Contents

 Abstract
 Statement of the problem
 Introduction to microwave
(i) Microwave frequency bands in radio spectrum
 Microwave transmission
 Introduction to wave guides
(i) Methods of propagation
(ii) Principle of operation
(iii) Propagation modes and cut-off frequencies

 Waveguide as microwave transmission line

 Uses
 Drawbacks
 Conclusion
 Reference
Abstract
This project presents an introduction to the basics of microwave transmission

and waveguide. It is important to understand the principles behind the

propagation and transmission of high-frequency signals, which are very

important in the area of communications. Designs at such high frequencies

require careful considerations to minimize losses and to ensure maximum

power transmission. This work presents the basics of microwave transmission

and waveguide propagation.

Statement of the problem
Communication system is an interesting topic in the modern day technology.

Different frequency ranges of the electromagnetic radiations are used in

different purposes which use different types of channels for communication.

Out of all the frequency ranges I find microwave to be the most interesting

range. A very important question is what the reason behind studying

microwaves? What do these have to offer, and how are they advantageous?

The answer is that most of modern electronic communication engineering

makes use of microwaves. Then again, what do microwaves have that makes

them suitable for use in communication engineering? The microwave is

widely used for telephone network, in microwave links, for space

communication, in satellite communications etc. Due to its diverse areas of

applications the principle of microwave transmission attracts the interests of

the scientific community. Actually the principle of microwave transmission

cannot be derived by the mere extensions of either low frequency radio or

high frequency optical wave, although they all are based upon the same

fundamental law of electromagnetism.

I therefore find that the microwave transmission and waveguide is an

interesting area of work.

Introduction to Microwave
Microwaves are a part of the electromagnetic spectrum. Usually, waves with

wavelengths ranging from as low as a few millimeters to almost a meter are

classified as microwaves. Conventional frequency for the microwave

transmission range is from 300MHz −300GHz.

Microwave frequency bands in radio spectrum:

The typical and unique applications of microwave may be summarized with

their corresponding frequency range in the following table.

Table: The Scopes of microwave applications in communication system.

Serial No. Applications Frequency range

1 Television, Satellite communication, 0.3-3 GHz

Surveillance radar, navigational aids,

point to point communication

2 Microwave links, common carrier 3-30 GHz

land mobile communication, satellite

communication.
 Microwave transmission
 The principles of microwave transmission cannot be derived by mere

extension of low frequency radio or high frequency optical concepts,

although they are all based upon the fundamental laws of

electromagnetism.

 If microwave is fed in a conventional two conductor line where the

longitudinal and transverse dimension of line are comparable to the wave

length of the propagating signal, it leads to a series of interesting effects

that fall outside the scope of problems examined by the classical theory of

long transmission lines.

 Such a line cannot be used for microwave transmission.

Thus hollow metal tubes called wave guides are used.

Introduction to wave guides:

A waveguide is a structure that guides waves, such as electromagnetic

waves or sound, with minimal loss of energy by restricting expansion to one or

two dimensions.
 Various Types of Wave guides

There is a similar effect in water waves constrained within a canal, or why guns

have barrels that restrict hot gas expansion to maximize energy transfer to their

bullets.

Without the physical constraint of a waveguide, wave amplitudes decrease

according to the inverse square law as they expand into three dimensional

spaces. A waveguide is simply a pipe of any constant cross section through

which an E.M wave travels by reflection and not by conduction.

Propagation

 Since the method of propagation in a waveguide is by means of

reflection, hence the interior surface of the guide should be smooth and

silvered and free of moisture.

 An abrupt change in the shape and direction is avoided to address the

cause of reflection to occur back towards the source.

Propagation of wave through waveguide is by means of reflection and not conduction

 There are different types of waveguides for each type of wave. The

original and most common meaning is a hollow conductive metal pipe

used to carry high frequency radio waves, particularly microwaves.

 The geometry of a waveguide reflects its function. Slab waveguides

confine energy in one dimension, fiber or channel waveguides in two

dimensions.

 The frequency of the transmitted wave also dictates the shape of a

waveguide. An optical fiber guiding high frequency light will not

guide microwaves of a much lower frequency.

 As a rule, the width of a waveguide needs to be of the same order of

magnitudes as of the wavelength of the guided wave.

 Principle of operation:

Example of waveguides and a diplexer in air traffic control

 Waves propagate in all directions in open space as spherical waves.

The power of the wave falls with the distance R from the source as the

square of the distance (inverse square law).

 A waveguide confines the wave to propagate in one dimension, so

that, under ideal conditions, the wave loses no power while

propagating. Due to total reflection at the walls, waves are confined to

the interior of a waveguide.

 Propagation modes and cutoff frequencies
A propagation mode in a waveguide is one solution of the wave equations, or,

in other words, a form of the wave. Due to the constraints of the boundary

conditions, there are only limited frequencies and forms for the wave function

which can propagate in the waveguide. The lowest frequency in which a

certain mode can propagate is the cut off frequency of that mode.

The mode with the lowest cutoff frequency is the fundamental mode of the

waveguide, and its cutoff frequency is the waveguide cutoff frequency.

Propagation modes are computed by solving the Helmholtz

equation alongside a set of boundary conditions depending on the geometrical

shape and materials bounding the region. The usual assumption for infinitely

long uniform waveguides allows us to assume a propagating form for the

wave, stating that every field component is knowingly dependent on the

direction of propagation (z).

The common approach is to first replace all unknown time-varying fields

u(x, y, z, t) (assuming for simplicity to describe the fields

in Cartesian components) with their complex phasors representation U(x, y,

z) , sufficient to fully describe any infinitely long single-tone signal at

frequency f and rewrite the Helmholtz equation and boundary conditions

accordingly. The term γ gets introduced which represents the propagation

constant (still unknown) along the direction along which the waveguide

extends to infinity.
For a lossless case, the propagation constant might be found to take on either real

or imaginary values:-

 When γ is purely real, the mode is said to be "below cutoff", since the

amplitude of the field phasors tends to exponentially decrease with

propagation;

 An imaginary γ, instead, represents modes said to be "in propagation" or

"above cutoff", as the complex amplitude of the phasors does not change

with ’z’.

Waveguides as microwave transmission

line
Any structure to guide the flow of electrical energy from one point to other or

from the source to the load is called the transmission line. Wave propagation in

unbounded media in infinite extent is unguided. Since in such medium the

uniform plane waves exists throughout all space and the electromagnetic energy

associated with the wave spreads over a wide area. Wave propagation in the

unbounded media is used in radio and T.V broadcasting.

But in telephone or data communication the information is being used by a single

person. The use of particular type of transmission line depends upon the
frequency, the power to be transmitted and the type of insulation. However, at

microwave frequencies waveguide are normally used.

Hollow wave guides are generally used as transmission lines at frequencies

around 1 GHz and above. Wave guides have certain advantages over the co axial

lines. These are as follows.

1. Higher power handling capacity

2. Lower loss per unit length

3. A simpler lower cost structure

It is because of the low loss factor, wave guides have edge over other kinds of

transmission lines at higher frequencies.

There are three transmission modes (how the wave is transmitted through the

waveguide)
 1. TEM:- Transverse Electric and Magnetic waves

2. TE:- Transverse electric waves

3. TM:- Transverse Magnetic Waves

Uses:-

 Optical fibers transmit light and signals for long distances with low
attenuation and a wide usable range of wavelengths.

 In a microwave oven a waveguide transfers power from the magnetron,

where waves are formed, to the cooking chamber.

 In a radar, a waveguide transfers radio frequency energy to and from

the antenna.

 Rectangular and Circular waveguides are commonly used to connect

feeds of parabolic dishes to their electronics, either low-noise receivers
or power amplifier/transmitters.
 Waveguide supplying power for the Argonne National Lab.

 Waveguides are used in transmitting power between the components of

a system.

 Waveguides are used in scientific instruments to measure optical,

acoustic and elastic properties of materials and objects. The waveguide
can be put in contact with the specimen (as in a medical
ultrasonography), in which case the waveguide ensures that the power
of the testing wave is conserved.

Drawbacks
 However, it has some problems; it is bulky, expensive to produce, and

the cutoff frequency effect makes it difficult to produce wideband

devices.

 Ridged waveguide can increase bandwidth beyond an octave, but a

better solution is to use a technology working in TEM mode (that is,

non-waveguide) such as coaxial conductors since TEM does not have a

cutoff frequency.
Conclusion
If a waveguide is compared to the microwave transmission line it is seen that
the transmission line consists of two or more conductors when waveguide
often consists of a single conductor. A transmission line supports the
transverse electromagnetic wave with zero longitudinal field components.
The transverse electromagnetic waves have a uniquely defined voltage
current and characteristics impedance. A waveguide supports the transverse
electric or transverse magnetic field along with one or both longitudinal
components.

The frequency for which the wave propagation ceases is called the cut off
frequency of the conducting plane wave guide. The cut off frequency depends
on the mode number, the separation between the two planes of the plane
waveguide and the velocity in the medium.

In case of a plane waveguide the phase velocity varies from the velocity of
the light in free space up to infinity as the frequency decreases below the
cutoff frequency.

The velocity of propagation in the waveguide is thus greater than the phase
velocity in free space. As the frequency increases over the cut off, the phase
velocity decreases from infinity and approaches to the velocity of light in free
space.

Study of wave propagation in the waveguide is an interesting area of current

research. Lots of works in these fields are going on. Different types of
waveguide are constructed with different geometrical shape and with different
cut off frequency which supports the high velocity wave propagation. This
area is an attractive area of current interest. Hope this small report will help to
increase the interest of other youngsters in this field.
 Reference
 Microwaves transmission line: An introduction to the basics; Debapratim
Ghosh; Dept. of Electrical Engineering, IIT Bombay.

 Microwaves: Introduction to circuits, devices and Antennas; M.L.Sisodia,

V.L.Gupta; New Age International Ltd.2001

 Bound States in Twisting Tubes, J Goldstone, R.L. Jaffe, MIT Department of

Physics

 Han, CC; Hwang, Y, "Satellite antennas", in, Lo, Y T; Lee, SW, Antenna
Handbook: Volume III Applications, chapter 21, Springer, 1993 ISBN
0442015941.

 Oliner, Arthur A, "The evolution of electromagnetic waveguides: from hollow

metallic guides to microwave integrated circuits", chapter 16 in, Sarkar et al.,
History of Wireless, Wiley, 2006 ISBN 0471783013.

 D. Pozar, "Microwave Engineering", Third Edition, John Wiley and Sons,

2005, Chapter 3.

 Ramo, Simon; Whinnery, John R.; Van Duzer, Theodore (1994). Fields and
Waves in Communication Electronics. New York: Joh Wiley and Sons. pp.
321–324. ISBN 0-471-58551-3.