-
8/3/2019 Microwaves Lesson Transmission Lines
1/35
TRANSMISSION
LINESMICROWAVES
LESSON 3
-
8/3/2019 Microwaves Lesson Transmission Lines
2/35
-
8/3/2019 Microwaves Lesson Transmission Lines
3/35
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
-
8/3/2019 Microwaves Lesson Transmission Lines
4/35
Some terms associated with
transmission lines:1. VSWR,
2. Reflection coefficient, and3. Return loss.
-
8/3/2019 Microwaves Lesson Transmission Lines
5/35
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
-
8/3/2019 Microwaves Lesson Transmission Lines
6/35
-
8/3/2019 Microwaves Lesson Transmission Lines
7/35
A. BASIC FLEXIBLE CABLE
-
8/3/2019 Microwaves Lesson Transmission Lines
8/35
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.
-
8/3/2019 Microwaves Lesson Transmission Lines
9/35
-
8/3/2019 Microwaves Lesson Transmission Lines
10/35
-
8/3/2019 Microwaves Lesson Transmission Lines
11/35
-
8/3/2019 Microwaves Lesson Transmission Lines
12/35
BNC CONNECTOR
For low power RF signal below 3 MHz; 50TO 75 ohms impedance
-
8/3/2019 Microwaves Lesson Transmission Lines
13/35
TNC CONNECTOR
It has a 50 impedance and operatesbest in the 011 GHz
-
8/3/2019 Microwaves Lesson Transmission Lines
14/35
SMA- Sub Miniature Version A
From DC to 18 GHz; 50 ohms impedance
-
8/3/2019 Microwaves Lesson Transmission Lines
15/35
N CONNECTOR
Carries RF signals up to 18 GHz; 50 to 75ohms impedance
50 ohms (bottom)
75 ohms (top)
-
8/3/2019 Microwaves Lesson Transmission Lines
16/35
B. SEMI-RIGID CABLE
-
8/3/2019 Microwaves Lesson Transmission Lines
17/35
-
8/3/2019 Microwaves Lesson Transmission Lines
18/35
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.
-
8/3/2019 Microwaves Lesson Transmission Lines
19/35
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.
-
8/3/2019 Microwaves Lesson Transmission Lines
20/35
-
8/3/2019 Microwaves Lesson Transmission Lines
21/35
-
8/3/2019 Microwaves Lesson Transmission Lines
22/35
Ground-Plane Spacing(GPS) is the spacingbetween the ground
planes, or copper on thecircuit boards.
-
8/3/2019 Microwaves Lesson Transmission Lines
23/35
-
8/3/2019 Microwaves Lesson Transmission Lines
24/35
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
-
8/3/2019 Microwaves Lesson Transmission Lines
25/35
-
8/3/2019 Microwaves Lesson Transmission Lines
26/35
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.
-
8/3/2019 Microwaves Lesson Transmission Lines
27/35
-
8/3/2019 Microwaves Lesson Transmission Lines
28/35
-
8/3/2019 Microwaves Lesson Transmission Lines
29/35
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
-
8/3/2019 Microwaves Lesson Transmission Lines
30/35
-
8/3/2019 Microwaves Lesson Transmission Lines
31/35
-
8/3/2019 Microwaves Lesson Transmission Lines
32/35
-
8/3/2019 Microwaves Lesson Transmission Lines
33/35
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.
-
8/3/2019 Microwaves Lesson Transmission Lines
34/35
THE END
ipv
-
8/3/2019 Microwaves Lesson Transmission Lines
35/35
