Role Of Waveguides In Em Waves Transmission.docx

WHAT IS THE ROLE OF WAVEGUIDES IN EM WAVES TRANSMISSION? DESCRIBE IMPEDANCE MATCHING & TINING IN WAVEGUIDES?

 HISTORY OF WAVEGUIDES: The 1890s During theorists did the first analyses of electromagnetic waves in ducts. Around 1893 J. J. Thomson derived the electromagnetic modes inside a cylindrical metal cavity. In 1897 Lord Rayleigh did a definitive analysis of waveguides; he solved the boundary-value problem of electromagnetic waves propagating through both conducting tubes and dielectric rods of arbitrary shape. He showed that the waves could travel without attenuation only in specific normal modes with either the electric field (TE modes) or magnetic field (TM modes), or both, perpendicular to the direction of propagation. He also showed each mode had a cutoff frequency below which waves would not propagate. [1]

 INTRODUCTION TO WAVEGUIDES: In electromagnetics and communications term waveguide may refer to any linear electromagnetic waves between its endpoints.

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A waveguide is a special form of transmission line consisting of a hollow, metal tube. The tube wall provides distributed inductance, while the empty space between the tube walls provide distributed capacitance.[2]

 ROLE OF WAVEGUIDES IN Transmission of EM Waves: Waveguides are practical only for signals of extremely high frequency, where the wavelength approaches the cross-sectional dimensions of the waveguide. Below such frequencies, waveguides are useless as electrical transmission lines.  When functioning as transmission lines, though, waveguides are considerably simpler than two-conductor cables—especially coaxial cables—in their manufacture and maintenance. With only a single conductor (the waveguide’s “shell”), there are no concerns with proper conductor-to-conductor spacing, or of the consistency of the dielectric material, since the only dielectric in a waveguide is air. Moisture is not as severe a problem in waveguides as it is within coaxial cables, either, and so waveguides are often spared the necessity of gas “filling.”

 Waveguides may be thought of as conduits for electromagnetic energy, the waveguide itself acting as nothing more than a “director” of the energy rather than as a signal conductor in the normal sense of the word. In a sense, all transmission lines function as conduits of electromagnetic energy when transporting pulses or high-frequency waves, directing the waves as the banks of a river direct a tidal wave. However, because waveguides are single-conductor elements, the propagation of electrical energy down a waveguide is of a very different nature than the propagation of electrical energy down a two-conductor transmission line.  At microwave signal frequencies (between 100 MHz and 300 GHz), two-conductor transmission lines of any substantial length operating in standard TEM mode become impractical. Lines small enough in cross-sectional dimension to maintain TEM mode signal propagation for microwave signals tend to have low voltage ratings, and suffer from large, parasitic power losses due to conductor “skin” and dielectric effects. Fortunately, though, at these short wavelengths there exist other modes of propagation that are not as “lossy,” if a conductive tube is used rather than two parallel conductors. It is at these high frequencies that waveguides become practical.  When an electromagnetic wave propagates down a hollow tube, only one of the fields—either electric or magnetic—will actually be transverse to the wave’s direction of travel. The other field will “loop” longitudinally to the direction of travel, but still be perpendicular to the other field. Whichever field remains transverse to the direction of travel determines whether the wave propagates in TE mode (Transverse Electric) or TM (Transverse Magnetic) mode. (Figure below)

 Like transmission line filters, waveguide filters always have multiple passbands, replicas of the lumped element prototype. In most designs, only the lowest frequency passband is useful (or lowest two in the case of band-stop filters) and the rest are considered unwanted spurious artefacts. This is an intrinsic property of the technology and cannot be designed out, although design can have some control over the frequency position of the spurious bands. Consequently, in any given filter design, there is an upper frequency beyond which the filter will fail to carry out its function. For this reason, true low-pass and high-pass filters cannot exist in waveguide. At some high frequency there will be a spurious passband or stopband interrupting the intended function of the filter. But, similar to the situation with waveguide cutoff frequency, the filter can be designed so that the edge of the first spurious band is well above any frequency of interest. IMPEDANCE MATCHING IN WAVEGUIDES: Waveguide transmission systems are not always perfectly impedance matched to their load devices.The standing waves that result from a mismatch cause a power loss, a reduction in powerhandlingcapability, and an increase in frequency sensitivity. Impedancechanging devices are therefore placed inthe waveguide to match the waveguide to the load. These devices are placed near the source of thestanding waves.Figure 1-42 illustrates three devices, called irises, that are used to introduce inductance orcapacitance into a waveguide.

An iris is nothing more than a metal plate that contains an opening throughwhich the waves may pass. The iris is located in the transverse plane.

An inductive iris and its equivalent circuit are illustrated in figure 1-42, view (A). The iris places ashunt inductive reactance across the waveguide that is directly proportional to the size of the opening.Notice that the edges of the inductive iris are perpendicular to the magnetic plane. The shunt capacitivereactance, illustrated in view (B), basically acts the same way. Again, the reactance is directlyproportional to the size of the opening, but the edges of the iris are perpendicular to the electric plane. Theiris, illustrated in view (C), has portions across both the magnetic and electric planes and forms anequivalent parallel-LC circuit across the waveguide. At the resonant frequency, the iris acts as a highshunt resistance. Above or below resonance, the iris acts as a capacitive or inductive reactance.POSTS and SCREWS made from conductive material can be used for impedancechanging devicesin waveguides. Figure 1-43A and 1-43B, illustrate two basic methods of using posts and screws. A post orscrew which only partially penetrates into the waveguide acts as a shunt capacitive reactance. When thepost or screw extends completely through the waveguide, making contact with the top and bottom walls,it acts as an inductive reactance. Note that when screws are used the amount of reactance can be varied.[3]

TUNING IN WAVEGUIDES:  Waveguide must be tuned just as resonant cavities or transmission lines must be tuned. Tuning waveguide means that you change its impedance to match the impedance of the device to which it is connected. This may be an antenna, a magnetron, or any other RF component.  Waveguide tuning methods are similar to those used in tuning resonant cavities. That is, you change the capacitive reactance so that it cancels the inductive reactance of the waveguide. inductive reactance so that it cancels the capacitive reactance of the waveguide. The first type of tuning we will consider is that done at the factory where the waveguide is constructed.  Waveguide attenuators. a. As you recall, an attenuator is a device used to reduce electrical energy. The two main types of attenuators used with waveguide are the shutter and resistive card. b. Part A of Figure 128 shows a shutter attenuator with its locking screw. As you lower the shutter into the waveguide, it reflects some of the energy traveling down the guide. The reflected energy is a loss, and the power on the other side of the shutter is less than that on the input side. We can say then, that the shutter attenuates (reduces) the power. The more you lower the shutter into the waveguide, the more attenuation you get. The locking screw holds the shutter at the desired position.

Notice that a shutter attenuator looks like the iris used in waveguide tuning. Actually, any tuning device results in some loss of power and can be used as an attenuator. The methods we use specifically for attenuation, however, are more convenient for this purpose.

 Fixed window tuner:  Figure shows two metallic fins or plates placed in a waveguide in such a way that they reduce the cross-section of the guide. The metal partitions are reactive elements called irises. The space between the metal plates is called a window; that's why we call it window tuning. The irises change the impedance characteristics of the waveguide by obstructing the electric and magnetic fields in the guide.

 Part A of Figure shows two metal partitions placed in a section of waveguide so they obstruct the electric field. This type of tuning is called capacitive window tuning. Here is how capacitive window tuning works. Suppose there is an undesirable inductive reactance at a particular point in a section of waveguide. You know that inductive and capacitive reactances are 180 degrees out of phase with each other. So, to get rid of the inductive reactance, all you have to do is add an equal capacitive reactance that cancels the inductive reactance. That is what happens when we place metal partitions across the width of the waveguide. The iriseobstruct the passage of the electric field and act as a capacitive reactance. The further we extend the irises into the waveguide, the more capacitive reactance we add at that point.[4]

REFERENCE: [1]https://en.wikipedia.org/wiki/Waveguide_(electromagnetism)

[2] https://www.allaboutcircuits.com/textbook/alternating-current/chpt-14/waveguides/ [3] http://electriciantraining.tpub.com/14183/css/Waveguide-Impedance-Matching-45.htm

[4] http://armymunitions.tpub.com/mm50058/mm500580191.htm