Untitled Quiz

Transmission Lines as Circuit Elements

• Transmission lines can act as resonant or reactive circuits with open and short-circuited loads.
• A shorted quarter-wavelength line acts as a parallel resonant circuit.
• A shorted half-wavelength line acts as a series resonant circuit.
• A shorted line less than a quarter wavelength acts like an inductor to a generator.
• A shorted line between a quarter and a half wavelength acts like a capacitor to the generator.
• Open lines behave oppositely to shorted lines.
• Open quarter wavelength acts as a series resonant circuit to the generator.
• Open half wavelength acts as a parallel resonant circuit to the generator.
• Lines less than a quarter wavelength act like a capacitance to the generator.
• Lines between a quarter and a half wavelength act like an inductance to the generator.

Resonant Circuits and Reactive Components

• Standing waves are often avoided in transmission lines but can be used as resonant or reactive circuits.
• Length of lines is in wavelengths on the horizontal axis of reactance graphs.
• Vertical axis shows reactance in ohms relative to the characteristic impedance of the line.
• Solid curves represent shorted lines.
• Dashed curves represent open circuit lines.
• If a line acts as a series resonant circuit, impedance is zero.
• If a line acts as a parallel resonant circuit, impedance is near infinity.
• Intermediate lengths result in intermediate impedance values.

Stripline and Microstrip

• At low frequencies (below 300MHz), lines are too long for use as reactive components.
• At higher frequencies (UHF 300-3000MHz and microwave 1GHz and above), one-half wavelength is short enough for use in tuned circuits, filters, etc.
• Special transmission lines (microstrip and stripline) are used at higher frequencies to realize small reactive components in circuit design.
• Microstrip uses a flat conductor separated by a dielectric from a ground plane.
• Characteristic impedance of microstrip calculated with formula: Z = √((ε + 1.41) / 0.8w + t/h)

Microstrip

• Microstrip is a flat conductor separated by a dielectric from a large ground plane.
• Microstrip is usually one-quarter or one-half wavelength long.
• Ground plane is a common circuit point
• Microstrip is an unbalanced line.
• Shorted lines are often preferred over open lines.
• Two-line balanced version also exists.

Examples of Microstrip

• Microstrip is used in electronic circuits.

Stripline

• Stripline is a flat conductor sandwiched between two ground planes.
• It is harder to produce than microstrip but does not radiate.
• Length is one-quarter or one-half wavelength.
• Shorted lines are usually preferred rather than open lines.

Microwave Integrated Circuits (MICs)

• Combination of diodes, transistors, and other components with microstrip and stripline techniques forms microwave-integrated circuits (MICs).

Seatwork

• Problem 1: Design parameters and calculation of a microstrip capacitor
• Problem 2: Calculate the length of the transmission line.