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TRANSMISSION
LINESMICROWAVES
LESSON 3
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DISTRIBUTED
COMPONENTS: The inductance in a transmission line comes
about because a current is flowing in a metallicconductor
The resistance is associated with the metallicconductor and current flow
The capacitive reactance, which is a result of the
line capacitance, decreases with an increase infrequency
Conductance is the amount of leakage throughthe dielectric
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Some terms associated with
transmission lines:1. VSWR,
2. Reflection coefficient, and3. Return loss.
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TYPES OF TRANSMISSION
LINES:1. Coaxial transmission lines
A transmission line in which one conductor
completely surrounds the other, the twobeing coaxial and separated by acontinuous solid dielectric or by dielectricspacers.
Types:
A. Flexible
B. Semi-rigid
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A. BASIC FLEXIBLE CABLE
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Parts of a basic flexible cable:
The center conductor can be either a solid wire or aseries of wires in a stranded configuration.
The outer conductor serves two functions. It is aground reference for the signal on the center conductorand also is used as a shield.
The braid construction may be single, double, ortriaxial (two braids separated by an insulator). Thesingle-braid construction consists of bare, tinned, orsilverplated copper wires. The double braid consists of
two single braids with no insulation between them. Thetriaxial consists of two single braids with a layer ofinsulation between them.
The outer coating provides protection for the cable.Such protection is mainly environmental. It plays no part
in the electrical performance of the cable.
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BNC CONNECTOR
For low power RF signal below 3 MHz; 50TO 75 ohms impedance
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TNC CONNECTOR
It has a 50 impedance and operatesbest in the 011 GHz
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SMA- Sub Miniature Version A
From DC to 18 GHz; 50 ohms impedance
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N CONNECTOR
Carries RF signals up to 18 GHz; 50 to 75ohms impedance
50 ohms (bottom)
75 ohms (top)
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B. SEMI-RIGID CABLE
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SEMI RIGID vs. FLEXIBLE
1. Semirigid cables cost considerably more thanflexible cables
2. For testing applications, semirigid cables arenot very practical. Most tests require manyconnect/disconnect operations, which can putstrain on the cables.
3. In some finished products, the cables mustmeander through the chassis to variouslocations. Semirigid cable would not fit thoseapplications in many cases.
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2. Strip transmission line(stripline)
It evolved from the circular coaxial deviceand still has all the original sections
(center conductor, dielectric, outsideshield, and electric fields) but now is in aform that will operate at much higher
frequencies and be more efficient for RFand microwave applications.
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Ground-Plane Spacing(GPS) is the spacingbetween the ground planes, or copper on thecircuit boards.
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Microstrip transmission line does awaywith the problem of inaccessibility thatstripline poses. Microstrip transmission
line, is similar to stripline transmission line,except that there is no top on thetransmission line. There is nothing but airon top of the circuitry and a dielectric
material underneath
3. Microstrip
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4. Coplanar waveguide
It is a modification of the microstripcircuitry
In a coplanar waveguide, there is still a
circuit trace on the top of the board that isa certain width and thickness, but thereare also ground planes on both sides ofthe circuit trace and there is no groundplane on the bottom of the circuit board. Aconductor surrounded by ground guides
the electromagnetic wave down the
transmission line.
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5. Waveguide
It provides a path to guide theelectromagnetic wave down the line.
Waveguide is used at microwave
frequencies (particularly at the highermicrowave frequencies) for tworeasons: They are often easier to
fabricate than coaxial lines; and theyoften can have much less attenuation
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APPLICATIONS:
In a microwave oven a waveguide leads powerfrom the magnetron where waves are formed tothe cooking chamber.
In a radar, a waveguide leads waves to theantenna
A waveguide created on a PCB and is used totransmit microwave signals on the board.
Waveguides are used in scientific instruments tomeasure optical, acoustic and elastic propertiesof materials and objects.
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THE END
ipv
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