1. Subject Name: Microwave and Radar Subject Code: 10EC54
Subject Name: Microwave and Radar-Unit 1
Subject Code: 10EC54
Prepared By: Lakshmi C R, Dharani K G
Department: ECE
Date:
Subject Name: Microwave and Radar
Subject Code: 10EC54
Prepared By: Lakshmi C R
Department: ECE
Date:
22/8/14

2. MICROWAVE TRANSMISSION LINES
UNIT 1
MICROWAVE TRANSMISSION LINES

3. Topic Details Transmission Line Introduction
Transmission Line Equation and Solution
Reflection Coefficient
Transmission Coefficient
Standing Waves And Standing Wave Ratio
Line impedance
Line admittance
Smith Chart
Impedance Matching Using Stubs
Microwave Coaxial Connectors
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4. Transmission Line Introduction
In an electronic system, the delivery of power requires the connection of two wires between the source and the load. At low frequencies, power is considered to be delivered to the load through the wire.
In the microwave frequency region, power is considered to be in electric and magnetic fields that are guided from lace to place by some physical structure. Any physical structure that will guide an electromagnetic wave place to place is called a Transmission Line
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5. Transmission Line Introduction
Properties
Has two conductors running parallel
Can propagate a signal at any frequency
Becomes lossy at high frequency
Can handle low or moderate amounts of power
Does not have signal distortion, unless there is loss
May or may not be immune to interference
Does not have Ez or Hz components of the fields TEMzi
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6. Types of Transmission Modes
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7. Transmission Line Equation and Solution
RDz
LDz
GDz
CDz z
v(z+z,t) + -
v(z,t)
i(z,t)
i(z+z,t)
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12. Reflection Coefficient
The reflection coefficient is a parameter that describes how much of an electromagnetic wave is reflected by an impedance discontinuity in the transmission medium. It is equal to the ratio of the amplitude of the reflected wave to the incident wave, with each expressed as phasors
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13. Transmission Coefficient
Transmission coefficient is defined as the ratio of transmitted voltage to the incident voltage
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14. Standing Waves And Standing Wave Ratio
A standing wave is formed by the addition of incident and reflected waves and has nodal points that remain stationary with time.
Voltage Standing Wave Ratio:
VSWR = Vmax/Vmin
Voltage standing wave ratio expressed in decibels is called the Standing Wave Ratio:
SWR (dB) = 20log10VSWR
The maximum impedance of the line is given by:
Zmax = Vmax/Imin
The minimum impedance of the line is given by:
Zmin = Vmin/Imax
or alternatively:
Zmin = Zo/VSWR
Relationship between VSWR and Reflection Coefficient:
VSWR = (1 + |G|)/(1 - |G|)
G = (VSWR – 1)/(VSWR + 1)
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15. Smith Chart
For complex transmission line problems, the use of the formulae becomes increasingly difficult and inconvenient. An indispensable graphical method of solution is the use of Smith Chart.
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16. The types of problems for which Smith charts are used include the following:
Plotting a complex impedance on a Smith chart
Finding VSWR for a given load
Finding the admittance for a given impedance
Finding the input impedance of a transmission line terminated in a short or open.
Finding the input impedance at any distance from a load ZL.
Locating the first maximum and minimum from any load
Matching a transmission line to a load with a single series stub.
Matching a transmission line with a single parallel stub
Matching a transmission line to a load with two parallel stubs.
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17. Line Impedence and Line Admittance
To locate a complex impedance, Z = R+-jX or admittance Y = G +- jB on a Smith chart, normalize the real and imaginary part of the complex impedance.
Locating the value of the normalized real term on the horizontal line scale locates the resistance circle. Locating the normalized value of the imaginary term on the outer circle locates the curve of constant reactance.
The intersection of the circle and the curve locates the complex impedance on the Smith chart
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18. Impedance Matching Using Single Stubs
transmission line used to match another (main) transmission line is called a stub. The length of a stub is chosen to make its admittance be equal –jB. This process of line matching is called single-stub matching.
The length of the stub can be made adjustable. Such adjustable-length transmission lines are sometimes called a trombone line.
Note that single-stub matching requires two adjustable distances: location of the stub d1 and the length of the stub d2. In some situations, only the stub’s length can be adjusted. In these cases, additional stub(s) may be used.
The distances mentioned here are normalized to the wavelength. Therefore, this method allows line matching at particular discrete frequencies.
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19. Microwave Coaxial Connectors
For high-frequency operation, the average circumference of a coaxial cable must be limited to about one wavelength in order to reduce multimodal propagation and eliminate erratic reflection coefficients, power losses, and signal distortion
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