Introduction Overview of Pertinent Electromagnetics.
In this microwave engineering course, we will focus primarily
on electrical circuits operating at frequencies of 1 GHz and
higher. In terms of band designations, we will be working with
circuits above UHF:
RF, microwave and millimeter wave circuit design and
construction is far more complicated than low frequency work.
So why do it?
Advantages of microwave circuits:
1. The gain of certain antennas increases (with reference to
an isotropic radiator) with its electrical size. Therefore,
one can construct high gain antennas at microwave
frequencies that are physically small. (DBS, for example.)
2. More bandwidth. A 1% bandwidth, for example,
provides more frequency range at microwave frequencies
that at HF.
3. Microwave signals travel predominately by line of sight.
Plus, they don’t reflect off the ionosphere like HF signals
do. Consequently, communication links between (and
among) satellites and terrestrial stations are possible.
4. At microwave frequencies, the electromagnetic properties
of many materials are changing with frequency. This is
due to molecular, atomic and nuclear resonances. This
behavior is useful for remote sensing and other
5. There is much less background noise at microwave
frequencies than at RF.
Examples of commercial products involving microwave circuits
include wireless data networks [Bluetooth, WiFi (IEEE Standard
802.11), WiMax (IEEE Standard 802.16), ZigBee], GPS,
cellular telephones, etc. Can you think of some others?
As was mentioned, microwave circuits are much more difficult
to analyze than low frequency ones. Why?
1. Voltage is not well defined if the distance between the
two points is not electrically small. At microwave
frequencies, “electrically large” distances may be just a
few millimeters! Moving the probe leads around will also
likely affect voltage measurements.
2. One must carefully choose lumped elements (L, C, R,
diodes, transistors, etc.) for use in the microwave region.
Typical low frequency components do not behave as
3. To “transport” electrical signals from one position to
another, one must use special “wires.” It is more common
to speak of “guiding” signals at these frequencies.
Electricity is an electromagnetic (EM) phenomenon involving
forces produced by stationary and moving charge.
Low frequency circuits are generally very, very small wrt the
smallest EM wavelength present in the circuit. Because of this,
simple lumped element circuit models can be used to describe
the EM effects of resistors, inductors, capacitors, voltage and
current sources, etc.
Conversely, at microwave frequencies the circuits may not be
electrically small. This requires a shift in our approach to the
EM analysis. Nevertheless, the electrical signals remain the
outcome of EM phenomenon. Perhaps the two most important of the Maxwell equations are
Much of our work in this course will be in the sinusoidal steady
state. With an assumed (and suppressed) e j?t time convention, these curl equations become:
where E , B , D, H and J are all vector phasors.
Of course, both the differential and integral forms are equally
valid. Which of these to use depends on the problem:
• To derive equations to solve for E and H , the differential
forms are often better.
• For circuit approximations of devices (or other physical
interpretations), the integral forms are often more useful.
Rather than using the full-blown Maxwell’s equations in
microwave circuit design, approximations are often made to
simplify the solutions. Transmission line theory, to be discussed
next, is one of these. We will not explicitly be seeing Maxwell’s equations much in this course.