Waveguides and Antennas

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This lectures gives details of theory of transmission of microwaves through waveguides, modes of propagation, TE and TM waves, Horn Antenna, Dish Antenna,


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Waveguides & Microwave Antennas:

Waveguides & Microwave Antennas 1 Subject: Microwave & Radar Engineering Class: 6 th Sem ECE Presented By Kulwinder Singh Sr. Lecturer ECE SGHS Govt Polytechnic College Ranwan Email: kulwinderpannu@gmail.com Mobile: 97813-00151


Waveguides 26-03-2012 2 PUNJAB EDUSAT SOCIETY (PES)

Shape of Waveguide:

Shape of Waveguide Theoretically any shape can be utilized. But practically shape of waveguides is chosen by using following two criteria: Ease in manufacturing. Ease in understanding the operation . 26-03-2012 3 PUNJAB EDUSAT SOCIETY (PES)

Shape of Waveguides:

Shape of Waveguides Thus most commonly used waveguide shapes are one with Constant rectangular cross section Constant circular cross section 26-03-2012 4 PUNJAB EDUSAT SOCIETY (PES)

Construction of a Waveguide:

Construction of a Waveguide The walls of a waveguides are made of conducting material. N ot for conduction of energy, but for reflection of waves. Conduction of energy takes place through the dielectric filling of the waveguide, usually air. 26-03-2012 5 PUNJAB EDUSAT SOCIETY (PES)

Propagation of Wave in Waveguide:

Propagation of Wave in Waveguide 26-03-2012 6 PUNJAB EDUSAT SOCIETY (PES)

Electromagnetic Wave:

Electromagnetic Wave Direction of Propagation 26-03-2012 7 PUNJAB EDUSAT SOCIETY (PES)

PowerPoint Presentation:

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Waveguide Terminology:

Waveguide Terminology While discussing the behaviour and properties of the waveguides, We talk about electric and magnetic fields Instead of currents and voltages. 26-03-2012 9 PUNJAB EDUSAT SOCIETY (PES)

Terminology of Waveguides:

Terminology of Waveguides 26-03-2012 10 PUNJAB EDUSAT SOCIETY (PES)

Waveguide Dimensions:

Waveguide Dimensions The cross sectional dimensions of waveguide should be of the order of wavelength. /2 26-03-2012 11 PUNJAB EDUSAT SOCIETY (PES)

Waveguide Dimensions:

Waveguide Dimensions Therefore use of waveguides is common in 3GHz to 100GHz range (10cm to 3mm). Within this range waveguides are superior to co-axial cables. To transmit multiple frequencies simultaneously, waveguides do not need filters to separate them. Instead propagation in different modes is used. 26-03-2012 12 PUNJAB EDUSAT SOCIETY (PES)

Waveguide Components:

Waveguide Components A large number of components used in waveguides are similar to transmission lines e.g. stubs, quarter-wave transformers, directional couplers and taper sections. Smith Chart can also be used for waveguides in the same way as is used in transmission lines. 26-03-2012 13 PUNJAB EDUSAT SOCIETY (PES)

Advantages of Waveguides:

Advantages of Waveguides As in construction, waveguides are similar to co-axial cable with inner conductor removed, hence waveguides are simpler to manufacture. Due to absence of inner conductor and supporting dielectric, there is no flashover, hence power handling capacity of waveguides is high (about 10 times). 26-03-2012 14 PUNJAB EDUSAT SOCIETY (PES)

Advantages of Waveguides:

Advantages of Waveguides As there is only air inside the waveguide and transmission is through reflection, hence losses in waveguides are low as compared to transmission lines. For example at 3GHz loss in case of Foam dielectric cable -10.8 dB/100m Air dielectric cable – 4dB/ 100m Copper waveguide – 1.9 dB /100m 26-03-2012 15 PUNJAB EDUSAT SOCIETY (PES)

Advantages of Waveguides:

Advantages of Waveguides Waveguides can handle much higher maximum operating frequency because of different method of propagation. For example waveguides can handle maximum operating frequency of 325 GHz compared with 18GHz of co-axial cable. 26-03-2012 16 PUNJAB EDUSAT SOCIETY (PES)

Reflection of Wave:

Reflection of Wave Incident Ray  p  n  Crests 26-03-2012 17 PUNJAB EDUSAT SOCIETY (PES)

Relationship b/w Various Wavelengths:

Relationship b/w Various Wavelengths  p = /sin  n = / cos  Wavelength depends on direction in which it is measured. Its value is greater when measured in directions other than the direction of propagation. 26-03-2012 18 PUNJAB EDUSAT SOCIETY (PES)

Cut-off Wavelength:

Cut-off Wavelength From the figure above it is clear that a = m  n /2 ; where m is an integer.  n is wavelength in direction normal to both walls. a 26-03-2012 19 PUNJAB EDUSAT SOCIETY (PES)

Cut off wavelength:

Cut off wavelength As a= m.  n /2 And  n will become  o when it represents highest wavelength. Then a = m  o /2 OR  o = cut-off wavelength= 2a/m 26-03-2012 20 PUNJAB EDUSAT SOCIETY (PES)

Propagation Constant:

Propagation Constant The propagation constant of an electromagnetic wave is a measure of the change in amplitude of the wave as it propagates in a given direction. The propagation constant itself measures change per metre but is otherwise dimensionless. 26-03-2012 21 PUNJAB EDUSAT SOCIETY (PES)

Propagation of Wave Inside Waveguide:

Propagation of Wave Inside Waveguide Hence wave is no longer transverse electromagnetic wave. Direction of Propagation Electric Vector Magnetic Vector 26-03-2012 22 PUNJAB EDUSAT SOCIETY (PES)

Propagation of Wave Inside Waveguide:

Propagation of Wave Inside Waveguide It can either be transverse magnetic wave, if magnetic field is perpendicular to direction of propagation; Known as TM wave. Or it can be transverse electric wave, if electric field is perpendicular to direction of propagation; Known as TE wave. 26-03-2012 23 PUNJAB EDUSAT SOCIETY (PES)

Modes of Propagation:

Modes of Propagation We have noted that the broad dimension of waveguide i.e. ‘a’ should contain integer multiple of half wavelengths say ‘m’. Similarly other dimension ‘b’ should contain say ‘n’ half lengths. a b 26-03-2012 24 PUNJAB EDUSAT SOCIETY (PES)

Modes of Propagation:

Modes of Propagation So if a TE wave has ‘m’ half wavelengths in ‘a’ direction and ‘n’ half wavelengths in ‘b’ direction then wave is said to in TE m,n mode. For example take m=1 and n=0 i.e. TE 1,0 mode b a b a 26-03-2012 25 PUNJAB EDUSAT SOCIETY (PES)

TE 1,1 mode:

TE 1,1 mode 26-03-2012 26 PUNJAB EDUSAT SOCIETY (PES)

Transverse Electric Mode:

Transverse Electric Mode 26-03-2012 PUNJAB EDUSAT SOCIETY (PES) 27 of 58 Magnetic Field Variation Electric Field Variation TE 1,0 Electric Field Variation TE 2,0

Transverse Magnetic Field:

Transverse Magnetic Field 26-03-2012 PUNJAB EDUSAT SOCIETY (PES) 28 of 58 Electric Field Variation Magnetic Field Variation TM 1,0 Magnetic Field Variation TM 2,0

Circular Waveguides:

Circular Waveguides In case of circular waveguides m= number of full wave variations along circumference. n= number of half wave variations along radius. 26-03-2012 29 PUNJAB EDUSAT SOCIETY (PES)

Circular Waveguides:

Circular Waveguides 26-03-2012 30 PUNJAB EDUSAT SOCIETY (PES)

Microwave Antennas:

Microwave Antennas The microwave antennas are highly directive. Due to small wavelength of these waves antennas of several half wavelengths are possible and hence there gain is high. Applications of microwaves in radars demand highly directional antennas. At microwaves the performance of active devices deteriorate, hence high gain antennas help. 26-03-2012 31 PUNJAB EDUSAT SOCIETY (PES)

Antenna with Parabolic Reflector:

Antenna with Parabolic Reflector A parabola is defined as locus of point which moves so that its distance from another point (focus) + from directrix is constant. FP+PP’= FQ+QQ’ = FR+RR’ =‘k’ R Q P A B D C FOCUS F R’ Q’ P’ 26-03-2012 32 PUNJAB EDUSAT SOCIETY (PES)

Parabolic Antenna:

Parabolic Antenna Aperture: The ratio of the focal length to the mouth diameter is called the aperture of the parabola. Aperture = AA’ / CD F Focal Length Mouth Diameter A A’ C D 26-03-2012 33 PUNJAB EDUSAT SOCIETY (PES)

Parabolic Antenna:

Parabolic Antenna When a source radiates from focus then all waves reaching at the directrix have covered same distance and will be in phase. R Q P A B D C FOCUS F R’ Q’ P’ 26-03-2012 34 PUNJAB EDUSAT SOCIETY (PES)

Parabolic Antenna:

Parabolic Antenna All waves thus reflected by parabola for transmission shall be in focus at directrix . And all waves received at focus after reflection from parabola are in focus. All such waves will be in phase. 26-03-2012 35 PUNJAB EDUSAT SOCIETY (PES)

Parabolic Antenna:

Parabolic Antenna As a result, radiation is very strong and concentrated along the axis. But cancellation will occur in all other directions, because of path differences. Thus parabola is seen to have properties that lead to the production of concentrated beams of radiation. 26-03-2012 36 PUNJAB EDUSAT SOCIETY (PES)

Dish Antenna:

Dish Antenna A practical reflector is a three dimensional surface, obtained by rotating parabola along the axis, known as paraboloid . This is called parabolic reflector or microwave dish. Such antenna have high gain because it collects radiation from large area and concentrates it on focal point. 26-03-2012 37 PUNJAB EDUSAT SOCIETY (PES)

Radiation Pattern :

Radiation Pattern Main Lobe Side Lobes  26-03-2012 38 PUNJAB EDUSAT SOCIETY (PES)

Beam Width:

Beam Width The directional pattern of dish antenna has a very sharp main lobe along the axis. The beam width of main lobe is given by  = 70. / D Where  is wavelength in m. D is mouth diameter in m. 26-03-2012 39 PUNJAB EDUSAT SOCIETY (PES)

Feed Mechanism:

Feed Mechanism 26-03-2012 40 PUNJAB EDUSAT SOCIETY (PES)

Feed Mechanism:

Feed Mechanism As dish is basically a reflector, another antenna is needed to feed it. This is known as feed mechanism. Feed mechanism basically radiates the electromagnetic energy and is known as primary antenna. An ideal feed mechanism should direct all energy towards reflector. 26-03-2012 41 PUNJAB EDUSAT SOCIETY (PES)

Axial or Front Feed:

Axial or Front Feed Reflector Primary Antenna/ feed mechanism 26-03-2012 42 PUNJAB EDUSAT SOCIETY (PES)

Cassegrain Feed Mechanism:

Cassegrain Feed Mechanism Reflector Convex Reflector 26-03-2012 43 PUNJAB EDUSAT SOCIETY (PES)

PowerPoint Presentation:

Cassegrain Feed Mechanism Reflector Spill-Over Zone Convex Reflector 26-03-2012 44 PUNJAB EDUSAT SOCIETY (PES)

Gregorian Feed Mechanism:

Gregorian Feed Mechanism Reflector Concave Reflector 26-03-2012 45 PUNJAB EDUSAT SOCIETY (PES)

PowerPoint Presentation:

Reflector Spill-Over Zone Concave Reflector Gregorian Feed Mechanism 26-03-2012 46 PUNJAB EDUSAT SOCIETY (PES)

Pros: Gregorian and Cassegrain:

Pros: Gregorian and Cassegrain Feed pattern reshaping, allowing use of efficient feedhorns Convenient feed location with shorter feedline Better illumination of very deep dishes At high elevations, little spillover toward ground – all sidelobes point at cold sky Large depth of focus A more compact structure 26-03-2012 47 PUNJAB EDUSAT SOCIETY (PES)

Cons: Gregorian and Cassegrain:

Cons: Gregorian and Cassegrain Greater blockage, particularly with small dishes Higher sidelobes (blockage increases sidelobes ) Larger feedhorns Not good with broadband feeds Tighter tolerance requirements (for equal path lengths) 26-03-2012 48 PUNJAB EDUSAT SOCIETY (PES)

Offset Feed Mechanism:

Offset Feed Mechanism 26-03-2012 49 PUNJAB EDUSAT SOCIETY (PES)

Horn Antenna:

Horn Antenna A waveguide is capable of radiating energy into the open space, if it is suitably excited at one end and open at other end. But due to impedance mismatch much energy is reflected back. Micro Wave Source Generated Energy Reflected Back Radiated Power 26-03-2012 50 PUNJAB EDUSAT SOCIETY (PES)

Horn Antenna:

Horn Antenna To overcome this difficulty i.e. to match the impedance of waveguide with open space, the mouth of the waveguide may be opened out. The resulting device is known as horn antenna 26-03-2012 51 PUNJAB EDUSAT SOCIETY (PES)

Basic Horn Antennas:

Basic Horn Antennas The most commonly used horn type antennas are: Sectoral Horn Pyramidal Horn Conical Horn. 26-03-2012 52 PUNJAB EDUSAT SOCIETY (PES)

Sectoral Horn:

Sectoral Horn The sectoral horn flares up in one direction only. 26-03-2012 53 PUNJAB EDUSAT SOCIETY (PES)

Pyramidal Horn Antenna:

Pyramidal Horn Antenna The Pyramidal Horn flares up in both directions and has a shape of truncated pyramid. Hence it is a combination of e-plane and h-plane sectoral horns. 26-03-2012 54 PUNJAB EDUSAT SOCIETY (PES)

Conical Horn Antenna:

Conical Horn Antenna Used in case of circular waveguides. It flares radially in all directions and has a cone type shape. 26-03-2012 55 PUNJAB EDUSAT SOCIETY (PES)

Horn Antenna Properties:

Horn Antenna Properties If flare angle  is too large, the radiated pattern will be spherical and will be poorly directed.  L 26-03-2012 56 PUNJAB EDUSAT SOCIETY (PES)

Horn Antenna Properties:

Horn Antenna Properties On the other hand if flare angle  is too small, so will be the mouth of horn and directivity will again suffer due to Diffraction at edges Reflection of signal due to impedance mismatch. Therefore flare angle must have optimum value. 26-03-2012 57 PUNJAB EDUSAT SOCIETY (PES)

Practical Values of Flare Angle:

Practical Values of Flare Angle In practice flare angle  is maximum at 40 0 when L/ = 6; In this case beamwidth is 66 0 , and maximum directive gain is 40. Minimum value of flare angle  is 15 0 , when L/ = 50. In this case beamwidth is 23 0 and gain=120. 26-03-2012 58 PUNJAB EDUSAT SOCIETY (PES)

Advantages of Horn Antennas:

Advantages of Horn Antennas Though Horn antennas are not as directive as dish antennas are, still they are Compact and easier to construct Easier to use with waveguides. 26-03-2012 59 PUNJAB EDUSAT SOCIETY (PES)

Application Areas of Horn Antennas:

Application Areas of Horn Antennas used as a feed element for large radio astronomy, satellite tracking and communication dishes A common element of phased arrays used in the calibration, other high-gain antennas used for making electromagnetic interference measurements 26-03-2012 60 PUNJAB EDUSAT SOCIETY (PES)

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