Podcast
Questions and Answers
When a signal is applied to a transmission line, it appears at the other end of the line some time later because of the propagation __________.
When a signal is applied to a transmission line, it appears at the other end of the line some time later because of the propagation __________.
delay
If a resistive load equal to the characteristic __________ of a line is connected at the end of the line, the signal is absorbed by the load.
If a resistive load equal to the characteristic __________ of a line is connected at the end of the line, the signal is absorbed by the load.
impedance
If the load is an antenna, the signal is converted to __________ energy and radiated into space.
If the load is an antenna, the signal is converted to __________ energy and radiated into space.
electromagnetic
When a line is not terminated properly, some of the energy is __________ from the end of the line and moves back toward the generator.
When a line is not terminated properly, some of the energy is __________ from the end of the line and moves back toward the generator.
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The composite voltage formed by the reflected and incident generator voltage constitutes a __________ wave.
The composite voltage formed by the reflected and incident generator voltage constitutes a __________ wave.
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Once the signal reaches the right end of the line, a reverse charging effect takes place on the __________ from right to left.
Once the signal reaches the right end of the line, a reverse charging effect takes place on the __________ from right to left.
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Ideally, a transmission line should be terminated in a load that has a resistive impedance equal to the characteristic impedance of the line; this is called a __________ line.
Ideally, a transmission line should be terminated in a load that has a resistive impedance equal to the characteristic impedance of the line; this is called a __________ line.
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One of the key objectives in designing antenna and transmission line systems is to ensure this __________.
One of the key objectives in designing antenna and transmission line systems is to ensure this __________.
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The ratio of the reflected voltage wave Vr to the incident voltage wave Vi is called the ______.
The ratio of the reflected voltage wave Vr to the incident voltage wave Vi is called the ______.
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The percentage of reflected power is also expressed by the term ______ loss.
The percentage of reflected power is also expressed by the term ______ loss.
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When the standing wave ratio (SWR) is ______, the percentage of reflected power is 0.
When the standing wave ratio (SWR) is ______, the percentage of reflected power is 0.
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As the line and load mismatch grows, reflected power ______.
As the line and load mismatch grows, reflected power ______.
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A standing wave ratio of 1.5 results in a percentage of reflected power of ______ percent.
A standing wave ratio of 1.5 results in a percentage of reflected power of ______ percent.
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The maximum voltage along a transmission line can be used to calculate the ______.
The maximum voltage along a transmission line can be used to calculate the ______.
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If the load and line impedances are matched, the SWR is ______.
If the load and line impedances are matched, the SWR is ______.
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The power applied to a transmission line with an SWR of 1.65 and 50 W is subject to reflected ______.
The power applied to a transmission line with an SWR of 1.65 and 50 W is subject to reflected ______.
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Alternating-current voltage at any point on a matched line is a constant value, disregarding ______.
Alternating-current voltage at any point on a matched line is a constant value, disregarding ______.
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If the load impedance is different from the line characteristic impedance, not all the power transmitted is ______ by the load.
If the load impedance is different from the line characteristic impedance, not all the power transmitted is ______ by the load.
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The power sent down the line toward the load is called forward or ______ power.
The power sent down the line toward the load is called forward or ______ power.
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In the case of a short at the end of a line, the voltage is ______ when the current is maximum.
In the case of a short at the end of a line, the voltage is ______ when the current is maximum.
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With an infinite impedance load, the voltage at the end of the line is maximum when the current is ______.
With an infinite impedance load, the voltage at the end of the line is maximum when the current is ______.
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When the load impedance does not exactly match the transmission line impedance, the line is said to be ______.
When the load impedance does not exactly match the transmission line impedance, the line is said to be ______.
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The magnitude of the standing waves on a transmission line is determined by the ratio of the maximum current to the minimum current, or the ratio of the maximum voltage to the minimum ______.
The magnitude of the standing waves on a transmission line is determined by the ratio of the maximum current to the minimum current, or the ratio of the maximum voltage to the minimum ______.
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These ratios are referred to as the standing wave ______ (SWR).
These ratios are referred to as the standing wave ______ (SWR).
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Study Notes
Standing Waves
- Standing waves occur when a signal on a transmission line is not fully absorbed by the load.
- When the load impedance is not equal to the characteristic impedance of the line, some of the energy is reflected back to the generator.
- This reflected signal combines with the incident signal, creating a pattern of voltage and current that appears to stand still along the line.
- Standing waves are undesirable.
- They indicate that power is not fully absorbed by the load. In cases like a shorted or open line, all the power is reflected, and none reaches the load.
- The impedance matching is crucial for maximum power transfer.
Relationship Between Reflections and Standing Waves
- When a signal reaches the end of a transmission line, it undergoes reverse charging which causes the signal to essentially travel backwards.
- This reversed signal is the reflected wave, and the original signal toward the load is the incident wave.
- The combined effect of reflected and incident waves creates standing waves.
Matched Lines
- Ideal transmission lines are terminated with a load equal to the line's characteristic impedance.
- This matched line ensures smooth power transfer without loss.
- A matched line has a constant voltage (or current) along its length (neglecting losses).
- Impedance matching is vital for designing antenna and transmission line systems to enable maximum power transfer.
Mismatched (Resonant) Lines
- In most real-world scenarios, load impedance does not precisely match the line's impedance.
- When the mismatch is significant, the load is characterized as resonant.
- While standing waves still occur in these mismatched scenarios, their amplitude is usually lower than in perfectly mismatched cases.
- A significant mismatch can damage the transmitter or the line itself.
Calculating Standing Wave Ratio
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Standing Wave Ratio (SWR) is a measure of reflections on a transmission line.
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It's calculated as the ratio of maximum to minimum voltage (or current) along the line.
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A lower SWR suggests better impedance matching.
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SWR = (Vmax / Vmin) or (Imax/ Imin).
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If Zo > Zl, SWR = (ZL / Zo)
Calculating Standing Wave Ratio and Reflection Coefficient
- Reflection coefficient (Γ, gamma) quantifies the ratio of reflected voltage to incident voltage.
- Γ = Vr / Vi where Vr is Reflected Voltage and Vi is Incident Voltage.
- SWR and reflection coefficient are interrelated. Formulae are given for calculating both from each other, or for directly calculating SWR using the load and line impedance.
Shorted Lines
- In a shorted line, voltage is zero when current is maximum, reflecting all the power back to the source.
Open Lines
- In an open line, voltage is maximum when current is zero; also all energy is reflected.
Quiz Questions
- Provides specific examples to calculate SWR and reflection coefficient from given values of impedance, voltage, or power
Reminder
- Prepare for oral recitation.
Thank You for Listening
- Provides contact information for further questions.
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Description
This quiz explores the concept of standing waves in transmission lines, including their formation and the effects of impedance mismatches. It also examines the relationship between incident and reflected signals, highlighting the importance of impedance matching for optimal power transfer.
