Radio Wave Propagation

RADIOWAVE PROPAGATION

Refers to the behavior of radio waves as they travel from one point to another.

WAVE PROPAGATION

It is the transfer of electromagnetic waves from one point to another.

radio waves

EM waves are also called

Poynting vector

• Represents: Average power density and instantaneous power flow

TRANSVERSE WAVES

• Propagation is PERPENDICULAR to its direction.

• Includes radio waves travelling through free space

LONGITUDINAL WAVES

Propagation is PARALLEL to its direction.

Elements of a wave

λ - length of 1 cycle

T - time for 1 cycle

f - cycles per 1 second

Wavelength

• Distance that a wave travels in the time of one cycle

• λ = c/f

Wave velocity

Speed of the wave depending on the type and nature of the propagation medium

refractive index of air

n = 1.0003

n = 1.33

refractive index of water

n = 1.55

refractive index of glass

Relationship of refractive index, density and velocity

↑n = ↑ρ = ↓v

Frequency range of ELFs

30-300Hz

EVVLMHVUSEI acronym meaning

Every, Very, Very, Loving, Mother, Has a, Very, Ugly, Son, Except, I

Frequency range of VFs

300-3000Hz

Frequency range of VLFs

3-30kHz

Frequency range of LFs

30kHz-300kHz

Frequency range of MFs

300kHz-3MHz

Frequency range of HFs

3MHz-30MHz

Frequency range of VHFs

30MHz-300MHz

Frequency range of UHFs

300MHz-3GHz

Frequency range of SHFs

3GHz-30GHz

Frequency range of EHFs

30GHz-300GHz

Medium Frequencies

AM is included in the frequency range

Very High Frequencies

FM is included in the frequency range

1GHz - 100GHz

Microwave frequency range

Ultra High, Super High, Extremely High

Microwave is included in the frequency range

ISOTROPIC ANTENNA

CAN RADIATE UNIFORMLY IN ALL DIRECTION

• Ideal antenna

WAVE IMPEDANCE

Also called the characteristic impedance for which the medium is alr.

POWER DENSITY

It is defined as the ratio of the transmitter power over the area at which it is distributed (P/A)

FIELD INTENSITY FIELD STRENGTH

The quality of the electric field at any given point in volt per meter or a potential of 1 volt induced in an antenna wire 1 meter long.

Wave impedance in air

Power density in terms of Energy

POLARIZATION

Refers to the property of certain electromagnetic radiations in which the direction and magnitude of the vibrating electric field are related in a specified way.

LINEAR

The orientation of a linearly polarized electromagnetic wave is defined by the direction of the electric field vector.

CIRCULAR

Electric vector rotates about the axis of the direction of propagation

ELLIPTICAL

Electric vector rotates about the axis of the direction of propagation but the amplitudes of its two linearly polarized components are unequal.

RANDOM

There is no fixed pattern of polarization variation

ATTENUATION

• Electromagnetic waves are attenuated as they travel outward from their source.

• Proportional to the square of the distance travelled

REFLECTION

The bouncing of waves as it strikes a conductive surface

DIFFRACTION

• An EM Wave that meets an obstacle tends to bend around it or a slit with a size comparable to their wavelength

SCATTERING

Diffusion of a wireless signal when it encounters an uneven surface of the object

ABSORPTION

Source of energy of the electromagnetic waves are transferred to the atoms and molecules of space.

INTERFERENCE

Occurs when two waves combine in such a way that the system performance was degraded (collision of waves)

SURFACE/GROUND WAVE

• Travels around the curvature of the earth, sometimes right around the globe.

• Utilizes frequencies below HF range, and losses with increasing frequency.

• The higher the frequency, the shorter the wavelength, means more prone to absorption.

• AM

• becomes highly impractical at higher frequencies (such as HF and above) primarily due to severe tilting of the wave front caused by earth induced losses and rapid ground attenuation

Vertical - to reduce attenuation

Polarization of surface wave

SPACE WAVE / LOS

• Travels in the troposphere; the portion of the atmosphere closest to the ground.

• Depends mostly on line-of-sight conditions; limited in propagation by the curvature of the earth.

• Caused by the varying density of the atmosphere due to diffraction around the curvature of the earth.

• FM, microwave

SKY WAVE

• The wave strikes the Ionosphere and is refracted back to the ground.

• It utilizes frequencies in the HF range and sometimes frequencies just above or below it.

IONIZATION

occurs when high energy ultraviolet light waves from the sun enter the ionospheric region of the atmosphere, strike a gas atom, and literally knock an electron free from its parent atom.

Thermosphere

• Layer of the atmosphere where ionization is highest

• While the wave is in the HIGHLY DENSE CENTER portion of the layer, refraction occurs more SLOWLY since the density of ionization is almost uniform.

Mesosphere to Thermosphere ionization pattern

• Increasing

• the Increase in velocity of the upper part of the wave causes it to be BENT BACK TOWARD THE EARTH

Thermosphere to Exosphere ionization pattern

• Decreasing

• As the wave enters into the upper part of the layer of DECREASING IONIZATION, the velocity of the upper part of the wave decreases, and the wave is BENT AWAY FROM THE EARTH

FREQUENCY OF THE WAVE IN IONOSPHERE

• ↓ frequency of a radio wave = the more rapidly the wave is refracted by a given degree of ionization = less distance is covered.

ANGLE OF THE WAVE

↓ angle of radiation = longer skip distances as a result of the geometry.

D-layer

• The lowest, existing at an average height of 70 km, with an average thickness of 10 km.

• Least important layer In HF propagation.

• This layer has the ability to refract signals of low frequencies. High frequencies pass right through it and are attenuated.

E-layer / KENELLY-HEAVISIDE LAYER.

• Existing at about 100 km in height, with thickness of 25 km.

• This layer has the ability to refract signals as high as 20 MHz. For this reason, it is valuable for communications in ranges up to about 1500 miles.

Es-Layer Sporadic

• Thin layer of very high ionization density, sometimes making an appearance with the E-layer.

• When it occurs, it often persists during the night.

F1-Layer

• Exist at a height of 180 km in daytime and combines with the F2 layer at night.

• Its daytime thickness is about 20 km.

• Main effect is to provide more absorption for HF waves.

F2-Layer

• The most important reflecting medium for HF radio waves.

• Approximate thickness is 200 km and its height ranges from 250 km to 400 km in daytime. At right, il falls to a height of about 300 km, where it combines with the F1-layer.

Tropospheric ducting

• When the refractive index of the atmosphere drops exceptionally fast with an increase in altitude(often due to a temperature Inversion or rapid moisture changes), signals can become trapped in a narrow layer.

• In this propagation method, when the signal encounters a rise In temperature in the atmosphere instead of the normal decreaso (known as a temperature inversion), the higher refractive index of the atmosphere there will cause the signal to be bent.

TROPOSPHERIC SCATTERING

When a radio wave passing through the troposphere meets a turbulence, it makes an abrupt change in velocity. This causes a small amount of the energy to be scattered in a forward direction and returned to Earth at distances beyond the horizon.

VIRTUAL HEIGHT

• The height of the projected path of the Sky Waves.

• It is measured by sending a wave vertical to the layer and measuring the time it takes to come back to the receiver.

• virtual height > actual height

CRITICAL FREQUENCY

The highest frequency that will be returned down to earth by a certain layer after having been beamed vertically upward.

MAXIMUM USABLE FREQUENCY

The highest frequency that will be returned down to the earth at a given distance when beamed at a specific angle other than the normal.

OPTIMUM WORKING FREQUENCY (OWF) / Frequency of Optimum Transmission (FOT) / Optimum Usable Frequency

Frequency chosen by practical to avoid the irregularities of the ionosphere. It is about 15% lower than MUF.

SKIP DISTANCE

The shortest distance from a transmitter, measured along the surface of the earth, at which a sky wave of fixed frequency will be returned to earth.

SKIP ZONE

The area where no signal can be heard.

INTERFERENCE FADING

mixing two or more signal components propagating along different paths

ABSORPTION FADING

caused by solar flare activities and particularly affects the lower frequencies

POLARIZATION FADING

caused by the so-called Faraday Effect or Faraday Rotation

SELECTIVE FADING

fading having different effects on different frequency ranges

11-YEAR SUNSPОТ CYCLE

About every 11 years, the sun's magnetic field does a flip. In other words, the north pole becomes the south pole, and vice versa.

27-DAY SUNSPОТ CYCLE

As the sun rotates on its own axis, these sunspots are visible at 27-day intervals, the approximate period required for the sun to make one complete rotation.

Birefringence / Ionospheric Splitting

When an electromagnetic wave enters the Ionosphere, the Earth's magnetic field splits the single wave into two distinct components with different propagation velocities and polarizations.

SID: Sudden Ionospheric Disturbance

Ionospheric storm causing temporary skywave blackout