Contents
Abstract Statement of the problem Introduction to microwave (i)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 a reas 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
z . transmission range is from 300MHz −300GH
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 land mobile communication, satellite communication.
3-30 GHz
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 wa ve 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
z ’. with ’
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 b y 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 wa veguide 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, b ut 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.
