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Waveguide

NEED OF WAVEGUIDE In the world of advanced communication system, the signal can be transmitted from one end to other with help of bonded media like coaxial cable, parallel conductor, twisted pair line, fiber optics cable etc. or with unbounded media like satellite, wireless communication. At frequencies of approximately 1 GHZ and above, the ability of cable to carry energy decrease effectively and quickly. Above 18 to 20 GHZ, coax is generally not usable except for very short distances. As a result of high losses and attenuation in cable from ‘skin effect’ and radiations, little of initial energy reaches the load, even if the line to load match is perfect. Also it is difficult to transfer large amount of power, because the voltage that occurs at high power level will break down dielectric barrier between conductors. So very wide spacing is required. The perfect solution over such losses is a system, which shows reliable performances at higher frequency named as ‘Waveguide’.

Waveguide

INTRODUCTION TO WAVEGUIDE Waveguide is nothing but a hollow conducting metallic tube used to guide electromagnetic wave. It can be also described as the waveguide is simple arrangement of conductors and isolators which carries electromagnetic field from one place to other. A waveguide is conducting tube through which energy is transmitted, in the form of electromagnetic (EM) wave. This waveguide tube is not carrying a current in same way as regular cable. Instead, it acts as boundary or enclosure for space through which EM wave propogates. The complete enclosure and skin effect prevents any energy from radiating outside the waveguide, so there are virtually no losses due to radiation. The signal energy is injected at one end of waveguide by single launcher and received at another end, where it is removed by signal absorber. The waveguide confines the propogation energy by reflecting it off the conducting walls. Any configuration electric and magnetic electric and magnetic field that exists inside waveguide is solution of Maxwell’s equation. The waveguide when filled with an inert gas such as nitrogen and pressurized in order to reduce losses for transporting RF energy over long distance. Actually traveling of wave inside waveguide takes place along wall to wall reflection i.e. in Zigzag fashion as shown in fig.

Fig: traveling of EM wave through waveguide.

Waveguide

MAXWELL’S EQUATION Electric and magnetic field that vary with time are governed by physical laws described by a set of equations known as the collectively as ‘Maxwell’s equation’. For the most part, these equations were arrived at experiment carried out by several investigators. It is not our purpose to justify the basis for these equations, but rather to gain some understanding to their physical significance and to learn how to obtain solution of interest in microwave engineering field. The electric field ‘r’ and magnetic field ‘B’ are vector field are in general have amplitude and directions that vary with these three spatial coordinates x, y, z and the time co-ordinate t. In mk3 unit, the electric field is measured in volts per meter and magnetic field in Weber per square meter. Since these field are vector fields the equations governing their behaviour are most conveniently written in vectors form. The electric field r and magnetic field ‘B’ are regarded as fundamental in that they give the force on charge ‘q’ moving with the velocity ν is given by,

F = q(r + v * β )

Waveguide

CLASSIFICATION OF WAVEGUIDES According to shape, waveguide are classified in two types broadly. 1. Rectangular waveguide. 2. Circular waveguide 1. Rectangular Waveguide The waveguide having the rectangular cross section is called as Rectangular waveguide. The cut off wave length ‘ λ c ‘ of rectangular waveguide is given by

λc =

2a m

Where, a – Spacing m – number of magnetic loop enclosed in conducting walls. The rectangular waveguide is shown as in fig. Direction of propogation

Waveguide Mouth

Fig. Rectangular waveguide

Waveguide 2. Circular Waveguide The construction of circular waveguide is similar to that of rectangular waveguide except that it has circular geometry. The cut off wavelength λ c is given by

λc =

2πr ' Kr

where r - inner radius of waveguide Kr – Solution of Bessel function. The circular waveguide is as shown in fig.

Fig. Cross-section of circular waveguide

Waveguide

PROPAGATION MODES IN WAVEGUIDE The electromagnetic waves in waveguide may travel in no. of various configurations such configurations are called as ‘Modes’ i.e. modes of propagation. Generally there are two modes i.e. transverse magnetic (TMmm) depending on electric or magnetic field is transverse to direction of propagation. Where m represent height of cross section of waveguide and n denotes number of half wavelength of intensity of electric field in TE mode while magnetic field intensity in TM mode.

Fig. Electric and magnetic field waveguide

Waveguide

COMPARISION OF RECTANGULAR AND CIRCULAR WAVEGUIDE Rectangular Waveguide Circular Waveguide i) The physical geometry of rectangular i) The physical geometry of circular waveguide i.e. it has rectangular cross wageguide i.e it has circular cross section. section. ii) The manufacture is comparitively ii) The manufacture is simple as compare to difficult. rectangular. iii) Cut – off Wavelength is 2r. iii) Cut – off Wavelength is 3. 2r. iv) TE10 mode shows dominant mode of iv) TE11 mode shows decimal mode of propogation. v) Losses are more than circular one vi) The waveguide coupling is complex. vii) It is used at very few applications.

propogation. v) Losses are less than Rectangular one. vi) The waveguide coupling is simple. vii) It is widely used in number of applications.

WAVEGUIDE COUPLING When two or more waveguide pieces or component joint together coupling is formed, which is generally by means of flanges. The function of such flange is to insure smooth mechanical junction and suitable electrical characteristics with low internal reflection.

Waveguide A) Flanges A typical piece of waveguide will have flange at either end such as shown in fig a and b. At lower frequency, flange will be branched or soldered on to waveguide where as at higher frequency much flatter butted plane flanges is used. When two pieces are joint flanges are bolted together, care been taken to insure perfect mechanical alignment which prevent an unwanted bend or step. Either of which would produce undesirable deflection. The flanges and guide end must be smoothly finished to avoid discontinuity at junction. With waveguide naturally, reduce in size when frequency rise.

A coupling

discontinuity become larger in proportion to signal wavelength and guide dimension. The discontinuity at higher frequency become more troublesome. To counteract small gap may be left between waveguide as shown in fig c. Fig d shows chock couple which is consist of ordinary flange and chock flange connected together to compenset for discontinuity.

Fig (a) Plain flange (b) Flange coupling

Fig. (a) Cross section of choke coupling (b) end view of choke flange.

Waveguide B) Bends and corner. :

Sometime it is required to change direction in which case bend

or corner may be used. Since these are discontinuity SWR will be increased either because of reflection for corner or because of different group velocity in piece of bend waveguide. H plane bend as shown in fig is piece of waveguide smoothly bend in plane parallel to magnetic field. In order to keep reflection in bend small, it’s wavelength. If it is undesirable because of size or bend must be sharp it is possible to minimize reflections by making mean length of bend and integral no. of guide wavelength. In this case, some cancellation of reflection takes place it must be noted that sharper bend greater mismatch included. For larger wavelength, bend is rather clumpsy & corner may be used instead. Because such a corner would introduce reflections

Fig. a) H-plane bend b)H-plane mitered corner c) E-plane double-mitered corner

if it were simply 900 corner. A part of it is cut, and corner is said to be mitered as shown in fig b. The dimension c depends on wavelength if it is correctly chosen reflection must be almostly completely eliminated. A H-plane corner is shown with E plane corner. There is risk of vtg. breakdown across distance c which would be naturally fairly small in such corner. Thus if a change of direction in E plane is required, double mitered corner is used. C) Taper and twists When it is necessary to couple waveguide having different dimension or having different cross-section shapes. Again some reflection will take place but they can be reduced if taper section is made gradually as shown in fig a. Final if change of polarization of direction is required a twist section may be used as shown in fig. b.

Waveguide

Fig. Waveguide transition a) Circular to rectangular taper b) 900 twist.

D) T- junctions Whenever it is required to combine two or more signal or spilt a signal into two or more parts in waveguide system, some form of T junction may be used.

For simpler

interconnection T shape junction are used where as more complex junction may be hybrid T. i) E plane T :- It is called as series T. It is as shown in fig a. It consists of main arm and side arm. Side arm is perpendicular to main arm and it is parallel to direction of field. It is also known as voltage junction.

Fig E plane T Fig. E plane T.

ii) H Plane T :- It is called as parallel or shunt T. It is shown in fig b. It also consists of main arm and side arm. Side arm is perpendicular to main arm and it is perpendicular to direction of field. It is also known as current junction.

Waveguide

Fig. H plane T

iii) Hybrid T :- It is also known as Magic T junction. Magic T is combination of E plane T and H plane T. Hence it is called as Hybrid T. This is called magic T because power gets divided into various arms depending on condition of entering power. In particular mode the various part of Magic T is shown in fig C. like part 1, part 2, part 3, part 4.

Fig. Hybrid T

ISOLATOR

Waveguide

Fig Faraday’s rotational isolator

This device is used to isolate one component in transmission line. An ideal isolator completely absorb power for propogation in one direction and provide loss less transmission in opposite direction, thus isolator is usually called uniline. Faraday’s rotational isolator consist of rectangular waveguide and one circular waveguide. The construction is as shown in fig a. The ferrite rod is place in circular waveguide at two end of circular waveguide two rectangular waveguide are place. The resistive pad are place as shown in fig. The magnetism is provided to this ferrite rod by using permanent magnet. A forward wave entering in rectangular waveguide is dominant mode, when this wave enter in circular waveguide it becomes dominant mode (TE11) when isolator is excited. The ferrite rod gives 450 rotation. The output end again gives 450 rotation to cancel the initial rotation. So far forward wave their is no attenuation while reflected wave is absorbed by resistive pads.

CIRCULATOR The circulator is a ferrite component. Ferrite circulator is three parts device. The reflected power is brought out from the separate port from the incident and transmitted power post and can be controlled or absorbed in the load. The basic circulator configuration is as shown in fig.

Waveguide

Fig. Basic Circulator

Circulators nowadays

used are Y junction circulator.

It can be used for both

waveguide and stripline. The ferrite material is placed equiangularly spaced transmission path and DC magnetic field is applied perpendicular to plane of these paths. The complex coupling results due to dielectric resonator and hence gives circulator characteristics that is low insertion loss, low VSWER, high port to port isolation etc. Circulators are used in diplexer as they provide greater than 20 dB isolation between transmitter and antenna. The power handling capacity is of the order of few megawatts.

Waveguide

CONCLUSION At higher frequency of same GHZ, range, the waveguide become eventually a perfect solution for guiding the wave. In spite of few losses it is a widely used media for high freq. communication. Different size, shape and properties make a waveguide popular for low noise

Waveguide and less distorted comm. The increasing use of waveguide brings a new revolution of technology in world of communication.

REFERENCE 1) Foundation for microwave engineering - Robert E collin 2) Microwave engineering - Sanjiv Gupta 3) Microwave devices & circuit - Samuel Y. LIAO 4) Electronic communication system - William Schweber 5) Electronic communication system - Frank R Dungan 6) Electronic communication system - Kennedy & Davis

COMPARISION OF RECTANGULAR AND CIRCULAR WAVEGUIDE Rectangular Waveguide Circular Waveguide i) The physical geometry of rectangular i) The physical geometry of circular

Waveguide waveguide i.e. it has rectangular cross wageguide i.e it has circular cross section. section. ii) The manufacture is comparatively ii) The manufacture is simple as compare to difficult. rectangular. iii) Cut – off Wavelength is 2r. iii) Cut – off Wavelength is 3. 2r. iv) TE10 mode shows dominant mode of iv) TE11 mode shows decimal mode of propogation. v) Losses are more than circular one vi) The waveguide coupling is complex. vii) It is used at very few applications.

propogation. v) Losses are less than Rectangular one. vi) The waveguide coupling is simple. vii) It is widely used in number of applications.

Waveguide

Fig. Electric and magnetic field waveguide

Waveguide

Fig Faraday’s rotational isolator

Fig. Basic Circulator

Fig. E plane T.

Waveguide

Fig. H plane T

Fig. Hybrid T

Fig (a) Plain flange (b) Flange

coupling

Fig. a) H-plane bend b)H-plane mitered corner c) E-plane double-mitered corner

Waveguide

Fig. Waveguide transition a) Circular to rectangular taper b) 900 twist.

Waveguide

Waveguide

Waveguide