Wave Optics: Diffraction Concepts

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Questions and Answers

What characterizes the first Fresnel zone in relation to line of sight?

It is always fully blocked by obstacles.

It corresponds to the line of sight. (correct)

It corresponds to phase differences of 90 degrees.

It has no phase difference from the source.

What happens when the line of sight (LOS) is partially blocked regarding the second Fresnel zone?

It enhances the received signal strength.

It eliminates diffraction loss completely.

It can cause destructive interference. (correct)

It results in constructive interference.

In terms of power propagation, how much power is typically found in the first Fresnel zone compared to free space propagation?

Always 30 dB below free space propagation.

Equal to free space propagation.

5 to 25 dB below free space propagation. (correct)

5 to 25 dB above free space propagation.

What is the correct description of scattering in relation to rough surfaces?

They cause scattering in all directions. Signup and view all the answers

What defines a surface as smooth in relation to the critical height for bumps?

If its minimum to maximum protuberance is less than critical height. Signup and view all the answers

What type of modeling is typically used for nearby metal objects?

Statistical modeling Signup and view all the answers

Which model is appropriate for predicting signal behavior averaged over large distances?

Path loss models Signup and view all the answers

What does the path loss in the free space model mainly depend on?

Distance to the transmitter Signup and view all the answers

What is considered to dominate small scale fading models?

Multipath effects Signup and view all the answers

In the path loss equation, what do you calculate when measuring PL(d)?

The path loss at a given distance Signup and view all the answers

Which factor is not typically included in large scale propagation models?

Frequency dependencies Signup and view all the answers

What is the purpose of the log-distance path loss model?

To generalize path loss with environmental factors Signup and view all the answers

The term 'free space model' refers to path loss valid only in what condition?

Far-field conditions Signup and view all the answers

How does the received power in a wireless communication system depend on the transmitter and receiver antennas?

It is a function of transmit power times gains of both antennas. Signup and view all the answers

What effect does reflection have on signal strength?

It causes the signal to decay more rapidly. Signup and view all the answers

What phenomenon describes the propagation of secondary wavelets when waves encounter an obstacle?

Diffraction Signup and view all the answers

What principle states that all points on a wavefront can be considered as point sources for new wavelets?

Huygen’s principle Signup and view all the answers

What must be kept clear to ensure effective line-of-sight microwave links according to Fresnel zones?

55% of the first Fresnel zone. Signup and view all the answers

In diffraction, what results from the excess path length?

A phase shift in the wave Signup and view all the answers

Fresnel-Kirchoff diffraction parameters are used to analyze what aspect of wave behavior?

The phase shifts related to obstacles in the path. Signup and view all the answers

Which statement about the signal strength decay is accurate?

Signal strength decays with distance to the power of -2. Signup and view all the answers

Study Notes

Diffraction

Diffraction occurs when waves hit the edge of an obstacle.
Secondary waves propagate into the shadowed region.
Diffraction results from the propagation of secondary wavelets into a shadowed region.
Excess path length leads to a phase shift.
The field strength of a diffracted wave in the shadowed region is the vector sum of the electric field components of all the secondary wavelets around the obstacle.
Huygens’ principle states that all points on a wavefront can be considered as point sources for the production of secondary wavelets, which combine to produce a new wavefront in the direction of propagation.

Diffraction Geometry

Equations 4.54-4.57 can be derived from the diffraction geometry.

Fresnel-Kirchoff Diffraction Parameters

Equation 4.56 defines Fresnel-Kirchoff diffraction parameters.

Fresnel Screens

Fresnel zones relate phase shifts to the positions of obstacles.
Equation 4.58 defines the Fresnel zone.
A rule of thumb for line-of-sight microwave links is to keep 55% of the first Fresnel zone clear.

Fresnel Zones

Bounded by elliptical loci of constant delay.
Zones alternate in phase by 180 degrees.
Line of sight (LOS) corresponds to the 1st zone.
If the LOS is partially blocked, the 2nd zone can destructively interfere, resulting in diffraction loss.
The amount of power propagated through the Fresnel zone can be estimated, with the 1st zone typically having 5 to 25 dB below free space propagation.

Knife-edge Diffraction

Equation 4.59 defines Fresnel integral.

Knife-edge Diffraction Loss

Diffraction loss is a measure of the attenuation of a signal due to the presence of an obstacle, such as a building or a hill.
It can be calculated using the Fresnel integral.
Exam 4.7 and 4.8 involve calculating diffraction loss.

Multiple Knife-edge Diffraction

Multiple knife-edge diffraction occurs when there are multiple obstacles in the path of a signal.
The diffraction loss in this case is the sum of the diffraction losses from each individual obstacle.

Scattering

Rough surfaces, such as lamp posts and trees, scatter waves in all directions.
The critical height for bumps is a function of the wavelength and incident angle, as defined by Equation 4.62.
A surface is considered smooth if its minimum to maximum protuberance is less than the critical height.
Scattering loss factor can be modeled using a Gaussian distribution (Equations 4.63 and 4.64).
Nearby metal objects (street signs, etc.) can scatter waves, usually modeled statistically.
Large distant objects can also scatter waves, and can be modeled using the Radar Cross Section (RCS).
Equation 4.66 defines the bistatic radar equation.

Measured Results

Measurements of radio wave propagation show that the signal strength can vary significantly, even over short distances. This variation is due to multipath propagation, which is the phenomenon of radio waves reflecting off objects and traveling along multiple paths.
The amount of variation in the signal strength is dependent on the environment, the frequency of the radio waves, and the distance between the transmitter and receiver.

Propagation Models

Large-scale models, which predict average behavior over distances much larger than the wavelength of the signal, are used to model the general trend of signal loss over distance.
These models are frequency-independent and useful for range calculations and capacity planning.
They are based on experimental data and often use path loss models, which include outdoor and indoor models.
Small-scale models focus on signal variability over short distances or time, known as fading. They model the rapid change in signal strength due to multipath effects.
Small-scale models are frequency and bandwidth-dependent.

Free Space Path Loss

Path loss is a measure of signal attenuation based on distance to the transmitter.
The free space model is only valid in the far field, which is defined by a distance greater than a certain threshold "d0".
Path loss models typically use a reference point d0 and define path loss relative to this point.
The log-distance model generalizes path loss to account for other environmental factors by introducing a distance-dependent exponent "beta".
The free space path loss can be measured or calculated for a reference point, and by taking measurements and deriving beta empirically, the path loss at other distances can be determined.

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Description

Explore the fascinating phenomenon of diffraction in wave optics. This quiz covers Huygens' principle, diffraction geometry, and Fresnel-Kirchoff parameters. Test your understanding of how waves interact with obstacles and the resulting patterns.