand side of Ampe`re’s law in a general case and got

∂D

= × H = J + ∂ (2.10) t

With this addition the principle of charge conservation holds, because by using Gauss’ law we obtain

The differential equations, (2.4) through (2.7), describe the fields locally or at a given point. In other words, they allow us to obtain the change of field versus space or time. Maxwell’s equations in integral form describe how the field integrals over a closed surface S (rS ) or along a closed loop G (rG ) depend on the sources and changes of the fields versus time. Maxwell’s equations in integral form are:

GS

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where d S is an element vector perpendicular to surface S having a magnitude equal to the surface element area, d l is a length element parallel to the loop, and dV is a volume element. The volume, V, is enclosed by the closed surface S. Equations (2.11) and (2.12) are obtained by applying Gauss’ theorem, according to which for any vector quantity A it holds

Maxwell’s equations would be symmetric in relation to electric and magnetic quantities if magnetic charge density rM [Wb/m3 = Vs/m3 ] and magnetic current density M [V/m2 ] were also introduced into them. However, there is no experimental evidence of their existence.

2.1.1 Maxwell’s Equations in Case of Harmonic Time Dependence

Time harmonic fields, that is, fields having a sinusoidal time dependence at angular frequency of v = 2pf , may be presented as

At a given point (x , y , z ), the field may be thought to be a vector rotating on the complex plane and having a constant amplitude, the real part of which is the field value at a given instant. Most phenomena in radio engineering are time harmonic or can be thought to be superpositions of several time harmonics. Therefore, in this book we will confine ourselves to the time harmonic cases. Assuming the e jvt time dependence, the time derivatives can be replaced by multiplications by jv. For such sinusoidal fields and sources, Maxwell’s equations in differential form are

where s is the conductivity [S/m = A/Vm] of the medium.

2.1.2 Interpretations of Maxwell’s Equations

Maxwell’s equations may be presented in words as follows:

IThe electric flux (surface integral of the electric flux) through a closed surface is equal to the total charge within the volume confined by the surface.

IIThe magnetic flux (surface integral of the magnetic flux) through any closed surface is zero.

IIIThe line integral of the electric field along a closed contour is equal to the negative time derivative of the magnetic flux through the closed contour.

IV The line integral of the magnetic field along a closed contour is equal to the sum of the total current through the closed contour and the time derivative of the electric flux.

Figure 2.1 illustrates Maxwell’s equations in integral form. These qualitative interpretations are as follows:

16 Radio Engineering for Wireless Communication and Sensor Applications

Figure 2.1 Maxwell’s equations (in integral form).

I The distribution of the electric charge determines the electric field.

IIThe magnetic flux lines are closed; in other words, there are no magnetic charges.

III A changing magnetic flux creates an electric field.

IV Both a moving charge (current) and a changing electric flux create a magnetic field.

The creation of an electromagnetic field is easy to understand qualitatively with the aid of Maxwell’s equations. Let us consider a current loop with a changing current. The changing current creates a changing magnetic field (IV); the changing magnetic field creates a changing electric field (III); the changing electric field creates a changing magnetic field (IV); and so on. Figure 2.2 illustrates the creation of a propagating wave.

Maxwell’s equations form the basis of radio engineering and, in fact, of the whole of electrical engineering. These equations cannot be derived from other laws; they are based on empirical research. Their validity comes from their capability to predict the electromagnetic phenomena correctly. Many books deal with fundamentals of the electromagnetic fields, such as those listed in [1–8].