Microwave Engineering

the area of science and technology concerned with the study and application of the properties of electromagnetic oscillations and waves in the frequency range from 300 megahertz (MHz) to 300 gigahertz (GHz). These limits are arbitrary; in some cases the lower limit of the microwave range is taken as 30 MHz and the upper limit as 3 terahertz. In English, the term “microwave” is sometimes used in a narrower sense.

According to the type of problems solved and the associated fields of application, microwave devices and systems (including radiating, transmitting, receiving, and measuring devices and systems) can be divided into information-transmission types and power types. The uses of information-transmission devices and systems include radio communication, television, radar, radio navigation, radio control, technical inspection, and computer technology. Power devices and systems are used, for example, in industrial technology, in household appliances, in medical, biological, and chemical equipment, and in power transmission. Microwave devices and systems are powerful tools for scientific research in such fields as radio-frequency spectroscopy, solid-state physics, nuclear physics, and radio astronomy.

The microwave range is extremely broad and is sometimes divided into separate parts, usually according to the wavelength λ. For meter waves, λ = 10–1 m; for decimeter waves, λ = 100–10 cm; for centimeter waves, λ = 10–1 cm; for millimeter waves, λ = 10–1 mm; and for decimillimeter waves (sometimes the broader term “submillimeter waves” is used for these waves), λ = 1–0.1 mm. The relation between the wavelength and the frequency f is given by the ratio λ = c/f, where c is the propagation velocity of electromagnetic waves in a vacuum.

The theory of microwave-frequency electromagnetic fields is based on the general laws of electrodynamics. The components of the electromagnetic field—the electric and magnetic field vectors E and H—depend on the coordinates and time. In accordance with the laws of electrodynamics, the relation between these components and the characteristics of the sources

Figure 1. Distribution of amplitudes of voltage U and current / in ideal, or lossless, microwave transmission lines of different lengths I: (A) short-circuited lines, (B) open-circuited lines; (a) when I < λ/4, (b) when I = λ/4, (c) when λ/4 < I < λ/2, (d) when I = λ/2; (λ) wavelength, (G) microwave oscillator, (L) inductance, (C) capacitance. Next to the curves are shown the equivalent circuits of the lines; the equivalent circuit indicates the nature of the line’s input impedance.

generating the field—the charge density and the density of the total current—is given by the Lorentz-Maxwell equations. By introducing the concept of the characteristic impedance ρ = E/H of a medium, it is possible to make use of what is called the telegrapher’s equation, or the equation of telegraphy. This equation gives the relation between, on the one hand, the voltages and currents in microwave equipment, which are functions of the coordinates and time, and, on the other hand, the electrical parameters of the equipment.

Figure 2. Maximum power levels of microwave electron-tube and semiconductor devices as of 1973–74: (1) electron-tube devices with grid control, (2) electron-tube devices with dynamic control, (3) semiconductor devices; (f) frequency, (λ) wavelength, (P) power. The solid curves correspond to continuous operation, and the broken curves correspond to pulse operation.

General properties and characteristics of microwave equipment. Microwave equipment, especially equipment operating at wavelengths from 30 cm to 3 mm, has characteristic properties that distinguish such devices and systems from equipment used in the parts of the electromagnetic spectrum adjacent to the microwave range. For example, the wavelength is generally comparable with the linear dimensions of the equipment and its elements, and the transit time of electrons in the electronic devices is comparable with the period of the microwave oscillations. Further examples can be given: the waves are weakly absorbed in the ionosphere but, at certain frequencies, are strongly absorbed in the surface layer of the earth; the coefficient of reflection from metal surfaces is high; microwave energy can be concentrated into a narrow beam; the wave energy can interact with a substance (molecules and atoms); and the microwave range has a large information-carrying capacity.

Microwave circuits, elements, and electronic devices. Passive circuits—that is, circuits not containing energy sources—and circuit elements in the microwave range are represented mainly by transmission lines and sections of transmission lines in the form of various wave guides, through which the electromagnetic energy is transmitted to a receiver so that the receiver can isolate microwave power or signals with useful information. Two-wire and coaxial lines are used for meter and decimeter waves; coaxial lines, hollow-pipe wave guides, and strip-lines are used for centimeter waves; and hollow-pipe dielectric and quasi-optical guides are used for millimeter and submillimeter waves. The length of the line usually is comparable with the wavelength or is longer than the wavelength. The propagation time of waves in the line is comparable with or greater than the period of the microwave oscillations. In electric circuits used to some extent at meter wavelengths but more often at longer wavelengths, the inductance is concentrated in a coil, the capacitance in a capacitor, and the ohmic resistance in a resistor; such circuits are known as lumped-constant circuits. Since the capacitance, inductance, and ohmic resistance in a transmission line can be regarded as being distributed along the entire conductor, transmission lines are referred to as distributed-pa-rameter circuits. Electrical processes in such circuits must be studied not only with respect to time but also with respect to space.

When one end of a line is connected to a generator of an alternating electromotive force and the other is connected to a load, a traveling wave, which carries energy, moves along the line from the generator to the load. Pure traveling waves are observed only when the line is loaded with an impedance equal to its characteristic impedance ρ; the input impedance of such a line (at the generator terminals) is also equal to the load impedance. For a lossless line, the effective values of the voltage and current along it are everywhere constant, and the transmitted energy is completely absorbed by the load impedance. In open-circuited and short-circuited lines (Figure 1), on the other hand, standing waves are established, and nodes and antinodes of the voltage and current alternate along the line. For any other value and type of load impedance, the impedance-matching condition is violated, and a more complicated process occurs in the line. In this case, mixed, or composite, waves are established: part of the energy of the incident wave is absorbed in the ohmic resistance of the load, and the remaining energy is reflected from the load—that is, standing waves are formed. The input impedance of such a line or of its sections may have a periodic character; the magnitude of the input impedance can vary over a wide range depending on the choice of the operating wavelength, the nature of the load, and the geometric length of the line. For example, the input impedance of a lossless line that is loaded with an ohmic resistance R_L is equal to ρ²/R_L when an odd number of quarter-wavelengths fit into the length of the line and equal to R_L when an even number do so. In order to characterize the waves in the line and to determine the amount of power delivered to the load, the standing-wave ratio is used. It is the ratio of the maximum voltage to the minimum voltage along the line. The inverse of the ratio is also used; it too is sometimes called the standing-wave ratio, and the term “traveling-wave ratio” is sometimes applied to it.

Transmission lines, sections of lines, and hollow metallic bodies with certain geometric dimensions and configurations are structures of great importance in microwave engineering. Various microwave elements and assemblies are designed on the basis of the properties exhibited by such structures with different input impedances. Examples are two-wire-line resonators, coaxial-line resonators, cavity resonators, impedance transformers, filters, hybrid junctions, directional couplers, attenuators, phase shifters, and stubs. The use of ferrites in lines has made possible the creation of microwave elements and assemblies with nonreciprocal (gating) properties—for example, isolators, directional phase shifters (seeGYRATOR), and circulators.

Active circuits contain sources of microwave energy along

Figure 3. Minimum noise levels of microwave electronic devices and equipment and environmental noise levels (data as of 1973–74): (1) triodes, (2) semiconductor diodes (mixers), (3) traveling-wave tubes, (4) parametric amplifiers, (5) masers, (6) noise of the galactic pole, (7) noise of the earth’s atmosphere; (f) frequency, (λ) wavelength, (T) noise temperature

Figure 4. Schematic of the operating chamber of a microwave oven for drying a ceramic mixture: (1) fixed hood, (2) wave guide, (3) open reservoir filled with an aqueous ceramic suspension, (4) channels filled with water to provide shielding from the microwave radiation, (5) removable bottom, (6) electromechanical drive, (7) tube through which water flows from under the hood during the condensation of the evaporated moisture, (8) canister containing the device that turns off the microwave oscillator when the drying process is completed

with passive elements. The sources are chiefly electronic devices, such as electron-tube, semiconductor, and quantum devices. In the principal types of electron tubes used for generation, amplification, conversion, and detection at microwave frequencies, a flux of electrons (a current) interacts with electrical oscillations or the field of an electromagnetic wave. Two groups of tubes are distinguished according to the way in which the current is controlled. In tubes with electrostatic control (grid control), the energy of the microwave oscillations is amplified by the action of the varying potential of a control grid on the cathode space charge; triodes, tetrodes, and pentodes are examples. In electronic devices with dynamic control, the energy of the microwave-frequency field is amplified as a result of the interaction of the field with the electrons. The interaction can be discrete, as in klystrons, or continuous, as in traveling-wave tubes, backward-wave tubes, magnetrons, and devices based on maser-cyclotron resonance (such as maser-cyclotron generators and amplifiers).

Electron inertia, interelectrode capacitances and lead inductances limit the maximum amplifying and generating frequencies. In devices of the first group, which are used chiefly at meter and decimeter wavelengths, a number of engineering and design measures are taken to decrease this effect and to reduce dielectric losses in the material of the envelope and base of the tube. For example, the interelectrode spacings and the electrode surfaces can be made smaller (the electrodes are made in the form of disks to provide a convenient connection with cavity resonators). Another example is the use of special ceramics that have low microwave energy losses. Such devices include metal-ceramic tubes, nuvistors, light-house tubes, resnatrons, and coaxitrons.

The devices of the second group, which are used chiefly at decimeter, centimeter, and millimeter wavelengths, are free of many of the shortcomings of devices of the first group, but their operating principles, design implementation, and tuning are generally more complicated. Their maximum amplifying and generating frequencies are limited by a number of factors. For example, as the operating frequency increases, the dimensions and the manufacturing tolerances of the individual microwave elements sharply decrease, losses increase, and coupling between the electron flux and the microwave-frequency field decreases.

All the basic types of semiconductor devices have found application throughout the microwave range. These devices include detector diodes, mixer diodes, transistors, varactors (varicaps), avalanche transit-time diodes, Gunn diodes, Schottky diodes, tunnel diodes, and parametric diodes. The generating and amplifying devices can produce up to several tens of watts (W) of continuous useful power at meter wavelengths and up to several W at centimeter wavelengths.

The operation of microwave electronic devices that are designed for the transmission or reception of information is characterized by the frequency-power characteristic, which represents the frequency dependence of the maximum achievable power level in the case of transmission (Figure 2) and of the minimum noise level in the case of reception (Figure 3). This characteristic, in particular, is associated with the attainment of the maximum power potential. The power potential is the ratio of the output power of the transmitting equipment to the minimum permissible (for normal operation) noise power of the receiving equipment. The operating range of radio-electronic systems depends on this rátio.

Microwave equipment and systems. Various combinations of passive or of active and passive microwave circuits are used to create such equipment as oscillators, amplifiers, radiation detectors, frequency multipliers, measuring instruments, and antenna feeder equipment, which couples an antenna by means of a feeder with the input circuit of a radio receiver or the output circuit of a radio transmitter. The use of superconducting resonators and hydrogen and cesium oscillators (seeQUANTUM FREQUENCY STANDARDS) in microwave equipment has made it possible to achieve the very small relative frequency instability 10^-10–10^-13.

Some radio-electronic systems with a high power potential make use of klystron magnetron, or magnetron-type oscillators. Other such systems—primarily phased antenna arrays with electronic control of the radiation pattern—involve a large number (up to 10,000) of comparatively low-power (up to several tens of W) electronic devices operating in parallel. Powerful microwave devices operating in parallel are used in accelerator technology. The problem of noise reduction in receiving equipment is handled most effectively by using parametric amplifiers (mostly uncooled) and quantum mechanical amplifiers—that is, masers, in which the active medium is cooled to the temperature of liquid helium or nitrogen, 4° or 77°K, respectively. Microwave ovens (Figures 4 and 5) are used in industry and in the preparation of food.

Figure 5. Microwave oven for preparing food: (1) glass plate on which the food is placed; (2) fan whose blades, by rotating, reflect the microwaves in all directions in order to heat the food on all sides; (3) wave guide; (4) magnetron; (5) timers

A radical solution of the problems of miniaturization and reliability in low-power-potential systems has been achieved by developing transmitting and receiving equipment consisting entirely of semiconductors (Figure 6), in particular, equipment making use of integrated circuits (seeMICROELECTRONICS and PLANAR PROCESS). The dimensions of the basic elements in hybrid and monolithic integrated microwave circuits are of the order of micrometers and tens of micrometers. Thus, such equipment, which is used chiefly at frequencies of 1 to 15 GHz, can be constructed from lumped-parameter circuits and two-wire lines. The greatest difficulties encountered in designing such equipment are the problems of heat removal and the elimination of spurious couplings. This area of microwave engineering and millimeter- and submillimeter-wavelength engineering are undergoing intensive development.

Safety. The increasing use of microwave equipment, particularly high-power equipment, has substantially raised the level of microwave energy on the earth. Local intensities of radiation of microwave energy by transmitting antennas, especially by antennas with a sharp radiation pattern, have increased. In addition, the supply of large amounts of microwave power to an antenna by a feeder involves high voltages, which can be harmful or fatal to people in the vicinity. Consequently, a special branch of occupational hygiene has come into being that studies the biological effects of radio-frequency radiation and develops measures against microwave hazards—not only against the harmful effects of microwave energy on man but also against injuries caused by the electric currents used in microwave equipment. The following maximum permissible power densities of a microwave field are considered to be safe: 10 milliwatts per cm² (mW/cm²) for seven or eight hours, 100 mW/cm² for two hours, and 1 W/cm² for 15 or 20 minutes (with the mandatory use of protective goggles). Maintenance personnel are permitted to work with industrial microwave equipment only after the required precautionary measures have been taken in conformity with the safety regulations for the equipment. The use of weak doses of microwave radiation in radiation therapy is known as microwave therapy.

Figure 6. Microwave transistor amplifier: (a) circuit diagram; (b) schematic design layout; (1) input; (2) input compensating circuit, which broadens the operating frequency range; (3) output compensating circuit; (4) output; (5) and (6) ground terminals; (7) lead to power source U; (Ch) microwave choke; (T) transistor; (R₁), (R₂), and (R₃) resistors; (C₁), (C₂), (C₃), and (C₄) capacitors; (L₁), (L₂), and (L₃) inductors

Outlook. The outlook for microwave engineering is closely bound up with the development of new as well as traditional areas of telecommunications, radar, electric power engineering, and industrial technology. An important role in the growth of microwave engineering is played by the study of the interaction of an electromagnetic field with matter and with plants and other living organisms. One notable direction of development is the further exploitation of the millimeter and submillimeter wavelength ranges—primarily in radio engineering, nuclear physics, chemistry, and medicine. Higher power potentials (see Figures 2 and 3) are needed, and the spectral characteristics of radiating microwave equipment must satisfy more stringent requirements.