Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Electronic Communications Systems (Tomasi) > Chapter 14: Electromagnetic Wave Propagation > Flashcards

Chapter 14: Electromagnetic Wave Propagation Flashcards

1
Q

Free-space propagation of electromagnetic waves is often called?

A

Radio-frequency (RF) Propagation or simply radio propagation.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What is an electromagnetic wave?

A

In essence, an electromagnetic wave is electrical energy that has escaped into free space.

Electromagnetic waves travel in a straight line at approximately the speed of light and are made of magnetic and electric fields that are at right angles to each other and at right angles to the direction of propagation.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What are radio waves? What are their essential properties?

A

Radio waves are form of electromagnetic radiation similar to light and heat. A radio wave consists of traveling electric and magnetic fields, with the energy evenly divided between the two types of fields.

The essential properties of radio waves are frequency, intensity, direction of travel, and plane of polarization.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What does the term polarization of a plane electromagnetic wave refer to?

A

The polarization of a plane electromagnetic wave is simply the orientation of the electric field vector in respect to the surface of the Earth.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Differentiate linear, circular and elliptical polarization.

A

If the polarization remains constant, it is described as linear polarization.

If the polarization vector rotates 360 degrees as the wave moves one wavelength through space and the field strength is equal at all angles of polarization, the wave is described as having a circular polarization

And when the field strength varies with changes in polarization, this is described as elliptical polarization.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What are the two forms of linear polarization?

A

Horizontal and vertical polarization.

If the electric field is propagating parallel to the Earth’s surface, the wave is said to be horizontally polarized. If the electric field is propagating perpendicular to the Earth’s surface, the wave is said to be vertically polarized.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Explain the right handed and left handed terms for a rotating wave.

A

A rotating wave can turn in either direction. If the vector rotates in a clockwise direction, it is right handed, and if the vector rotates in a counterclockwise direction, it is considered left handed.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Explain the concepts of rays and wavefronts.

A

The concepts of rays and wavefronts are aids to illustrating the effects of electromagnetic wave propagation through free space.

A ray is a line drawn along the direction of propagation of an electromagnetic wave. Rays are used to show the relative direction of electromagnetic wave propagation.

A wavefront shows a surface of constant phase of electromagnetic waves. A wavefront is formed when points of equal phase on rays propagated form the same source are joined together.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What is a point source?

A

A point source is a single location from which rays propagate equally in all directions (an isotropic source)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What is the shape of a wavefront generated from a point source?

A

The wavefront generated from a point source is simply a sphere with radius R and its center located at the point of origin of the waves.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What are magnetic fields and electric fields?

A

They are both invisible force fields. A magnetic field is produced by a magnet, such as a conductor when current is flowing through it. An electric field is produced by a difference in voltage potential between two conductors.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What is permittivity?

A

Permittivity is the dielectric constant of the material separating the two conductors.

The permittivity of air or free space is approximately 8.85 x 10^-12 F/m

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What is the formula for magnetic field strength?

A

…

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What is the formula for electric field strength?

A

…

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What is power density?

A

Power density is the rate at which energy passes through a given surface area in free space.

Power density is energy per unit time per unit area and is usually given in watts per square meter.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What is field intensity?

A

Field intensity is the intensity of the electric and magnetic fields of an electromagnetic wave propagating in free space. Electric field intensity is usually given in volts per meter and magnetic field intensity in ampere turns per meter.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

What is the formula(s) for power density?

A

…

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

What the characteristic impedance for a lossless transmission medium?

A

The characteristic impedance for a lossless transmission medium is equal to the square root of the ratio of its magnetic permeability to its electric permittivity.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

What is the characteristic impedance of free space?

A

377 ohms

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

An isotropic radiator is closely approximated by a/an?

A

An isotropic radiator is closely approximated by an omnidirectional antenna.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

Describe a spherical wavefront.

A

A spherical wavefront with radius R is produced by an isotropic radiator. All points distance R from the source lie on the surface of the sphere and have equal power densities.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

The power density at any point on the surface of a spherical wavefront is? (formula)

A

…

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

Explain the inverse square law.

A

The inverse square law refers to the inversely proportional relationship of power density and the square of the distance from the source.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

Differentiate attentuation and absorption (loss).

A

The reduction in power density with distance is equivalent to a power loss and is commonly called wave attenuation or space attenuation.

Absorption is the reduction in power density due to nonfree-space propagation. Wave absorption is analogous to an I2R power loss.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

Formula for wave attenuation:

A

…

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

Say something about absorption (Figure 14-4a)

A

Absorption of radio frequencies in a normal atmosphere depends on frequency and is relatively insignificant below approximately 10GHz.

Wave attenuation due to absorption depends not on the distance from the radiating source but, rather, on the total distance that the wave propagates through the atmosphere.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

Describe the effects of rain on electromagnetic wave propagation. (Figure 14-4b)

A

The effect of rain on electromagnetic wave propagation is insignificant below approximately 6GHz. At higher frequencies, however, rain attenuates radio transmission much more severely.

The electromagnetic energy is absorbed and scattered by the raindrops, and this effect becomes even more pronounced when the length of the wave approaches the size of the raindrop.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
28
Q

Define refraction.

A

Refraction is sometimes referred to as the bending of the radio-wave path.

Electromagnetic refraction is actually the changing of direction of an electromagnetic ray as it passes obliquely from one medium into another with different velocities of propagation.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
29
Q

What is the relationship between velocity and density?

A

The velocity (v) at which an electromagnetic wave propagates is inversely proportional to the density of the medium in which it is propagating.

30
Q

How do we the refractive index? (formula)

A

rafractive index is the square root of the dielectric constant.

It is also the ratio of the velocity of propagation of a light ray in free space to the velocity of propagation of a light ray in a given material.

31
Q

Snell’s law: (formulas)

A

…

32
Q

Refraction also occurs when a wavefront propagates in a medium that has?

A

Refraction also occurs when a wavefront propagates in a medium that has a density gradient that is perpendicular to the direction of propagation.

33
Q

What is electromagnetic reflection?

A

Electromagnetic reflection occurs when an incident wave strikes a boundary of two media and some or all of the incident power does not enter the second material.

The angle of reflection equals the angle of incidence, however, the reflected voltage field intensity is less than the incident voltage field intensity.

34
Q

What is the reflection coefficient?

A

Reflection coefficient is the ratio of the reflected to the incident voltage intensities.

35
Q

What is the power transmission coefficient (T)?

A

The portion of the incident power that is not reflected is called the power transmission coefficient (T).

36
Q

What is the difference when a wave strikes an irregular, smooth or a semirough surface? (reflection)

A

When an incident wavefront strikes an irregular surface, it is randomly scattered in many directions (diffuse reflection), whereas reflection from a perfectly smooth surface is called specular reflection. Semirough surfaces cause a combination of diffuse and specular reflection.

37
Q

State the Rayleigh criterion.

A

The Rayleigh criterion states that a semirough surface will reflect as if it were smooth surface whenever the cosine of the angle of incidence is greater than the ratio of the wavelength of the incident wave and eight times the depth of the surface irregularity.

38
Q

What is diffraction?

A

Diffraction is defined as the modulation or redistribution of energy within a wavefront when it passes near the edge of an opaque object.

Diffraction is the phenomenon that allows light or radio waves to propagate (peek) around corners.

39
Q

What does the Huygen’s principle state?

A

The Huygen’s principle states that every point on a given spherical wavefront can be considered as a secondary point source of electromagnetic waves from which other secondary waves (wavelets) are radiated outward.

40
Q

How does radio wave interference occur?

A

Radio wave interference occurs when two or more electromagnetic waves combine in such a way that system performance is degraded.

Interference is subject to the principle of linear superposition of electromagnetic waves and occurs whenever two or more waves simultaneously occupy the same point in space.

For frequencies below VHF, the relatively large wavelengths prevent interferences from being a significant problem. However, with UHF and above, wave interference can be severe.

41
Q

What are terrestrial waves?

A

Terrestrial waves are electromagnetic waves travelling within Earth’s atmosphere.

42
Q

What are the four propagation modes in terrestrial radio communications?

A

surface wave, ground-reflected wave, space wave, and sky wave

43
Q

What is a surface wave?

A

A surface wave is an Earth-guided electromagnetic wave that travels over the surface of Earth.

Surface wave propagation is commonly used for ship-to-ship and ship-to-shore communications, for navigation and for maritime mobile communications. Surface waves are used at frequencies as low as 15 kHz.

44
Q

Relative conductivity of Earth’s surfaces.

A 
Seawater....good
Flat, loamy soil...fair
large bodies of freshwater...fair
rocky terrain...poor
desert...poor
jungle...unusable
45
Q

What are the disadvantages of surface waves?

A
  • Surface waves require a relatively high transmission power
  • Ground waves are limited to VLF, LF and VHF, requiring large antennas.
  • Ground losses vary considerably with surface material and composition.
46
Q

What are some advantages of ground wave propagation?

A
  • Given enough transmit power, ground waves can be used to communicate between any two locations in the world
  • Ground waves are relatively unaffected by changing atmospheric conditions
47
Q

What are space waves?

A

Space waves include both direct and ground-reflected waves. Space wave propagation of electromagnetic energy includes radiated energy that travels in the lower few miles of Earth’s atmosphere.

48
Q

What are direct waves?

A

Direct waves travel essentially in a straight line between the transmit and receive antenna.

Space wave propagation with direct waves is commonly called line-of-sight (LOS) transmission.

49
Q

What are ground-reflected waves?

A

Ground-reflected waves are waves reflected by Earth’s surface as they propagate between the transmit and receive antennas.

50
Q

What is the radio horizon?

A

The radio horizon is the horizon to space wave propagation presented by the curvature of the Earth. The radio horizon is approximately four-thirds that of the optical horizon.

The radio horizon can be lengthened simply by elevating the transmit or receive antennas (or both) above Earth’s surface with towers or by placing them on top of mountains or high buildings.

51
Q

Formula for the maximum distance between a transmitter and a receiver over average terrain:

A

…

52
Q

How does duct propagation occur?

A

Duct propagation occurs when the density of the lower atmosphere is such that electromagnetic waves are trapped between it and Earth’s surface. The layers of the atmosphere act as a duct, and an electromagnetic wave can propagate for great distances around the curvature of Earth within this duct.

53
Q

What are sky waves?

A

Sky waves are electromagnetic waves that are directed above the horizon level. Sky waves are radiated toward the sky, where they are either reflected or refracted back o the Earth by the ionosphere. Because of this, sky wave propagation is sometimes called ionospheric propagation.

54
Q

Where is the ionosphere located?

A

The ionosphere is the region of space located approximately 50 km to 400 km (30 mi to 250 mi) above Earth’s surface. The ionosphere is the upper portion of Earth’s surface.

55
Q

What are the layers of the ionosphere and where are they located?

A 
D layer (50 km to 100km)
E layer (100 km to 140 km)
F1 layer (140 km to 250 km)
F2 layer (250 km to 350 km), winter
   (140 km to 300 km), summer
56
Q

Say something about the D layer of the ionosphere.

A

The D layer is the lowest layer of the ionosphere and is located approximately 50 to 100 km above Earth’s surface. The amount of ionization in the D layer depends on the altitude of the sun above the horizon.

The D layer reflects VLF and LF waves and absorbs MF and HF waves.

57
Q

Say something about the E layer of the ionosphere

A

The E layer is located between (100 km to 140 km) above Earth’s surface. It is sometimes called the Kennelly-Heaviside layer. The E layer has its maximum density at approximately 70 miles at noon, when the sun is at its highest point.

The E layer aids MF surface wave propagation and reflects HF waves somewhat during the daytime.

The upper portion of the E layer is sometimes considered separately and is called the sporadic layer because it seems to come and go rather unpredictably.

58
Q

Say something about the F layer of the ionosphere.

A

The F layer is actually made of two layers, the F1 and F2 layers.

During the night, the F1 layer combines with the F2 layer to form a single layer.

The F1 layer absorbs and attenuates some HF waves, although most of the waves pass through the F2 layer, where they are refracted back to Earth.

59
Q

What is critical frequency and critical angle?

A

Critical frequency is defined as the highest frequency that can be propagated directly upward and still be returned to Earth by the ionosphere.

Critical angle is the maximum vertical angle at which it can be propagated and still be refracted back by the ionosphere.

60
Q

What is ionospheric sounding?

A

Ionospheric sounding is sometimes used to determine the critical frequency.

61
Q

What is virtual height?

A

Virtual height is the height above Earth’s surface from which a refracted wave appears to have been reflected.

62
Q

What is the maximum usable frequency (MUF)? (formula)

A

the maximum usable frequency is the highest frequency that can be used for sky wave propagation between two specific points on Earth’s surface.

63
Q

What does the secant law assume?

A

The secant law assume assumes a flat Earth and a flat reflecting layer which, of course, can never exist.

64
Q

What is the optimum working frequency?

A

It has been proven that operating at a frequency 85% of the MUF provides more reliable communications, this frequency it sometimes called the optimum working frequency.

65
Q

What is skip distance?

A

Skip distance is defined as the minimum distance from a transit antenna that a sky wave at a given frequency will be returned to Earth. The frequency must be less than the maximum usable frequency and propagated at its critical angle.

66
Q

What happens when the radiation angle exceeds the critical angle?

A

When the radiation angle exceeds the critical angle, the wave penetrates the ionosphere and escapes Earth’s atmosphere.

67
Q

Explain the concept of lower ray and the upper, or the Pedersen, ray.

A

At distances greater than the skip distance, two rays can take different paths and still be returned to the same point on Earth. The two rays are called the lower ray and the upper, Pedersen, ray.

The Pedersen ray is usually of little significance, as it tends to be much weaker than the lower ray because it spreads over a much larger area than the lower ray. It becomes important when circumstances prevent the lower ray from reaching a particular point.

68
Q

What is the skip zone?

A

It is the area where the surface waves are completely dissipated and the point where the first sky wave returns to Earth. It is an area where there is no reception.

69
Q

What is the free-space path loss? (formula)

A

The free-space path loss is often defined as the loss incurred by an electromagnetic wave as it propagates in a straight line through a vacuum with no absorption or reflection of energy from nearby objects. (spreading loss)

70
Q

Describe fading.

A

Fading is the variation is signal loss, it can be caused by natural weather disturbances, such as rainfall, snow-fall, fog, hail and extremely cold air over a warm Earth.

71
Q

Describe fade margin. (formula)

A

Fade margin is the additional loss that is adder to the normal path loss to accommodate temporary fading.

Electronic Communications Systems (Tomasi) (1 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions