Modern microwave engineering involves predominantly distributed circuits and design in contrast to the waveguide, field theory and EM orientations of earlier generations. Microwave CAD software and network analyzers are the essential tool for today’s microwave engineer even though a general basic understanding of EM theory is a pre-requisite.
Microwave and RF technology is more pervasive than ever. This is especially true in the commercial sector, where modern applications include cellular telephony, personal communications systems, wireless local area data networks, millimeter wave collision avoidance vehicle radars, direct broadcast satellites for radio and television, global positioning systems, radio frequency identification tagging, ultra wideband radio and radar systems, and microwave remote sensing systems for the environment. Defense systems continue to rely heavily on microwave technology for passive and active sensing, communications, and weapons control systems. This state of affairs suggests that there will be no shortage of challenging problems in RF and microwave engineering in the foreseeable future, and a clear need for engineers having both an understanding of the fundamentals of microwave engineering as well as the creativity to apply this knowledge to problems of practical interest.
The term microwaves refers to alternating current signals with frequencies between 300MHz (3 х 108 Hz) and 300 GHz (3 х 1011), with a corresponding electrical wavelength between λ = c/f = 1m and λ = 1mm, respectively. Signals with wavelength on the order of millimeters are called millimeter waves. Because of the high frequency (and short wavelengths), standard circuit theory generally cannot be used directly to solve microwave network problems. In a sense, standard circuit theory is an approximation or special use of the broader theory of electromagnetic as described by Maxwell’s equations. This is due to the fact that, in general, the lumped circuit element approximations of circuit theory are not valid at microwave frequencies. Microwave components are often distributed elements, where the phase of a voltage or current changes significantly over the physical extent of the device, because the device dimensions of a component. The other extreme of frequency can be identified as optical engineering, in which the wavelength is much shorter than the dimensions of the component. In this case Maxwell’s equations can be simplified to the geometrical optics regime, and optical systems can be designed using geometrical optics. Such techniques are sometimes applicable to millimeter wave systems, where they are referred to as quasioptics.
In microwave engineering, then, one must often begin with Maxwell’s equations and their solutions. It is in the nature of these equations that mathematical complexity arises, since Maxwell’s equations involve vector differential or integral operations on vector field quantities and these fields are functions of spatial coordinates.
Goal: to try to reduce the complexity of a field theory. A field theory solution generally provides a much more information than we really need for most practical purposes. We are typically more interested in terminal quantities such as power, impedance, voltage, and current, which can often be expressed in terms of circuit concepts. It is this complexity that adds to the challenge, as well as the rewards, of microwave engineering.