radio wave propagation
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RADIO WAVE PROPAGATION RADIO WAVE
PROPAGATION
EE 182/ECE 121 – Communications System
Free-space/RF or Radio Propagation
Propagation – How radio waves travel from point A to point B
Free-space/RF or Radio Propagation
Radio - the transmission of signals through free space by EM waves with frequencies below visible light, in the RF range, from about 3 kHz to 300 GHz. These waves are called RADIO WAVES.
Free-space – a space that does not interfere with normal radiation and propagation of radio waves
Radio waves travel as electromagnetic waves with its velocity≈ speed of light!
Electromagnetic Waves
Are forms of radiant energy like heat, light, radio, x-ray and TV waves that are considered to be oscillatory disturbances in free space
Consist of co-travelling electrical and magnetic fields oscillating 90° out of phase with each other and arranged orthogonally to each other
The direction of propagation is mutually perpendicular
Polarization The orientation of
the electric field with respect to the Earth’s surface and is determined by the physical structure of the antenna and by its orientation
Velocity of Propagation for any medium
Electromagnetic Radiation
Consider an Isotropic Source, the theoretical construct in propagation
Power is radiated uniformly at a constant rate in all directions
Closely resembles an OMNIDIRECTIONAL antenna
All points distance R from the source lie on the surface of the sphere and have equal power densities
At any instant of time, total radiated power is uniformly distributed over the total surface of the sphere
r
Power Density=
Total radiated power over
area of the sphere
Electromagnetic Quantities and Parameters
Ohm’s Law for Electromagnetic Waves
Characteristic Impedance for a lossless medium
Characteristic Impedance Of free space
Characteristic Impedance For a non-magnetic medium
Electromagnetic Quantities and Parameters
PD = PT
4πR2
Power Density at a distance R from the source
Field Strengths at a distance from the source
PD = ε Η = Η2 Zs Power Density with E and H
Examples
A power of 100 W is supplied to an isotropic radiator. What is the power density at a point 10 km away?
Find the electric field strength for the signal in the previous example.
Find the characteristic impedance of polyethylene, which has a dielectric constant of 2.3.
IMPORTANT TERMS IN WAVE PROPAGATION CALCULATIONS
Transmitting Antenna Gain
If transmitting antenna has a gain in a given direction, Power Density is…
PD = PT GT
4πR2
EIRP = PT GTEffective Isotropic Radiated Power
the amount of power that would have to be emitted by an isotropic antenna to
produce the peak power density observed in the direction of
maximum antenna gain.
In practical communications, it is very important to know the signal strength at the
receiver input. This depends on the transmitter power and the distance from the
transmitter and receiver.
Receiving Antenna Gain
Effective Area of an Antenna- All the power in the wave is extracted
and delivered to the receiver
Aeff = PR
PD
Effective Area of a Receiving AntennaAeff = λ2GR
4π
Free Space Attenuation
Attenuation of Free Space PR = λ2GTGR
PT 16π2R2
Attenuation as expressed in dB
PR = GT (dBi) + GR (dBi) – (32.44 PT +20 log d(km) + 20 log f (MHZ)
(dB)
Free Space Loss (FSL)
Free Space Loss (FSL)
Example
A transmitter has a power output of 150 W at a carrier frequency of 325 MHz. It is connected to an antenna with a gain of 12 dBi. The receiving antenna is 10 km away and has a gain of 5 dBi. Calculate the power delivered to the receiving, assuming free-space propagation. Assume no losses or mismatches in the system.
Example
A satellite transmitter operates at 4GHz with an antenna gain of 40 dBi. The receiver 40,000 km away has an antenna gain of 50 dBi. If the transmitter has a power of 8W, find
a) EIRP in dBWb) The power delivered to the receiver
Role of Environment on Wave Propagation
Reflection (Bouncing of Signals)
Reflection
• occurs when a wave hits a reflective/smooth surface
• When the wave hits the surface at an angle, the rebound of the wave will be equal to that wave on the other side of the normal.
• Complete reflection occurs only for a theoretically perfect conductor and when the electric field is perpendicular to the reflecting element
Refraction
• bending of a ray as it passes from one medium to another at an angle
• occurs when EM waves pass from one propagating medium to another medium having different density
• degree of bending of a wave at boundaries increases with frequency
Refraction (Bending of Signals)
Refraction
• Angles involved are given by Snell’s Law:
n1 sin θ1 = n2 sin θ2
Where n = index of refractionΘ = angle
sin θ1 = √ϵR2 sin θ2 √ϵR1
Refraction (Bending of Signals)
total internal reflection – occurs when the angle of incidence is large and wave travels into a region of considerably lower dielectric constant, the angle of refraction can be greater than 90°, so that the wave comes out of the second medium and back into the first.
Refraction (Bending of Signals)
critical angle – the angle of incidence that results in the angle of refraction of exactly 90° (so that the wave propagates along the boundary between the two media)
Interference - is when two waves of the same power combine with each other and either cancel each other out or increase the amplitude. This can occur with light, sound or electromagnetic waves.
- It occurs when two waves that left one source and traveled by different paths arrive at a point
Interference (Collision of Signals)
Diffraction - bending of a ray that is traveling in a straight path as it hits an obstacle
- occurs after a waves passes an object and starts to curve around it. Waves when let into a larger space tend to spread out.
“Every point of a wave front may be considered the
source of secondary wavelets that spread out in all
directions with a speed equal to the speed of propagation
of the waves.” - Huygen’s principle
Diffraction(Scattering of Signals)
Examples
Find the critical angle when a wave passes from glass with ϵR = 7.8, into air.
A radio signal moves from air to glass. The angle of incidence is 20°. Calculate the angle of reflection. Relative permittivity of the glass is 7.8
A certain antenna has a gain of 7 dB with respect to an isotropic radiator. What is the effective area if it operates at 200 MHz? How much power would it absorb from a signal with a field strength of 50µV/m?
Types of Wave Propagation
Ground Wave ( f < 3 MHz)
Sky Wave (3 to 30 MHz)
Space Wave (f > 30 MHz)
Ground or Surface Wave Propagation
Earth-guided EM waves that travel close to the surface of the earth
Must be vertically polarized to prevent short-circuiting the electric component
As signal moves away from the transmitter, the ground wave eventually disappears due to tilting. Radio waves lose energy as they are forced to bend to follow the earth’s curvature.
Attenuation due to absorption depends on the conductivity of the earth’s surface and the frequency of the EM wave.
Ground or Surface Wave Propagation
Ground losses increase rapidly with increasing frequency.
Used in ship-to-ship and ship-to-shore communication , for radio navigation and for maritime mobile communications.
Relative Conductivity of Earth Surfaces
Surface Relative Conductivity
Seawater Good
Flat, loamy soil Fair
Large bodies of freshwater Fair
Rocky Terrain Poor
Desert Poor
Jungle Unusable
Sky Wave or Ionospheric Propagation
EM waves that are directed above the horizontal level
Waves radiated from the antenna transmitter in a direction that produces a large angle with reference to earth.
Sky waves are radiated toward the sky, and are either reflected or refracted back to earth by the ionosphere.
Layers of the Atmosphere
Ionosphere Uppermost part of the atmosphere
which absorbs large quantities of radiant energy from the sun, hence,
it is an IONIZED region.
Ionization is converting an atom or molecule into an ion by light (heating up or charging) from the sun on the upper atmosphere.
• Creates a horizontally stratified
medium where each layer has a peak density and a definable width or profile.
• Thus, it influences radio propagation.
Layers of the Ionosphere
Layer Height(km) Thickness(km)
Single-Hop Range (km)
D 50-90 (70 ave) 10
E 110 25 2350
F1 175-250(180 ave) 20 3000
F2 250-400 200
3840 (daytime)
4130 (nighttime)
Layers of the Ionosphere D Layer
Lowest ionized region whose ionizations depend on the altitude of the sun above the horizon
Ionization begins at sunrise, peaks at local noon, and disappears at sundown
Layer disappears at night It reflects VLF and LF waves; It absorbs MF and HF
waves At very low frequencies, the D layer and the ground
combine to act as a huge waveguide, making worldwide communication possible with large antennas and high power transmitters
Layers of the Ionosphere E Layer
Also called the “Kennelly-Heaviside Layer” the lowest portion of the ionosphere that is useful
for long distance communication ionization increases rapidly after sunrise, reaches
maximum around noon, and drops off quickly after sundown. Minimum ionization is after midnight.
Layer almost totally disappears at night, too. Reflects some HF waves in daytime and aids MF-
surface wave propagation
Layers of the Ionosphere F Layer
Also called “Appleton Layer” The region where most of long-distance
communications capability stems Consists of two layers: F1 and F2 Ionization is at its maximum during the afternoon
hours. Atoms in this layer remain ionized for a longer time after sunset
At night, F1 combines with F2 to form a single layer ≈ 300 km
Ionospheric Propagation Terms
Critical Frequency
Highest frequency that will be returned down to earth (by a layer) when beamed straight up
It is layer dependent (depends on its ionization density) and varies with time of the day and the season
Angle of incidence is normal In practice, it is 5-12 MHz in F2 layer and is used as
a point of reference for comparison purposes or “benchmarking”.
Ionospheric Propagation Terms
Virtual Height
Apparent height of the ionized layer and is measured by sending a wave vertically to the layer and measuring the time it takes to come back to the receiver.
Critical Angle
Highest angle of radiation or the maximum vertical angle that a wave can be propagated and still be refracted back by the ionosphere
Ionospheric Propagation Terms Maximum Usable Frequency
Highest frequency that will be returned down to earth at a given distance when beamed at a specific angle other than the normal.
Normal values of MUF reach about 8-35 MHz but may rise as high as 50 MHz under unusual solar activities.
Secant Law- This assumes a flat Earth and
a flat reflecting layer
Ionospheric Propagation Terms
Optimum Working Frequency
Frequency that provides the most consistent communication.
Chosen by practical experience and is 85% if the MUF considering the instability of the ionospheric conditions
Ionospheric Propagation Terms
Skip Distance
Minimum distance from a transmit antenna that a sky wave at a given frequency will be returned to Earth
Frequency must be less than the MUF and propagated at its critical angle
Example
A VHF radio is to be established via the ionosphere. Assuming the earth is flat with a critical frequency of
5MHz, the angle of elevation is 45°. Calculate the OWF.
Space Wave or Direct Wave
Also called “LOS” or “Tropospheric Propagation”
Depend mostly on LOS conditions, a space wave is limited in propagation by the curvature of the earth
The horizon is theoretically the limit of the communications distance
Employed mostly on VHF, UHF and SHF bands or the microwave frequencies
Tropospheric Propagation Terms
Radio Horizon
4/3 farther than the optical horizon due to bending in the atmosphere
It can be lengthened by elevating the transmit and receive antennas above Earth’s surface with towers or by placing them on top of mountains or high buildings.
Tropospheric Propagation Terms
Maximum Radio Horizon Distance orMaximum Radio Range
Example
A radio tower has a UHF radio antenna mounted 150 ft above the earth. Calculate the radio horizon in miles.
What is the distance to the radio horizon for an antenna located 80 ft above the top of a 5000-ft mountain?
assignment
Other RF Propagation Modes
