Chapter One
INTRODUCTION
The purpose of this introductory chapter is to provide a short, and admittedly incomplete, survey of what the microwave engineering field encompasses. Section 1.2 presents a brief discussion of many of the varied and sometimes unique applications of microwaves. This is followed by a third section in which an attempt is made to show in what ways microwave engineering differs from the engineering of communication systems at lower frequencies. In addition, a number of microwave devices are introduced to provide examples of the types of devices and circuit elements that are examined in greater detail later on in the text.
1.1 MICROWAVE FREQUENCIES
The descriptive term microwaves is used to describe electromagnetic waves with wavelengths ranging from 1 cm to 1 m. The corresponding frequency range is 300 MHz up to 30 GHz for 1-cm-wavelength waves. Electromagnetic waves with wavelengths ranging from 1 to 10 mm are called millimeter waves. The infrared radiation spectrum comprises electromagnetic waves with wavelengths in the range 1 m ([10.sup.6] m) up to 1 mm. Beyond the infrared range is the visible optical spectrum, the ultraviolet spectrum, and finally x-rays. Several different classification schemes are in use to designate frequency bands in the electromagnetic spectrum. These classification schemes are summarized in Tables 1.1 and 1.2. The radar band classification came into use during World War II and is still in common use today even though the new military band classification is the recommended one.
In the UHF band up to around a frequency of 1 GHz, most communications circuits are constructed using lumped-parameter circuit components. In the frequency range from 1 up to 100 GHz, lumped circuit elements are usually replaced by transmission-line and waveguide components. Thus by the term microwave engineering we shall mean generally the engineering and design of information-handling systems in the frequency range from 1 to 100 GHz corresponding to wavelengths as long as 30 cm and as short as 3 mm. At shorter wavelengths we have what can be called optical engineering since many of the techniques used are derived from classical optical techniques. The characteristic feature of microwave engineering is the short wavelengths involved, these being of the same order of magnitude as the circuit elements and devices employed.
The short wavelengths involved in turn mean that the propagation time for electrical effects from one point in a circuit to another point is comparable with the period of the oscillating currents and charges in the system. As a result, conventional low-frequency circuit analysis based on Kirchhoff s laws and voltage-current concepts no longer suffices for an adequate description of the electrical phenomena taking place. It is necessary instead to carry out the analysis in terms of a description of the electric and magnetic fields associated with the device. In essence, it might be said, microwave engineering is applied electromagnetic fields engineering. For this reason the successful engineer in this area must have a good working knowledge of electromagnetic field theory.
There is no distinct frequency boundary at which lumped-parameter circuit elements must be replaced by distributed circuit elements. With modern technological processes it is possible to construct printed-circuit inductors that are so small that they retain their lumped-parameter characteristics at frequencies as high as 10 GHz or even higher. Likewise, optical components, such as parabolic reflectors and lenses, are used to focus microwaves with wavelengths as long as 1 m or more. Consequently, the microwave engineer will frequently employ low-frequency lumped-parameter circuit elements, such as miniaturized inductors and capacitors, as well as optical devices in the design of a microwave system.
1.2 MICROWAVE APPLICATIONS
The great interest in microwave frequencies arises for a variety of reasons. Basic among these is the ever-increasing need for more radio-frequency-spectrum space and the rather unique uses to which microwave frequencies can be applied. When it is noted that the frequency range [10.sup.9] to [10.sup.12] Hz contains a thousand sections like the frequency spectrum from 0 to [10.sup.9] Hz, the value of developing the microwave band as a means of increasing the available usable frequency spectrum may be readily appreciated.
At one time (during World War II and shortly afterward), microwave engineering was almost synonymous with radar (RAdio Detection And Ranging) engineering because of the great stimulus given to the development of microwave systems by the need for high-resolution radar capable of detecting and locating enemy planes and ships. Even today radar, in its many varied forms, such as missile-tracking radar, fire-control radar, weather-detecting radar, missile-guidance radar, airport traffic-control radar, etc., represents a major use of microwave frequencies. This use arises predominantly from the need to have antennas that will radiate essentially all the transmitter power into a narrow pencil-like beam similar to that produced by an optical searchlight. The ability of an antenna to concentrate radiation into a narrow beam is limited by diffraction effects, which in turn are governed by the relative size of the radiating aperture in terms of wavelengths. For example, a parabolic reflector-type antenna produces a pencil beam of radiated energy having an angular beam width of 140/(D/[[lambda].sub.0]), where D is the diameter of the parabola and [[lambda].sub.0] is the wavelength. A 90-cm (about 3 ft) parabola can thus produce a 4.7 beam at a frequency of [10.sup.10] Hz, i.e., at a wavelength of 3 cm. A beam of this type can give reasonably accurate position data for a target being observed by the radar. To achieve comparable performance at a frequency of 100 MHz would require a 300-ft parabola, a size much too large to be carried aboard an airplane.
In more recent years microwave frequencies have also come into widespread use in communication links, generally referred to as microwave links. Since the propagation of microwaves is effectively along line-of-sight paths, these links employ high towers with reflector or lens-type antennas as repeater stations spaced along the communication path. Such links are a familiar sight to the motorist traveling across the country because of their frequent use by highway authorities, utility companies, and television networks. A further interesting means of communication by microwaves is the use of satellites as microwave relay stations. The first of these, the Telstar, launched in July 1962, provided the first transmission of live television programs from the United States to Europe.
Since that time a large number of satellites have been deployed for communication purposes, as well as for surveillance and collecting data on atmospheric and weather conditions. For direct television broadcasting the most heavily used band is the ITLITL band. The up-link frequency used is in the 5.9- to 6.4-GHz band and the receive or down-link frequency band is between 3.7 and 4.2 GHz. For home reception an 8-ft-diameter parabolic reflector antenna is commonly used. A second frequency band has also been allocated for direct television broadcasting. For this second band the up-link frequency is in the 14- to 14.5-GHz range and the down-link frequencies are between 10.95 and 11.2 GHz and 11.45 and 11.7 GHz. In this band a receiving parabolic antenna with a 3-ft diameter is adequate. At the present time this frequency band is not being used to any great extent in the United States. It is more widely used in Europe and Japan.
Terrestrial microwave links have been used for many years. The TD-2 system was put into service in 1948 as part of the Bell Network. It operated in the 3.7- to 4.2-GHZ band and had 480 voice circuits, each occupying a 3.1-kHz bandwidth. In 1974, the TN-1 system operating in the 10.7- to 11.7-GHz band was put into operation. This system had a capacity of 1,800 voice circuits or one video channel with a 4.5-MHz bandwidth. Since that time the use of terrestrial microwave links has grown rapidly.
At the present time most communication systems are shifting to the use of digital transmission, i.e., analog signals are digitized before transmission. Microwave digital communication system development is progressing rapidly. In the early systems simple modulation schemes were used and resulted in inefficient use of the available frequency spectrum. The development of 64-state quadrature amplitude modulation (64-QAM) has made it possible to transmit 2,016 voice channels within a single 30-MHz RF channel. This is competitive with FM analog modulation schemes for voice. The next step up is the 256-QAM system which is under development.
For the ready processing and handling of a modulated carrier, modulation sidebands can be only a few percent of the carrier frequency. It is thus seen that the carrier frequency must be in the microwave range for efficient transmission of many television programs over one link. Without the development of microwave systems, our communications facilities would have been severely overloaded and totally inadequate for present operations.
Even though such uses of microwaves are of great importance, the applications of microwaves and microwave technology extend much further, into a variety of areas of basic and applied research, and including a number of diverse practical devices, such as microwave ovens that can cook a small roast in just a few minutes. Some of these specific applications are briefly discussed below.
Waveguides periodically loaded with shunt susceptance elements support slow waves having velocities much less than the velocity of light, and are used in linear accelerators. These produce high-energy beams of charged particles for use in atomic and nuclear research. The slow-traveling electromagnetic waves interact very efficiently with charged-particle beams having the same velocity, and thereby give up energy to the beam. Another possibility is for the energy in an electron beam to be given up to the electromagnetic wave, with resultant amplification. This latter device is the traveling-wave tube, and is examined in detail in a later chapter.
Sensitive microwave receivers are used in radio astronomy to detect and study the electromagnetic radiation from the sun and a number of radio stars that emit radiation in this band. Such receivers are also used to detect the noise radiated from plasmas (an approximately neutral collection of electrons and ions, e.g., a gas discharge). The information obtained enables scientists to analyze and predict the various mechanisms responsible for plasma radiation. Microwave radiometers are also used to map atmospheric temperature profiles, moisture conditions in soils and crops, and for other remote-sensing applications as well.
Molecular, atomic, and nuclear systems exhibit various resonance phenomena under the action of periodic forces arising from an applied electromagnetic field. Many of these resonances occur in the microwave range; hence microwaves have provided a very powerful experimental probe for the study of basic properties of materials. Out of this research on materials have come many useful devices, such as some of the nonreciprocal devices employing ferrites, several solid-state microwave amplifiers and oscillators, e.g., masers, and even the coherent-light generator and amplifier (laser).
The development of the laser, a generator of essentially monochromatic (single-frequency) coherent-light waves, has stimulated a great interest in the possibilities of developing communication systems at optical wavelengths. This frequency band is sometimes referred to as the ultramicrowave band. With some modification, a good deal of the present microwave technology can be exploited in the development of optical systems. For this reason, familiarity with conventional microwave theory and devices provides a good background for work in the new frontiers of the electromagnetic spectrum.
The domestic microwave oven operates at 2,450 MHz and uses a magnetron tube with a power output of 500 to 1000 W. For industrial heating applications, such as drying grain, manufacturing wood and paper products, and material curing, the frequencies of 915 and 2,450 MHz have been assigned. Microwave radiation has also found some application for medical hyperthermia or localized heating of tumors.
It is not possible here to give a complete account of all the applications of microwaves that are being made. The brief look at some of these, as given above, should convince the reader that this portion of the radio spectrum offers many unusual and unique features. Although the microwave engineering field may now be considered a mature and well-developed one, the opportunities for further development of devices, techniques, and applications to communications, industry, and basic research are still excellent.
1.3 MICROWAVE CIRCUIT ELEMENTS AND ANALYSIS
At frequencies where the wavelength is several orders of magnitude larger than the greatest dimensions of the circuit or system being examined, conventional circuit elements such as capacitors, inductors, resistors, electron tubes, and transistors are the basic building blocks for the information transmitting, receiving, and processing circuits used. The description or analysis of such circuits may be adequately carried out in terms of loop currents and node voltages without consideration of propagation effects. The time delay between cause and effect at different points in these circuits is so small compared with the period of the applied signal as to be negligible. It might be noted here that an electromagnetic wave propagates a distance of one wavelength in a time interval equal to one period of a sinusoidally time-varying applied signal. As a consequence, when the distances involved are short compared with a wavelength [[lambda].sub.0] ([[lambda].sub.0] = velocity of light/frequency), the time delay is not significant. As the frequency is raised to a point where the wavelength is no longer large compared with the circuit dimensions, propagation effects can no longer be ignored. A further effect is the great relative increase in the impedance of connecting leads, terminals, etc., and the effect of distributed (stray) capacitance and inductance. In addition, currents circulating in unshielded circuits comparable in size with a wavelength are very effective in radiating electromagnetic waves. The net effect of all this is to make most conventional low-frequency circuit elements and circuits hopelessly inadequate at microwave frequencies.
If a rather general viewpoint is adopted, one may classify resistors, inductors, and capacitors as elements that dissipate electric energy, store magnetic energy, and store electric energy, respectively. The fact that such elements have the form encountered in practice, e.g., a coil of wire for an inductor, is incidental to the function they perform. The construction used in practical elements may be considered just a convenient way to build these devices so that they will exhibit the desired electrical properties. As is well known, many of these circuit elements do not behave in the desired manner at high frequencies. For example, a coil of wire may be an excellent inductor at 1 MHz, but at 50 MHz it may be an equally good capacitor because of the predominating effect of interturn capacitance. Even though practical low-frequency resistors, inductors, and capacitors do not function in the desired manner at microwave frequencies, this does not mean that such energy-dissipating and storage elements cannot be constructed at microwave frequencies. On the contrary, there are many equivalent inductive and capacitive devices for use at microwave frequencies. Their geometrical form is quite different, but they can be and are used for much the same purposes, such as impedance matching, resonant circuits, etc. Perhaps the most significant electrical difference is the generally much more involved frequency dependence of these equivalent inductors and capacitors at microwave frequencies.
(Continues...)
Excerpted from Foundations for Microwave Engineeringby Robert E. Collin Excerpted by permission.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.