radio propagation

1. RADIO WAVE
PROPAGATION
EE 182/ECE 121 – Communications System

2. Free-space/RF or Radio Propagation
 Propagation – How radio waves travel from
point A to point B

3. 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!

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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.

10. IMPORTANT TERMS IN WAVE
PROPAGATION CALCULATIONS

11. Transmitting Antenna Gain
If transmitting antenna has a gain in
a given direction, Power Density is…
PD = PT GT
4πR2
EIRP = PT GT
Effective 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.

12. 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

13. 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)

14. Free Space Loss (FSL)

15. Free Space Loss (FSL)

16. 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.

17. 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 dBW
b) The power delivered to the receiver

18. Role of Environment on
Wave Propagation

19. 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

20.  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)

21.  Refraction
• Angles involved are given by
Snell’s Law:
n1 sin θ1 = n2 sin θ2
Where n = index of refraction
Θ = angle
sin θ1 = √ϵR2
sin θ2 √ϵR1
Refraction (Bending of Signals)

22.  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)

23.  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)

24.  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)

26. 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?

27. Types of Wave Propagation
 Ground Wave ( f < 3 MHz)
 Sky Wave (3 to 30 MHz)
 Space Wave (f > 30 MHz)

28. 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.

29. 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

30. 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.

31. Layers of the Atmosphere

32. 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.

33. 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)

34. 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

35. 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

36. 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

37. 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”.

38. 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

39. 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

40. 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

41. 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

42. 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.

43. 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

44. 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.

45. Tropospheric Propagation Terms
Maximum Radio Horizon Distance or
Maximum Radio Range

46. 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?

47. assignment
 Other RF Propagation Modes