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Microstrip transmission lines
By:- MR. HIMANSHU DIWAKARAssistant Professor
JETGI
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Microstrip line
Quasi-TEM line Easy fabrication: etching Substrate Characteristic impedance
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• In recent years-with the introduction of monolithic microwave integrated circuits (MMICs)- Microstrip lines and coplanar strip lines have been used extensively, because they provide one free and accessible surface on which solid-state devices can be placed.
• All electrical and electronic devices with high-power output commonly use conventional lines, such as coaxial lines or waveguides, for power transmission.
• However, the microwave solid-state device is usually fabricated as a semiconducting chip with a volume on the order of 0.008-0.08 .
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MICROSTRIP LINES (cont’d)
• Inhomogeneous structure:• Due to the fields within two guided-wave media, the microstrip does not support a pure TEM wave.
• When the longitudinal components of the fields for the dominant mode of a microstrip line is much smaller than the transverse components, the quasi-TEM approximation is applicable to facilitate design.
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MICROSTRIP LINES (cont’d)• Can be fabricated using PCB technology.• It consists of a conducting strip separated from a ground plane by a
dielectric known as substrate. • It’s Components are couplers, filters • Power divided can formed using microstrip lines• Less expensive as traditional waveguide technology as well as lighter and
compact.• Low power handling capacity and higher radiation losses. • Like waveguide microstrip is not enclosed.• Microwave integrated circuits with microstrip lines are commonly• used with the chips. The microstrip line is also called an open-strip line.MR. HIMANSHU DIWAKAR
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• Radiation loss in microstrip lines is a problem, particularly at such discontinuities as short circuit posts, corners, and so on.• However, the use of thin, high-dielectric materials considerably
reduces the radiation loss of the open strip.
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Characteristic Impedance of Microstrip Lines
• Microstrip lines are used extensively to interconnect high-speed logic circuits in digital computers because they can be fabricated by automated techniques and they provide the required uniform signal paths.
A microstrip line A wire-over-ground line.
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Characteristic Impedance (cont’d)
• You can see that the characteristic impedance of a microstrip line is a function of the strip-line width, the strip-line thickness, the distance between the line and the ground plane, and the homogeneous dielectric constant of the board material.
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Characteristic Impedance in the form of thickness
Where
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A microstrip transmission line is shown in cross-section in the following figure. Its physical characteristics include the microstrip width (w), the microstrip thickness (t), the substrate height (d), and the relative permittivity constant (ε)
d
w
t
Microstrip Tx line
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Substrate
Relative permittivity
Thickness of a substrate: mil (inch/1000)
Thickness of a metal: oz (almost 1.4 mils)
Loss: loss tangent
Temperature
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Coupled Transmission Lines
• Coupling between two transmission lines is introduced by their proximity to each other.• Coupling effects may be undesirable, such as crosstalk in printed
circuits, or they may be desirable, as in directional couplers where the objective is to transfer power from one line to the other
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Electrical wall
Even mode Magnetic Wall
Electric field Magnetic field
Coupled line StructureThe coupled line structure supports two quasi-TEM modes: odd mode and even
modeOdd mode
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Strip lines
• Combination of two wire lines and co-axial lines.• These are basically planer transmission lines and are widely used for
frequencies from 100 MHz to 100 GHz.
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• A Strip line consists of a central thin conducting strip of width ω which is greater than its thickness t. It is placed inside the low loss dielectric (εr) substrate of thickness b/2 between two wide ground plates.
• The width of the ground plates is five times greater than the spacing between the plates.
• The fundamental and dominant mode in Strip lines is TEM mode.
• For b<λ/2, there will be no propagation in the transverse direction.
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Coplanar strip lines• A Coplanar strip line is formed by two conducting strips with one
strip grounded, both being placed on the same substrate surface, for convenient connections. The following figure explains this.
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Thank youMR. HIMANSHU DIWAKAR
