Introduction

ETT 06202 Microwave Technology

What are microwaves?
• Microwaves are electromagnetic waves whose
frequencies range from about 300 MHz – 300 GHz
• The word Microwave means very short wave, which is
the region of the radio spectrum and a part of the
electromagnetic spectrum.
— For example, a 100-GHz microwave signal has a wavelength of
0.3 cm
Frequencies above 20GHz are referred to as millimetre waves
How microwaves differ from other EM
waves?
• Microwaves are distinct because at microwave
frequencies the wavelength approximates the
physical size of ordinary electronic components.
• Therefore components behave differently at
microwave frequencies than they do at lower
frequencies.
• For example: At microwave frequencies, a
half-watt metal film resistor, looks like a complex
RLC network with distributed L and C values and
a surprisingly different R value.
• Distributed L and C are crucial at microwave
frequencies but negligible at lower frequencies
 How microwaves differ from other EM
waves?
• why ordinary lumped constant electronic
components do not work well at microwave
frequencies?
1. component size and lead lengths approximate
microwave wavelengths.
2. distributed values of inductance and
capacitance become significant at these
frequencies.
3. phenomenon called skin effect produces an
ac resistance that is greater than the dc
resistance.
Cont’d
— Skin effect is the phenomenon in which alternating
currents tend to flow on the surface of a conductor.
While dc currents flow in the entire cross section of the
conductor, ac flows in a narrow band near the surface.
— Current density falls off exponentially from the surface of
the conductor toward the center
— Because current only flows in a small part of the
conductor (instead of the entire conductor, or nearly so
as at lower frequencies), the ac resistance of a
conductor at microwave is higher than the dc resistance
by a considerable amount.
— Because skin effect is a function of frequency, we
find that conductors will perform as they do at dc for
very low-frequency ac, but may be useless at microwave
frequencies
Cont’d
— For these reasons, microwave circuits must consider
the distributed constants of R, L, and C for electronic
components.
— In addition, specially designed lumped
constant components, such as thin-film inductors,
strip-line inductors, chip resistors, and capacitors,
must be used.
— A combination of these will be found
in many microwave circuits.

Chip resistor & capacitors Thin film inductor

Strip line inductor
 Cont’d
• a common resistor that looks like pure resistance at low frequencies
does not exhibit the same characteristics at microwave frequencies.
• The short leads of a resistor, although they may be less than an
inch, represent a significant amount of inductive reactance at very
high frequencies. A small capacitance also exists between the leads.
• These small stray and distributed reactances are sometimes called
residuals. Because of these effects, at microwave frequencies a
simple resistor looks like a complex RLC circuit. This is also true of
inductors and capacitors.

Equivalent circuits of components at microwave frequencies. (a) Resistor. (b) Capacitor. (c) Inductor
• To physically realize resonant circuits at microwave frequencies, the
values of inductance and capacitance must be smaller and smaller.
Physical limits become a problem. Even a 0.5-in piece of wire
represents a significant amount of inductance at microwave
frequencies.
• Tiny surface-mounted chip resistors, capacitors, and inductors have
partially solved this problem. Furthermore, as integrated-circuit
dimensions have continued to decrease, smaller and smaller on-chip
inductors and capacitors have been made successfully
• Another solution is to use distributed circuit elements, such as
transmission lines, rather than lumped components, at microwave
frequencies. When transmission lines are cut to the appropriate
length, they act as inductors, capacitors, and resonant circuits.
Special versions of transmission lines known as striplines, microstrips,
waveguides, and cavity resonators are widely used to implement
tuned circuits and reactances
Microwave History
• The microwave spectrum was relatively undeveloped until
World War II, when radar and other electronics
technologies needed for the war effort caused the U.S.
government (and its allies) to pour a large amount of
money into research.
• The two main problems that prevented operation at those
frequencies were interelectrode capacitance and electron
transit time between the cathode and anode. Attempts to
reduce electrode size and spacing resulted in other
problems that proved unacceptable in practical circuits.
Microwave History
• By 1921, The magnetron was the first practical
high-power microwave generator and is still used today
in some radars and in microwave ovens.
• Unfortunately, the magnetron is a narrow-band device,
so one often has to make a trade-off between output
power and frequency agility.
• By 1937, he first klystron device, a vacuum tube that
amplified microwaves
• In the 1950s, vacuum tubes began to be
replaced with solid-state devices such
as bipolar transistors. Semiconductor material
exhibits a phenomenon similar to the vacuum
tube transit time limitation: electron saturation
velocity.
Microwave History
• Modern electronics designers have a wide variety
of solid-state devices to select from, and
microwaves are no longer the stepchild of
electronics technology.
Radar, wireless communications, electronic
navigation, and satellite TV links all
operate in the microwave region. In addition,
modern medical diathermy (tissue
heating) equipment
— Ordinary vacuum tubes do not work at
microwaves because of excessive interelectrode
capacitance and electron transit times greater
than the period of the microwave signal.
 Properties of Microwaves
1. Microwave is an electromagnetic radiation of
short wavelength (ranging 33cm to 1mm)
2. They can reflect by conducting surfaces just
like optical waves since they travel in straight
line.
3. Microwave currents flow through a thin outer
layer of an ordinary cable.
4. Microwaves are easily attenuated within short
distances.
5. They are not reflected by ionosphere

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Advantages and Limitations

1. Increased bandwidth availability:

Microwaves have large bandwidths compared to the
common bands like short waves (SW), ultrahigh
frequency (UHF) waves, etc.
For example, the microwaves extending from λ = 1 cm -
λ = 10 cm (i.e) from 30,000 MHz – 3000 MHz, this
region has a bandwidth of 27,000 MHz.
2. Improved directive properties:
The second advantage of microwaves is their ability to
use high gain directive antennas, any EM wave can be
focused in a specified direction (Just as the focusing of
light rays with lenses or reflectors)

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Advantages and
Limitations

3. Fading effect and reliability:

Fading effect due to the variation in the transmission
medium is more effective at low frequency.
Due to the Line of Sight (LOS) propagation and high
frequencies, there is less fading effect and hence
microwave communication is more reliable.
4. Power requirements:
Transmitter / receiver power requirements are pretty low
at microwave frequencies compared to that at short
wave band.

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Advantages and Limitations

5.Transparency property of microwaves:

Microwave frequency band ranging from 300

MHz – 10 GHz are capable of freely propagating
through the atmosphere.

The presence of such a transparent window in a

microwave band facilitates the study of
microwave radiation from the sun and stars in
radio astronomical research of space.

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 Applications

Microwaves have a wide range of applications in modern

technology, which are listed below

1. Telecommunication: Intercontinental Telephone and

TV, space communication (Earth – to – space and space
– to – Earth), telemetry communication link for railways
etc.
2. Radars: detect aircraft, track / guide supersonic
missiles, observe and track weather patterns, air traffic
control (ATC), burglar alarms, garage door openers,
police speed detectors etc.

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3.Commercial and industrial
applications

Microwave oven
Drying machines – textile, food and paper industry for drying
clothes, potato chips, printed matters etc.
Food process industry – Precooling / cooking, pasteurization /
sterility, hat frozen / refrigerated precooled meats, roasting of
food grains / beans.
Rubber industry / plastics / chemical / forest product industries
Mining / public works, breaking rocks, tunnel boring, drying /
breaking up concrete, breaking up coal seams, curing of
cement.
Drying inks / drying textiles, drying / sterilizing grains, drying /
sterilizing pharmaceuticals, leather, tobacco, power
transmission.
Biomedical Applications ( diagnostic / therapeutic ) –
diathermy for localized superficial heating, deep
electromagnetic heating for treatment of cancer, hyperthermia
( local, regional or whole body for cancer therapy).

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 Other Applications

4. Identifying objects or personnel by non –

contact method.

5. Light generated charge carriers in a

microwave semiconductor make it possible to
create a whole new world of microwave
devices, fast jitter free switches, phase
shifters, HF generators, etc.

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Microwaves frequency
• receivers need an
unobstructed view
of the sender to
successfully receive
microwaves
• microwaves are
ideal when large
areas need to be
covered and there
are no obstacles in
the path
Advantages of microwaves over radio waves
• because of high frequency, more data can be sent
through microwaves -> increased bandwidth, higher
speeds
• because of their short wave length, microwaves use
smaller antennas
• smaller antennas produce a more focused beam which is
difficult to intercept
Disadvantages of microwave communication
• they require no obstacle present in the transmission path
• the cost of implementing the communication infrastructure
is high
• microwaves are susceptible to rain, snow, electromagnetic
interference
Microwaves usages

▪ carrier waves in satellite communications

▪ cellular communication
▪ bluetooth
▪ wimax
▪ wireless local area network
▪ GPS (Global Positioning System)
Microwave communication concepts
• microwaves are generated by magnetrons through
vibration of electrons
• LoS (Line of Sight) – is a visible straight line between
the sender and the receiver
• LoS propagation – propagation of microwaves in a
straight line free from any obstructions
• Fresnel zone – eliptical area around the LoS between a
sender and receiver; microwaves spread into this area
once are generated by an antenna; this area should be
free of any obstacles:
Review questions
• Define microwaves
• Why microwave technology differs from ordinary
high frequency technology?
• Give the designations of microwave frequency
bands
• Why ordinary lumped constant components
don’t work properly at microwave frequencies?
Review questions
• What is the general microwave frequency
range?
• Briefly explain skin effect
• Calculate the wavelength of a 11.7 GHz
microwave signal in Teflon, which has a direct
electric constant of 9.6
Bibliography
• Sadie Ammons (2011), “Microwave Technology”, First
Edition, The English Press
• Joseph Carr (1997), “Microwave &wireless communication
technology”, Newnes
• Louis E. Frenzel Jr (2016), “principles of electronic
communication systems”, 4th ed, Mc-Graw Hill.
• Wayne Tomasi, Advanced electronic communications, 6th
edition