Chapter-1
INTRODUCTION TO MICROWAVE
“A radio wave operating in the frequency range of
300 MHz to 300 GHz that requires printed circuit
components be used instead of conventional lumped
components.”
The term microwaves refers to alternating current signals
with frequencies between 300MHz (3 x 108Hz) and 300
GHz (3 x 1011Hz), with a corresponding electrical
wavelength between λ=c/f=1m and λ=1mm, respectively.
Signals with wavelengths on the order of millimeters are
called millimeter waves.
FIGURE 1.1 The electromagnetic spectrum.
Because of the high frequencies (and short wavelengths), standard
circuit theory generally cannot be used directly to solve microwave
network problems.
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 are on the order of
the microwave wavelength. At much lower frequencies, the wavelength
is large enough that there is insignificant phase variation across the
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 with
the theory of geometrical optics. Such techniques are sometimes
applicable to millimeter wave systems, where they are referred to as
quasioptical.
 The lumped circuits: carbon resistors, mica capacitors, and small
inductors:
 The reason those lumped components can not be used in microwave
frequency application is a phenomenon called Skin effect.
 Skin effect: high frequency energy travels only on the outside skin of
a conductor and does not penetrate into it any great distance.
 As frequency gets higher, a centrifugal force also be present. The force
is inductance that is set is set up in the transmission line simply
because a current is flowing in that transmission line. This force ,
which we refer to as a microwave centrifugal force, keeps the energy
from penetrating the surface of the Transmission line and makes it
follow a path along the skin of the line rather than down into the entire
cross-sectional area, as in low frequency circuits.
skin depth
-how far the microwave energy actually penetrates a conductor.
- This depth is dependent on the material being used and on the frequency
at which you are operating.
For example,
-the skin depth in copper at 10 GHz is 0.000025 inch
- for aluminum at 10 GHz, it is 0.000031 inch; for silver, it
is 0.000023 inch; and for gold, it is 0.000019 inch.
Since the high-frequency signals and transmission lines do not allow
energy to penetrate very far into a conductor, it makes no sense to have
round (radial) wire leads on components for microwave applications.
The energy would travel only on the skin of the lead and be very
inefficient. That is why you see ribbon leads or no leads with solder
termination points on most microwave components. It also is why you
do not see many physical components on a microwave circuit board.
They are there, but they are distributed over a large, thin area and result
in the same values as a lumped device that would be used at lower
frequencies; hence, the term distributed element components.
Lead reactance
At RF and microwave frequencies
Physical size of circuit approaches to the wave- length - the phase of ac
signal must be considered
At higher frequency range
For larger size of the circuits
Voltage and Current must be treated as waves
Phasor notation is very convenient
On the circuit board one dimensional analysis is possible
Distributed circuit approach must be used
Lumped element equivalent circuit approach enable us to use Basic Circuit Theory
Impedance is very important as in the Circuit Theory
Advantages of Microwaves
There are many advantages of Microwaves such as the following −
Supports larger bandwidth and hence more information is transmitted. For
this reason, microwaves are used for point-to-point communications.
More antenna gain is possible.
Higher data rates are transmitted as the bandwidth is more.
Antenna size gets reduced, as the frequencies are higher.
Low power consumption as the signals are of higher frequencies.
Effect of fading gets reduced by using line of sight propagation.
Provides effective reflection area in the radar systems.
Satellite and terrestrial communications with high capacities are possible.
Low-cost miniature microwave components can be developed.
Effective spectrum usage with wide variety of applications in all available
frequency ranges of operation
Just as the high frequencies and short wavelengths of microwave energy
make for difficulties in analysis and design of microwave components and
systems, these same factors provide unique opportunities for the application
of microwave systems.This is because of the following considerations
Antenna gain is proportional to the electrical size of the antenna. At
higher frequencies, more antenna gain is therefore possible for a given
physical antenna size, which has important consequences for implementing
miniaturized microwave systems.
More bandwidth (information-carrying capacity) can be realized at
higher frequencies. A I% bandwidth at 600 MHz is 6 MHz (the bandwidth
of a single television channel)
and at 60 GHz a l % bandwidth is 600 MHz (100 television channels).
Bandwidth is critically important because available frequency bands in the
electromagnetic spectrum are being rapidly depleted.
Microwave signals travel by line of sight and are not bent by the ionosphere
as are lower frequency signals. Satellite and terrestrial communication links
with very high capacities are thus possible, with frequency reuse at
minimally distant locations.
The effective reflection area (radar cross section) of a radar target is usually
proportional to the target's electrical size. This fact, coupled with the
frequency characteristics of antenna gain, generally makes microwave
frequencies preferred for radar systems.
Various molecular, atomic, and nuclear resonances occur at microwave
frequencies, creating a variety of unique applications in the areas of basic
science, remote sensing, medical diagnostics and treatment, and heating
methods
Higher frequency operation has several advantages, including:
Terminologies of microwaves and wireless
technology
1. Decibel (dB)
A decibel, which is a relative term with no units, is a ratio of two
powers (or voltages).
If an output power of a device (or system) is measured, an input
power is measured, the ratio of the two taken, and the log of the
ratio is multiplied by 10, you have a decibel value for that
particular gain or loss. (When using voltages, the multiplication
factor is 20.)
The dB notation compresses the wide
range of power values that occur in
microwave equipment into a practical
range of numbers. It allows picowatt and
megawatt to be dealt within the same
calculation.
The dB notation also allows addition to be
used instead of multiplication, when
tracing a microwave signal through a
microwave system or test set up.
The term decibel tells you only how much a device increases or decreases
a power or voltage level. It does not tell you what that power or voltage
level actually is. That is valuable in determining a system’s overall gain or
loss.
2. Decibels referred to milliwatts (dBm)
is an absolute number, that is, decibels referred to milliwatts are specific
powers (milliwatts, watts, and so forth). To determine decibels referred to
milliwatts you need only one power. If you have a power of
10mW(0.010W), for example, you would take that power, divide it by 1
mW, take the log of the result, and multiply it by 10 (+10 dBm, in this
case).
As can be seen, the value of +10 dBm tells you that a definite 10 mW
of power are available from a source or are being read at a specific
point. That differs greatly from +10 dB, which only means that there is
a gain of 10 dB (gain of 10). So whenever you require absolute power
readings, use decibels referred to milliwatts.
3. Characteristic impedance.
Characteristic impedance is an impedance (in ohms) that
determines the flow of high-frequency energy in a system or through a
transmission line.
The characteristic impedance most often used in high-frequency
applications is 50Ω.
This value is a dynamic impedance in that it is not an ohmic value
measured with an ohmmeter but rather an alternating-current (ac)
impedance, which depends on the characteristics of the transmission line
or component being used.
characteristic impedance is not a direct-current (dc) parameter but
one that “characterizes” the system or transmission line at the
frequencies with which it is designed to work.
We have mentioned earlier that the characteristic impedance most often
used in high-frequency applications is 50Ω. The question that comes
about is:Where does this 50-Ω figure come from?
the maximum power handling capability of a particular transmission line
or system is 30Ω, while the lowest attenuation for a transmission line or
system is 77Ω. The ideal characteristic impedance, therefore, is a
compromise between these two values, or 50Ω.
The six main components of microwave system:
1. Oscillator
2. Amplifier
3. Signal control components
4. Microwave Tubes
5. Microwave Antennas
6. Low noise Receiver.
The two main causes of signal power loss in microwave
communication systems:
1. When a signal passes through a microwave component some portion
of the signal power is get absorbed by the component.
2. Due to impedance mismatch part of the input signal power is
reflected back and it never passes the component.
4. Insertion Loss
5. Return loss.
The return loss (in decibels) indicates the level of power being reflected
from a device due to a mismatch.
If we have a perfect match between a transmission line and a load at
its output, very little, if any, power is reflected, and the difference
between the input level and the reflected power is a large number of
decibels.
The return loss for a matched, or near-matched, condition is a large
negative number of decibels; the value for a large mismatch is basically
0 dB. It is important to point out that the return loss is a negative
number, because it is a loss.
6. Reflection coefficient.
The reflection coefficient is the percentage of power reflected from a
mismatch at the end of a transmission line or at the input or output of a
circuit.
If there is a perfectly matched condition, the reflection coefficient is 0
(0%); if there is an open circuit or a short circuit at the end of a
transmission line, the reflection coefficient is 1 (100%).
Any mismatch condition between those two extremes is between 0 and 1.
The designation for the reflection coefficient is either ρ or Γ, depending
on the text you are using.
7. Voltage Standing Wave Ratio (VSWR)
- is used to characterize many areas of microwaves.
- It is a number between 1.0 and infinity.
The best value you can get for the VSWR is 1:1 (notice that it is
expressed as a ratio), which is termed a matched condition. (A matched
condition is one in which systems have the same impedance, so no
signals are reflected back to the source of energy.)
The amplitude of a standing wave depends on how well the output is
matched to the input. In high-frequency microwave applications, the
standing wave ratio depends on the value of the impedance at the output
of a transmission line compared to the characteristic impedance of the
transmission line.
A perfect match is indicated by no standing waves.
A drastic mismatch like an open circuit or a short circuit shows a large
amplitude standing wave on the transmission line or device. That would
indicate a very large mismatch between devices or between the
transmission line and the load that was at its output.
The larger the mismatch, the larger the VSWR on the transmission line
or at the input or output of a device.
Applications of Microwave Engineering
The majority of applications of today's microwave technology are:
 Communications system
 radar system
 environmental remote sensing
 medical systems
RF and microwave communications systems are pervasive,
especially today when wireless connectivity promises to
provide voice and data access to "everyone, anywhere, at
any time,"
 cellular telephone systems
 Satellite system
 Global Positioning Satellite (GPS) system
 Direct Broadcast Satellite (DBS) system
 Wireless Local Area Networks (WLANs)
Revision
1. High frequency energy travels only on the outside skin or surface of a conductor
and does not penetrate into it any great distance. And it causes higher losses and
field radiation. To overcome this problem flat and field confined cables are used.
This phenomenon is called _________.
a. Phase shift
b. lead reactance c. skin effect d. return loss
2. Which of the following is an advantage of microwave frequencies?
a. Reduced dimension or size of antennas and other components.
b. More bandwidth (information-carrying capacity) can be realized at higher
frequencies.
c. Microwave signals travel by line of sight and are not bent by the
ionosphere as are lower frequency signals.
d. Less interference from nearby applications.
e. All of the above
3. If 10% of the microwave power is reflected at a mismatch. Find the return loss,
reflection coefficient and SWR.
4. If the return loss is 20dB, find percent reflected power, reflection coefficient
and SWR.
5. Complete the table.