Radio Wave Propagation2

RADIO WAVE PROPAGATION

LAYERS USED

POWER DENSITY

ELECTRIC FIELD INTENSITY

hug the contour of the earth and are affected by the

terrain

antenna must be vertically polarized

antenna must project the signal at a very small

radiation angle so that the energy is not transmitted

toward the atmosphere instead along the ground

/4

Ground (Surface) Waves

Typical Ground ConstantTERRAIN TYPE DIELECTRIC

CONSTANT

CONDUCTIVITY

Sea water 81 4.5 x 10 –11

Fresh water 80 10 x 10 –14

Pastoral low hills, rich soil 20 10 x 10 –14

Pastoral medium hills, forestation

13 5 x 10 –14

Rocky soil 12 2 x10 -14

Cities & industrial areas 5 1x 10-14

SOMMERFELD ANALYSIS on GWP

d

HT

TX

E =A E 1

d

Where:

E - Ground Wave Field Strength

A = factor affecting ground conductivity

d - Distance from transmitting antenna

E1 = electric field intensity at a unity distance

RX

HR

Problem

8. A police radio transmitter operating at a frequency of 1690 kHz

is required to provide a ground-wave having a field strength of at

least 0.5 mV/m at a distance of 10 miles. The transmitter antenna

is expected to have an efficiency of 50 %, thus, it radiates 50 % of

the energy actually delivered to it and produces a radiated field

that is proportional to the cosine of the angle of elevation. The

ground is such that a conductivity of 5 x 10 -14 emu and dielectric

constant of 15 can be expected resulting to a ground constant of 0.15.

Determine the transmitter power required.64.24 watts

Direct (Space) Waves

are useful primarily only in VHF and UHF bands.

VHF and UHF signals are easily reflected by

buildings, hills, and even airplanes, so some of your

signal reach the other station by a direct path and

some may be reflected

Direct or space wave travel in a straight line from

transmitting antenna to receiving antenna

is referred to as line-of-sight propagation

TV and FM radio broadcasts

Problem

10. In a VHF mobile radio system, the base station transmits

100 watts at 150 MHz, and the antenna is 20 m above ground.

The transmitting antenna is /2 dipole for which the gain is 1.64.

Calculate the field strength at a receiving antenna of height 2 m

at a distance of 40 km.

11. Using the data in no. 1, find the a) path difference of the

radiated wave and b) their phase difference.

11 V/m

2 mm ; 6.28 x10 -3 rad/s

d2

D

R R

R


Path Distance

d1

d2

D

R’ R’

R’

A

B

C

D


Path Distance

d1


D = d1 + d2

D = 2 HT 2 HR+

Where:

D – path distance, miles

HT – height of transmitting antenna, ft

HR – height of receiving antenna, ft

D = d1 + d2

D = 17 HT 17 HR+

Where:

D – path distance, Km

HT – height of transmitting antenna, m

HR – height of receiving antenna, m

Path Distance

Problem

9. Calculate the maximum distance at which a receiving

antenna of 200 ft be constructed away from a transmitting

antenna of 400ft considering line-of-sight condition with

k=4/3.

48.28 mi

Direct and Ground Reflected Waves

CHARACTERISTICS:

The received power is a function of the frequency of the radiated field.

The received power is a function of the (transmit and receive) antenna heights.

This model is valid for relatively low antenna elevations and frequencies of 50

MHz and above

This model assumes a plane earth and does not take into account the curvature

of the earth

Propagation Over Plane Earth

HT

HR

D

Direct Path

Reflected Path

ER

Propagation Over Plane Earth

ER =4 Ht Hr

D Ed

Where:

ER - Resulting Field Strength ; v/m

Ed - Direct Ray Field strength; v/m

HT - Height of transmitting antenna ; m

HR - Height of receiving antenna ; m

D - Path distance ; m

Tropospheric Propagations

Ed =

30 G Pt

D


is the region of the earth's atmosphere immediately

adjacent to the earth surface

Tropospheric scatter is the method of propagating

microwave energy beyond LOS or over the horizon

takes advantage of the refraction and reflection

phenomena

causes signals to return to Earth beyond the

geometric horizon, and allows you to contact stations

that are farther away than would otherwise be

possible

TROPOSPHERE

This radio path horizon is generally about 15% farther

away than the visible horizon.

TROPOSPHERE


Radio signals can be trapped in the troposphere,

traveling a longer distance than normal before

coming back to the Earth's surface

The area between the Earth and the warm air mass is

known as a duct

Ducts usually form over water, but they can also form

over land

Tropospheric ducting is the most common type of

enhanced propagation at UHF

Ducting-Super refractions


Tropospheric Ducting

Ionospheric Propagations - Sky Waves

is aimed not at the intended receiver but at the sky

takes advantage of the ionosphere that surrounds the

earth to provide worldwide communications

radiate signal toward ionosphere and have it refract

and return to earth

Some of the signal passes through the layers of the

ionosphere and out into space, but enough returns to

earth to be picked up by a sensitive receiver

SKY WAVE

Transmittedwave

Reflectedwave

Refractedwave

Skip distance

Ionosphere

SKY WAVE


is the layer of partially ionized gas that is above the

oxygen-rich layer we live

absorbs large quantities of radiant energy from the

sun, becoming heated and ionized

The ionization is caused by ultraviolet radiation's from the sun

The amount of ionization depends on many factors:

amount of sunlight, seasons of the year, sunspot, weather conditions and local terrain

If the wave is bent enough, it returns to Earth

If the wave is not bent enough, it travels off into space

IONOSPHERE


IONOSPHERE

The ionosphere consist of a series of layers

varying ion density at different height (D, E, F1 & F2 layers)


Layer D:

30 to 60 Miles (50 - 95 Km) above the earth's surface

Exist during daytime only and it ceases to exist after sundown

Least important layer from the point of view of HF

propagations

Reflects some VLF and LF waves and

Absorb MF and HF waves to a certain extent

IONOSPHERE


SPORADIC E - Is a thin layer of very high ionization density,

sometimes making an appearance with the E-layer

LAYER E:

60 to 80 miles (95 - 130 Km) above the earth's surface

Also a day light phenomenon (disappear at night)

This layer depend on the suns ultraviolet radiation

The main effect of the E layer are to aid MF wave a little

and to reflect HF waves in daytime

Layer F:

F1 100 to 155 miles (160 - 250 Km)

Provide more absorption for HF waves but some are reflected

from it

F2 155 to 250 miles (250 -400 Km)

Most important reflecting medium for HF radio waves

Most HF waves pass through to F2 where they are refracted

back to Earth

F1 and F2 are separate during daylights and merge after

sunset/nighttime (F-layer)

Most highly ionized, and hence there is some chance for the ionization to remain at night

Higher ion density means, the more refraction (bending)

IONOSPHERE


Effects of Ionospheric Density on Radio waves


Relative Permittivity( R)

Where:

R - Relative Permittivity (dielectric constant of the ionized layer)

N - Electron Density ; m -3

m - Electron Mass ; 9.11 x 10 -31 Kg

qe - Electron Charge ; 1.6 x 10 -19 C

- Angular frequency ; rad/sec

o - Permittivity of Free Space

R =N qe2

1 -o m 2


Plasma Frequency (Fp)

Angular velocity of the wave can have a value that makes R

equal to zero, this is the plasma angular frequency p

FP = 9 N

p = N qe2

o m


Critical Frequency (FC)

A wave of low frequency that is sent vertically toward

the ionosphere will be reflected back to the

transmitter

The highest frequency wave that will be reflected from

a given layer when the wave is at vertical incidence

Is a plasma frequency from a given layer

The equipment used to measure the virtual height of

the ionosphere is called an ionosonde

FC = 9 Nmax


Maximum Usable Frequency (MUF)

The critical frequency for the layer is an indication of

the highest frequency, called MUF

MUF is the highest frequency that will be reflected

from a given layer and return to earth at a given

angle of radiation

The frequency at which communication just starts to

fail is known as the Maximum Usable Frequency (MUF)

MUF = FC Sec i


Optimum Working Frequency (OWF)

Because of the general instability of the ionosphere, the optimum working frequency is used instead.

The highest frequency that can be used for sky wave

propagations between two points on the earth

OWF = 0.85 MUF

Frequency versus Refraction and Distance

Incidence angles of Radio Waves

Effect of Frequency on the Critical Angle

Problem

12. Two points on earth 1500km apart are to

communicate by means of HF. Given that this is to

be a single hop transmission, the critical frequency

at that time is 6 MHz and the conditions are

idealized. Calculate the maximum usable frequency

if the height of the ionosphere is 350 km.

14.188 MHz

Problem

13. A wave travelling in free space undergoes refraction

after it enters a denser region such that the original

25º angle of incidence at the boundary of the two

media is changed to 20º. What is the velocity of the

EM wave in the denser medium?

2.43 x 10 8 m/s

Problem

14. Calculate the transmission path distance for an

ionospheric transmission that utilizes a layer of

virtual height 200 km. The angle of elevation of the

antenna beam is 20º.

d = 2R ( R

R + h

2) sin -1 cos )(

1100 km

966 km

d = 2h / tan

TERMS• If the radiated energy comes from another radio

transmitter, then it is considered radio-frequency

interference (RFI)

• If the energy comes from else where, then it is

electromagnetic interference (EMI)

• Isothermal region Temperature in the stratosphere is

believed to be fairly constant and is not subject to

temperature changes or inversions and will not cause

significant refractions

TERMS

• Sunspots are relatively cool areas that appear as dark

blemishes on the face of the sun. They are formed

when magnetic field lines just below the sun's surface

are twisted and poke though the solar photosphere.

• Solar flares emit high-speed particles which cause

auroras, known in the northern hemisphere as

Northern Lights. From the ground auroras appear as

shimmering curtains of red and green light in the sky.

Radio Wave Propagation2

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