Supplementary Readings: What Are Microwaves?

“Microwaves” is a descriptive term used to identify electromagnetic waves in the frequency spectrum ranging approximately from 1 Giga Hertz (109Hertz) to 30 Giga Hertz. This corresponds to wavelengths from 30 cm to 1 cm. Higher frequencies (extending up to 600 GHz) are also called “microwaves”. These waves present several interesting and unusual features not found in other portions of the electromagnetic frequency spectrum. These features make “microwaves” uniquely suitable for several useful applications.

Characteristic features of microwaves

The main characteristic features of microwaves originate from the small size of wavelengths (1 cm to 30 cm) in relation to the sizes of components or devices commonly used. Since the wavelengths are small, the phase varies rapidly with distance; consequently the techniques of circuit analysis and design, of measurements and of power generation and amplification at these frequencies are distinct from those at lower frequencies.

For dealing with these small wavelengths, methods of circuit representation and analysis need to be modified. The phase difference caused by the interconnection between various components or various parts of a single component is not negligible. Consequently, analysis based on Kirchhoff's laws and voltage current concepts are not adequate to describe the circuit behavior at microwave frequencies. It is necessary to analyze the circuit or the component in terms of electric and magnetic fields associated with it. For this reason microwave engineering is also known as electromagnetic engineering or applied electromagnetics. A background of electromagnetic theory is a prerequisite for understanding microwaves.

Not only analytical techniques, the methods of measurement also become specialized at microwave frequencies. Measurements are carried out in terms of field amplitudes, phase differences and powers carried by the waves. A very commonly used method of microwave measurements is based on the study of a standing wave pattern formed along the line because of the interference of incident and reflected waves. Ratio of the amplitudes and phase relationship between incident and reflected waves tell us about impedance characteristics of the components causing the reflection. Several other special techniques have been developed for use at microwave frequencies.

Microwave circuit components also have a different look. Use of lumped elements at microwave frequencies becomes difficult because of small wavelengths involved. For realizing the lumped behavior of a capacitor, an inductor or a resistor, the component size must be much smaller than the wavelength. Because of this reason microwave systems employ distributed circuit elements very often. These elements are made up of small sections of transmission lines and waveguides. For example, a quarter wavelength section of a transmission line is used as an impedance transformer. A half wavelength section, on the other hand, constitutes a resonant circuit to be used in place of an L-C resonator. Use of miniature lumped elements at microwave frequencies has been made possible by the advancement in microelectronics technology during the last ten years. These techniques allow us to fabricate inductors, capacitors and resistors that are about a millimeter or two in size; much smaller than small wavelengths at microwave frequencies. Another aspect unique to microwave circuits is the possibility of radiation from discontinuities in distributed circuits. This necessitates a more careful and accurate circuit design.

The challenge of generating microwaves has resulted in a variety of devices—both in vacuum tube and in semiconductor device areas. When an attempt is made to use a lower frequency source at microwave frequencies, the operation is limited by the fact that transit time of the carriers through the device (i.e., electrons in triodes and electrons or holes in transistors) becomes comparable to the time period of the wave. This problem has been solved by technological innovations (in case of transistors) and by totally novel ideas (as in case of klystrons, magnetrons, transferred electron devices and avalanche diodes).

Applications of microwaves

Study and research in microwaves has not only been an interesting and challenging academic endeavor, it has led to several useful applications in communications, in radar, in physical research, in medicine and in industrial measurements and also for heating and drying of agricultural and food products.

A significant advantage associated with the use of microwaves for communications is their large bandwidth. A ten percent bandwidth at 3 GHz implies availability of 300 MHz spectrum. This means all the radio, television and other communications that are transmitted in the frequency spectrum from DC to 300MHz can be accommodated in a 10% bandwidth around 3 GHz (say from 2850 to 3150 MHz). Since the lower frequency part of the radio spectrum is getting crowded, there is a trend to use more and more of microwave region (and beyond) for various different services.

Short wavelengths also simplify the design and installation of high directivity antennae. Antenna directivity depends on the ratio of antenna aperture to the wavelength of the signal to be transmitted. At 10 GHz, a pencil beam with 1° beamwidth can be obtained by using a 6.9 feet diameter antenna. At 10 MHz, this will require an antenna diameter of 6,900 feet. This becomes impractical, especially if it is desired to rotate the antenna so that the beam can look in various directions.

Small antenna size and the property of reflection of microwaves from metallic surfaces make it practical to operate radar systems at these frequencies. Radar is an electronic method of detecting the presence of aircraft (or other objects) at ranges and in circumstances where other means of detection are not possible. Operation of the radar is based on the measurement of the time it takes for a pulse transmitted from an antenna to get reflected by the object to be detected and to return at the antenna and the receiver. Also, in many radar systems, a shift in the frequency of reflected signal caused by the Doppler effect can be recorded. Velocity of the target can be calculated from this measurement. The reflection from the object to be measured is significant only when the wavelength is much smaller than the size of the object. For this reason, the radar could not become practicable at lower frequency and had to wait for the development of microwave technology during the Second World War period. Today, radars constitute about 70% of microwave equipment. There are a whole variety of radars: early-warning radar, missile-tracking radar, missile-guidance radar, fire-control radar, weather-detection radar, air-traffic control radar and even radars to detect and control the speed of automobiles.

There are other advantages associated with the small wavelengths at microwave frequencies. Unlike lower radio frequencies, these waves are not reflected and practically not absorbed by the ionosphere. This has led radio astronomers to use these frequencies to study electromagnetic radiations originating from stars and other astronomical objects. Also, this property makes microwaves suitable for space communication and satellite communication.

Microwaves exhibit another interesting feature. Molecular, atomic, and nuclear systems display various resonance phenomena when placed in periodic electromagnetic fields. Several of these resonance absorption lines lie in the microwave frequency range. The resonance absorption is due to rotational transitions in the molecules and the absorption spectra provide information on the molecular structure and intramolecular energies. Thus microwaves become a very powerful experimental tool for the study of some of the basic properties of materials. Besides scientific research, absorption of microwaves by molecular resonances is well suited for various industrial measurements. It can be used to measure the concentration of different gases, e.g., in an exhaust chimney in order to control the emission of pollutants, or in chemical processes in order to record continuously the concentration of gases evolved in the process.

The study of microwave resonances in molecules has led to several useful devices. The most significant ones are the non-reciprocal devices employing ferrites and solid-state microwave amplifiers and oscillators called masers. The magnetic properties of microwave ferrites are due to the electron spins in solids. The coupling between spins is such as to divide the magnetic atoms into groups having oppositely oriented magnetic dipoles. When placed in an external static magnetic field these materials exhibit non-reciprocal behavior at microwave frequencies. The maser is a microwave amplifier or oscillator which employs, as its working substance, a paramagnetic material having a suitable set of electron-spin energy levels separated by energy intervals that correspond to the frequencies in the microwave range. Stimulated transition from a higher energy state to a lower state results in radiation at microwave frequencies. Masers have lower noise figure than any other type of amplifier known and are used in the communication systems where extremely low noise characteristics are desired.

Just like any other form of energy, microwave energy can also be used for heating. Thermal effects produced by microwaves have a variety of industrial applications. Microwave ovens for cooking follow the principle of dielectric heating. Cooking is done very quickly and uniformly by microwaves since the food is cooked by the waves on the inside at the same time as on the outside. Like transfer of heat by conduction, convection and radiation, microwave heating can be considered as another mode of heat transfer. In this mode, heat is produced directly at the locations of the dielectric losses. Water has higher dielectric loss than the other ingredients in food products. Thus water pockets get heated first which is exactly where heat is required for cooking purposes. Microwave diathermy machines produce heat inside the muscles without heating the tissues and skin outside. Also, microwave drying machines are used in printing, textile, and paper industries.