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TRANSMISSION LINES
What is transmission line
Transmission line is a specialized cable or other
structure designed to carry alternating current of radio
frequency
This lines are consider to be impedance matching circuits
design to deliver power from Tx to antenna and maximum
signal from antenna to the receiver
 Used for, Electricity distribution, trunk lines, cable
television signals, , computer network connections and
many more…
Fundamental of transmission lines
Common types of transmission lines:-
There are two types of commonly used transmission lines:The parallel-wire (Balanced line)
The coaxial (Unbalanced)
Parallel-wire Line
It is employed where balance properties are required
The impedance between each leg above the earth is the
same
Balanced lines do not have a common.
The signal information is carried on both wires.
One wire carries the signal called the positive (+ve)
signal and the other carries a signal 180 degrees out of
phase called the negative (-ve) signal.
Often the wires are twisted together in order to
tightly couple the wires electrically
The goal is to have any noise that appears on one
wire to appear on the other wire. Because the signals
are 180 degrees out of phase, the noise will cancel
In connection of folded-dipole antenna to a TV receiver
or a rhombic antenna to an HF transmitter
These lines are never used for microwaves, since it
likely to radiate RF energy
Losses are more as frequency increases
Dielectric heating also more
In parallel lines characteristic impedance is restricted
to a range of 100 to 600 ohms
Coaxial Lines
It is employed where unbalance properties are required
Unbalanced lines consist of two wires. One wire carries the signal and the other is the
reference line called the common. The common wire is usually at ground potential.
Often the common wire will also be used as a shield for noise immunity
Unbalanced lines have difficulty with noise immunity
as any EMI noise will appear on the signal lead.
Unbalanced lines are used for short distances
because of the inherent problem with noise immunity
It is used at UHF and microwave frequencies, to avoid
the risk of radiations from transmission lines itself
Between 1 and 18 GHz, coaxial lines are used
Dielectric heating is less as compare to parallel-wire
lines
Different losses in Transmission lines
The energy losses that happen in case of transmission
lines are shown below:
1. Conductor Heating
2. Dielectric heating
3. Radiation Losses
It is observed that the radiation loss in parallel wire lines is much more than that of the
coaxial cables
Conductor heating:
The heating rate of a conductor is directly proportional
to the square of the current.
It is inversely proportional to the characteristic
impedance (Zo).
Conductor heating will also increase with the increase in
frequency.
It is also referred as I²R losses
It can be minimized in an RF line by plating the line with
silver. Since silver is a better conductor than copper,
most of the current will flow through the silver layer
Radiation Loss:
The transmission lines act as antennas when the
separation distance between the conductors is very
small as compared to their wavelength. Then the
conductor starts radiating energy.
This applies more to parallel-wire lines than the
coaxial lines.
Radiation losses are difficult to estimate, being
normally measured rather than calculated.
They increase with frequency for any given
transmission line.
Dielectric heating:
It directly depends upon the voltage flowing across the
dielectric
Similar to conductor heating, it is also inversely
proportional to the characteristic impedance of the line
In this case the loss also increases with the increase in
frequency
If we use air as the dielectric medium then the loss will be
almost zero
Conductor and Dielectric losses are proportional to
length and given by manufacturers in charts, expressed in
db/100mts.
The Double Stub
Transmission-line matching device must have variable
parameters or degree of freedom, as SW pattern
Single stub is useful at frequencies below microwave
range
At Microwave frequencies to provide second degree of
freedom a second stub of adjustable position is added to
first stub
The resultant is double stub as shown in figure
The two stubs are placed 0.375 ʎ. Two variables are provided and very good matching is
possible
Such matcher is normally connected between load and main transmission line to ensure
shortest possible length
It has a same characteristic impedance as the main
line
Method for adjustment for matching is trial and error.
In this stub nearest to the load is set at a number of
points and the further stub is move back and forth until
the best possible match has been achieved
The SWR is monitored constantly while adjustment is
taking place
If perfect matching is required, a triple-stub tuner
should be used, but the stubs are placed 0.125 ʎ apart
Directional Couplers
It is necessary to measure the power being deliver
to load or antenna through transmission line
Only forwarded wave in line is measured, not the
reflected wave
The coupling units used for such purpose are known
as directional couplers
The two hole coupler being among the most popular
is shown in figure
It consist of a piece of transmission line to be connected
in series with main line together with auxiliary line
coupled to main line via two-probes through slots
The probes do not touch the inner conductor of auxiliary
line. They couple sufficient energy into it simply by being
near it
If they touch most of the energy from main to auxiliary
line would be coupled but a fraction is all that is needed
The energy flow in auxiliary line is mostly same in
direction as in the main line and provision is made to deal
with energy flowing in “wrong” direction
The distance between probe is ʎ/4 (can be any odd
number of quarter-wavelength)
Auxiliary line is terminated at one end by a resistive load
It absorb all energy fed to it and often term as
nonreflecting termination, other end goes to a detector
probe for measurment
Any wave launched from right to left will be absorbed
by load, and not measured
The wave entering at auxiliary line at A, proceeding
toward detector, will meet at B (another sample of the
forward wave), get added and travel toward detector
Various mechanical inaccuracies prevent ideal
operation of directional coupler
The directivity of coupler is standard method of
measuring the extent of this unwanted wave
If the ratio of forward and reversed power measured
by the detector is 30dB, then the directional coupler is
said to have a directivity of 30dB. This value is
common in practice
Other quantity is directional coupling :It is the ratio of forward wave in the main line to the
forward wave in the auxiliary line.
It is measured in decibels, and 20dB (100:1) is a
typical value
Baluns
It is Balance -to- Unbalance transformer used to
connect a balanced line to an unbalanced line or antenna
It is used to connect an unbalanced (coaxial) line to
balanced antenna such as dipole
For high frequencies, several baluns are exist for
differing purposes and narrowband and wideband
applications
The most common balun, a narrow-band one is shown in
figure. It is known as the choke, sleeve, or bazooka balun
As shown, a quarter-wavelength sleeve is placed around
the outer conductor of coaxial line and connected to it at
x
The outer conductor of coaxial line no longer has zero
impedance to ground at y
One of the wires of the balanced line may be connected
to it without fear of being short- circuited to ground
The other balanced wire is connected to the inner
conductor of the coaxial line
Any balanced load, such as the simple dipole antenna,
may now be placed upon it as shown in figure
The Slotted Line
Slotted line simply permits convenient and accurate
measurement of the position and size of the first
voltage maximum from the load and any subsequent
ones as may be described
Slotted line is a piece of coaxial line with slot in outer
conductor
A flat plate is mounted on outer conductor, with a
corresponding slot in it to carry a detector probe
carriage
• It has a rule on the side, with a vernier for microwave frequencies to indicate the exact
position of the probe
• The probe extends into the slot, coming quiet close to the inner conductor of the line but not
touching it as shown in figure
In this fashion, loose coupling between line and probe is
obtained which is adequate for measurement, but small
enough so as not to interfere unduly
The slotted line must have the same characteristics
impedance as the main line to which it is connected in series
It allows the calculation of:
Load impedance
Standing-wave ratio
Frequency of generator being used
The practical measurement and calculations methods are
normally indicated in instructions that comes with a particular
slotted line
Q.1 Compare parallel and coaxial transmission line.
Q.2 Define the term standing wave Discuss its pattern for open and short
circuited transmission lines.
Q.3 Define the characteristic impendence of a transmission line and show
that : Zc = underroot R+jwL/G+jwC
Q.4 Explain the fundamental of smith chart with application. Why smith
chart covers distance of only half wavelength?
Q.5 Explain working principle of balance to unbalance transformer with
neat sketch.
Q.6 what is standing wave ratio? Explain impedance inversion by quarter
wavelength line. Explain impedance matching by quarter wave
transformer.
Q.7 Define term Characteristic impedance of transmission line.
determine the char. impedance of coaxial line with following specification
:
L=0.118 µH/ft, C=21 pf/ft , d=0.025 inch , D= 0.15 inch, Є=2.23
Q.8 Assignment problems of smith chart.
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