CH:5 RADIO WAVE PROPAGATION

Wave Propagation

It is the process by which the waves travel through a medium

Radiation

It is the loss or escape of energy into free space.

Emission

It is the term for broadcasting using EM Waves with intention

Electromagnetic Waves

These are also known as radio waves, which are forms of radiant energy like heat, light, x-ray waves that are considered to be oscillatory disturbances in free space.

Nomenclature of Frequency Band:
Metric Designation - Myriametric

Frequency Range: 3-30KHz
Frequency Band Designation: VLF
Propagation Mode: Ground Waves

Nomenclature of Frequency Band:
Metric Designation - Kilometric

Frequency Range: 30-300KHz
Frequency Band Designation: LF
Propagation Mode: Ground Waves

Nomenclature of Frequency Band:
Metric Designation - Hectometric

Frequency Range: 300-3000KHz
Frequency Band Designation: MF
Propagation Mode: Ground Waves

Nomenclature of Frequency Band:
Metric Designation - Decametric

Frequency Range: 3-30MHz
Frequency Band Designation: HF
Propagation Mode: Sky Waves

Nomenclature of Frequency Band:
Metric Designation - Metric

Frequency Range: 30-300MHz
Frequency Band Designation: VHF
Propagation Mode: Space Waves

Nomenclature of Frequency Band:
Metric Designation - Decimetric

Frequency Range: 300-3000MHz
Frequency Band Designation: UHF
Propagation Mode: Space Waves

Nomenclature of Frequency Band:
Metric Designation - Centimetric

Frequency Range: 3-30GHz
Frequency Band Designation: SHF
Propagation Mode: Space Waves

Nomenclature of Frequency Band:
Metric Designation - Millimetric

Frequency Range: 30-300GHz
Frequency Band Designation: EHF
Propagation Mode: Space Waves

HF signals

are commonly used for sky wave propagation, since these signals can be refracted (bent) by the ionosphere.

Frequencies lower than HF signals

are usually absorbed by the ionosphere, thus they are commonly used for ground (surface) wave propagation.

Frequencies higher than HF signals

usually passes through the ionosphere, thus they are commonly used for space wave (line - of - sight) propagation

Electromagnetic Wave Characteristics:

Components of an Electromagnetic Wave

an electromagnetic wave has two components, namely an electric field and a magnetic field which are perpendicular with each other. Each of these components varies sinusoidally with time at a fixed point in space.

Wave propagation Velocity

waves travel at characteristic speeds depending on the type of wave and the nature of the propagation of the medium (electromagnetic wave velocity in the atmosphere or free space is approximately equal to the speed of light in vacuum).

Polarization

it is the orientation of the electric field vector in respect to the surface of the earth.

Linear Polarization, Rotating Polarization, and Random Polarization

Types of polarization

Linear Polarization

the direction or orientation of the electric field is constant, and it propagates in only one direction

Type of Linear Polarization:
Horizontal Polarization

The electric field is parallel to the surface of the earth, while the magnetic field is perpendicular to the surface of the earth

Type of Linear Polarization:
Vertical Polarization

The electric field is perpendicular to the surface of the earth, while magnetic field is parallel to the surface of the earth.

Rotating Polarization

the direction or orientation of the electric field is varying or rotating while propagating in space.

Type of Rotating Polarization:
Circular Polarization

the amplitudes of the electric and magnetic field are equal (i.e., the electric field has a constant strength)

Type of Rotating Polarization:
Elliptical Polarization

the magnitude of the electric field and magnetic field are not equal (i.e., the electric field has a varying strength).

Random Polarization

if the polarization cannot be classified as linear or rotating

Faraday’s rotation (Faraday’s effect)

causes the polarization of radio waves to rotate as it passes through the ionosphere and is a complex process involving the presence of ionized particles and the Earth’s magnetic field.

Rays & Wavefronts

Electromagnetic waves are invisible, so they must be analyzed by indirect method using schematic diagrams.

Rays & Wavefronts

are aids to illustrating the effects of electromagnetic wave propagation through the space.

Ray

It is the line drawn along the direction of propagation of a wave that is used to show the relative direction of electromagnetic wave propagation, however it does not necessarily represent the propagation of a single electromagnetic wave

Wavefront

it is a surface of constant phase of the wave that is formed when two points of equal phase on rays propagated from the same source are joined together.

Permeability and Permittivity

these are the wave propagation parameters

Permeability

It is the ability of a magnetic material to concentrate magnetic flux, and it is measure of the superiority of a material over vacuum as a path for magnetic lines of force.

μ_o= 4π X 10^-7H/m

Permeability of free space

Permittivity

It is the ability of an insulator to concentrate electric flux, and it determines the capacity of a medium to store electrostatic energy.

ε_o = (1/36π) x 10^-9 F/m

Permittivity of free space

Free Space Characteristic Impedance

• it is related to the electric and magnetic field intensities of an electromagnetic wave in free space

• it is equal to the square root of the ratio of the magnetic permeability to the electric permittivity of a lossless transmission medium.

Power Density

It is the rate at which energy flows through a unit area of surface in space, and it is inversely proportional to the square of the distance from the source (Inverse Square Law).

Point Source

It is also known as isotropic radiator or isotropic source, and it is a single location from which rays propagate equally in all directions.

Electric Field Intensity

It is directly proportional to the square root of power density and inversely proportional to the distance from the source.

Attenuation

It is the reduction of power density with distance from the source due to the spherical spreading of the wave, wherein the signal strength decreases while the distance from the source increases.

Absorption

it occurs when some of the energy from the electromagnetic waves is transferred to the atoms and molecules of the atmosphere causing some radio waves to be absorbed (i.e., signal energy is absorbed by the obstacles encountered by the signal during propagation)

Absorption

It depends on frequency of a radio wave (insignificant below 10 GHz), and also dependent on the collision of particles (the greater the particle density, the greater the probability of collision, the greater the absorption).

Vacuum (free space)

only attenuation can occur in a certain signal, while in Earth’s atmosphere, attenuation and absorption can both occur.

Refraction, reflection, diffraction, and interference

Optical Properties of Radio Waves

Refraction, reflection, diffraction, and interference

are also possible for radio waves and follow rules similar to those for light.

Refraction

can be thought as bending

Reflection

can be thought as bouncing

Diffraction

can be thought as scattering

Interference

can be thought as colliding

Refraction

it is defined as bending of an electromagnetic wave as it passes at an angle from one propagating medium to another medium having different intensity.

Refraction

the degree of bending of a wave at boundaries increases with frequency, and the amount of bending or refraction depends on the refractive index (index of refraction) of the medium.

Snell’s Law

It explains on how an electromagnetic wave reacts when it meets the interface of two mediums that have different indexes of refraction

Normal line

is an imaginary line drawn perpendicular to the interface at the point of incidence.

Angle of incidence

is the angle formed between the incident wave and the normal.

Angle of refraction

is the angle formed between the refracted wave and the normal.

Refractive index (index of refraction)

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

less dense to a denser (n1 < n2)

When an incident wave travel from ____ medium, the refracted wave moves towards the normal line and its velocity decreases (slower).

denser to a less dense (n1>n2)

When an incident wave travel from _____ medium, the refracted wave moves away from the normal line and its velocity increases (faster)

Reflection

It is the bouncing of an electromagnetic wave in a smooth surface, and it occurs at any boundary between materials of differing dielectric constants.

Specular (Mirror - like) Reflection, Diffused Reflection, Reflection in Semi - rough Surfaces

Types of reflection

Specular (Mirror-like) Reflection

it is the reflection from a perfectly smooth surface.

Diffused Reflection

it is a reflection wherein the incident wave strikes an irregular surface, and it is randomly scattered in many directions.

Reflection in Semi-rough Surfaces

it is a reflection which follows the Rayleigh Criterion.

Rayleigh Criterion

it states that semi-rough surface will reflect as if it were a smooth surface whenever the cosine of the angle of incidence is greater than λ/8d

always equal

The angle of incidence is _______ to the angle of reflection (Law of Reflection)

Same

The velocity of the incident and reflected wave is _____

always less than

The signal strength of the reflected wave is _______ the signal strength of the incident wave since some energy is absorbed due to the bouncing of the wave.

Diffraction

it is the redistribution (scattering) of energy within a wavefront when it passes near the edge of an object (the diffracted signal is very weak compared to the original signal

Huygens’ Principle

it states that every point on a given spherical wavefront may be considered as a secondary point of source of EM waves from which the other secondary waves (wavelets) are radiated outward.

Interference

It occurs when two waves that left one source travelled different paths and arrive at one point, also, it usually occurs in high frequency sky wave propagation and in microwave space wave propagation

Constructive Interference, Destructive Interference

Two types of interference

Constructive Interference

it is when two waves are in phase with each other, they form an addition in amplitude.

Destructive Interference

it is when two waves are antiphase with each other, they form a cancellation in amplitude.

Sunspots, Solar Flux, and Solar Flare

Solar Effects on Wave Propagation

Sunspots

It is the tendency of the sun to have a greyish - black blemishes, seemingly at random times and at random places, on its fiery surface.

The 11 - year sunspot cycle

This affects propagation conditions because there is a direct correlation between sunspot activity and ionization.

Solar Flux

• It is a measure of energy received per unit time, per unit area, per unit frequency interval.

• The radio fluxes change gradually day to day, in response to the activity causing sunspots (daily solar flux information is of value in determining current propagation conditions).

Solar Flare

it is a sudden eruption on the sun that causes high-speed atomic particles to be ejected far into space from the surface of the sun.

Ground Wave (Surface Wave) Propagation, Sky Wave (Ionospheric) Propagation, Space Wave (Tropospheric) Propagation

Wave Propagation Modes

Ground Wave (Surface Wave) Propagation

• It is the propagation mode wherein the radio waves travel or progress along the surface of the earth.

• It must be vertically polarized because the electric field in horizontally polarized wave would be parallel to Earth’s surface and would be short - circuited by the conductivity of the ground.

Disadvantages of Ground Wave Propagation

• it requires a high Transmisson power

• it is limited to VLF, LF, and MF, requiring large antennas.

• its ground losses vary considerably with surface material and composition.

• it is affected by Tilting

Advantages of Ground Wave Propagation

• Used to communicate between any two locations in the world when given enough transmit power.

• it is unaffected by changing atmospheric conditions.

Relative Conductivity of Earth’s Surfaces

Surface: Seawater

Relative conductivity: Good

Relative Conductivity of Earth’s Surfaces

Surface: Flat, loamy soil

Relative conductivity: Fair

Relative Conductivity of Earth’s Surfaces

Surface: Large bodies of water

Relative conductivity: Fair

Relative Conductivity of Earth’s Surfaces

Surface: Rocky terrain

Relative conductivity: Poor

Relative Conductivity of Earth’s Surfaces

Surface: Desert

Relative conductivity: Poor

Relative Conductivity of Earth’s Surfaces

Surface: Jungle

Relative conductivity: Unusable

Ground wave propagation

• it gives good results over a long distances for signals in the MF range and below. It is limited only for those frequencies below 2 MHz for practical operation because as the frequency increases, the ground loss also increases. Also, the direction of the wave cannot be controlled.

• Also affected by tilting, since Earth’s atmosphere has a gradient density (density decreases gradually with distance from Earth’s surface), which causes the wavefront to tilt progressively forward.

Ground wave propagation

Is commonly used for ship-to-ship and ship-to-shore communications, for radio navigation, and for maritime mobile communications.

Sky Wave (Ionospheric) Propagation

It is the propagation mode wherein the radio waves are radiated from the transmitting antenna in a direction that produces a large angle with reference to the earth

Atmospheric Layers:
Troposphere

It is the lowest of the atmosphere where all weather disturbances take place. it extends to a height of approximately 8 to 10 miles above sea level.

Atmospheric Layers:
Stratosphere

it is a region directly above the troposphere where no weather is seen but circulation does occur. It extends above the troposphere at about 40 miles. (This layer is where the airplanes go)

Atmospheric Layers:
Ionosphere

it is the region in the atmosphere above the stratosphere where the several ionized layers of low-density gas are found. It absorbs large quantities of the sun’s radiant energy, which ionizes the air molecules, creating a free electron. It extends from 30 to 250 miles (50 km to 400km) above the ground. Because of the ionosphere’s non-uniform composition and its temperature and density variations, it is stratified. Essentially three layers make up the ionosphere: D, E, and F layer.

Ionosphere layer

consists of many ions (charged particles) and receives the most radiation from the sun. Also, the air molecules in the ionosphere absorb this radiation, causing them to ionize. And as the signal reacts with the ionized air molecules. the layer changes density, causing the signal to bend (refract).

The Upper atmosphere

it has a higher percentage of ionized molecules than the lower atmosphere. The higher the ion density, the more refraction.

Ionospheric Layers:
D Layer

It is the lowest layer of the ionosphere and is located approximately between 30 miles and 60 miles (50km to 100km) above Earth’s surface. Ionization begins at sunrise, peaks at local noon, and disappears at sundown. In this region, atoms are broken up into ions by sunlight recombine quickly, it does not bend HF and MF waves back to earth but bends VLF and LF signals. it disappears at night.

Ionospheric Layers:
E Layer

it is also known as Kennelly - Heaviside layer. it is located approximately between 60 miles and 85 miles (100km to 140km) above Earth’s surface, Ionization varies with the angle above the horizon. It aids the MF surface wave propagation and reflects some of the HF waves in daytime. it also disappears at night.

Ionospheric Layers:

E_s (Sporadic E) Layer

it is the upper portion of the E layer. It is a thin layer of very high ionization density, sometimes making an appearance with the E layer. it is caused by solar flares and sunspot activity. It often persists during the night, and when it appears, there is generally an unexpected improvement in long distance radio transmission.