TAS 1 - radiowave propagation

first part

4 elements of communication

transmitter

receiver

channel

message

Why is distance near the skip distance should be used for sky-wave propagation?

To prevent sky wave and upper ray interference

Window

A range of microwave frequency more easily passed by the atmosphere than the other

Radio wave Propagation

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

Wave Propagation

transfer of electromagnetic waves from one point to another

can be perpendicular, right, normal, or 90 degrees

Electromagnetic Waves (EM waves)

Also called radio waves

2 types of waves (wrt DIRECTION)

transverse and longitudinal

transverse waves

PERPENDICULAR (light, em waves) propagation to its direction

longitudinal waves

PARALLEL (sound waves) propagation to its direction

elements of a wave

peak-to-peak

crest (+ peak)

trough (- peak)

wavelength

alternation (at x axis)

frequency

#of cycles per second of a wave

wavelength

distance that wave travels in time of 1 cycle

λ = c / f

wave velocity

speed of wave depends on the type and nature of propagation of the medium

Electromagnetic Spectrum

the complete range of electromagnetic radiation, arranged by wavelength or frequency, from radio waves to gamma rays

isotropic antenna

can radiate uniformly in all direction

ideal antenna/radiator/reference (hypothetical)

gain = 1 (0db)

wave impedance

also called characteristic impedance for w/c medium is air

wavelength

λ = c / f

power density

ratio of tx power over area w/c it is distributed

field intensity/field strength

quality of electric field at any given point (volt/m)

potential of 1 volt induced in a 1m antenna wire

E and H fields

perpendicular to each other and direction

poynting vector

quantifies energy flow of EM wave

energy per unit area, per unit time

how does electric field intensity (E) affects the power density (Pd) when it is doubled?

the power density will quadruple

polarization

orientation of E-field wrt Earth’s surface

linear, circular, elliptic, random

linear polarization

defined by direction of the electric field vector

circular polarization

electric vector rotates in horizontal + vertical (equal in magnitude)

elliptical polarization

electric vector rotates about the axis of direction, but amplitude of its 2 linearly polarized components is not equal

random polarization

stochastic

random polarization

no fixed pattern of polarization variation

attenuation

attenuates the EM waves as it travels outward from its source

proportional to the square of distance travelled

dB

reflection

bouncing of waves as it strikes conductive surface

180 degrees phase shift

refraction

bending of waves as it passes boundary of 2 media having diff density

HLA LHT

uses Snell’s law and refractive index

diffraction

ability of an EM wave to pass through obstacles

bends around obstacle

low freq - bass

high freq - mid treble

low freq diffract more than high freq

scattering

diffusion of signal when it encounters an uneven surface of the object

absorption

signal is absorbed & converted to heat

source of energy of waves is transferred to atoms and molecules of space

interference

occurs when 2 waves combine which results to degraded performance

(collision of waves)

absorption loss increases with freq due to

molecular resonance

Rayleigh fading assumes

multipath interference, no LOS

Rayleigh fading

statistical model for wireless comms

phase shift upon reflection from denser medium is

180 deg

at grazing incidence on sea water, vertical polarization reflection coefficient approaches

-1

increases the attenuation of a radio signal

high freq of signal

critical angle occurs when refracted ray is

90 deg

surface wave

travels around curvature of earth, sometimes right around the globe

soil conductivity

surface wave

vertical polarization to reduce attenuation

freq below HF

loss with increase freq

high freq short wavelength

AM

535-1605 kHz

(Marconi Antenna)

surface wave

VLF

3-30 kHz

sky wave

HF

3-30 MHz

space wave

SHF

3-30 GHz

space wave

SHF

microwave

GHz

space wave

LOS & P2P

travels in troposphere (atm portion closest to the ground)

caused by varying density of atm due to diffraction around curvature of earth

sky wave

HF

3-30 MHz

FM/TV

sky wave

strikes at the ionosphere and refracted back to the ground

Increasing Ionization

Bends waves toward Earth.

Decreasing Ionization

Bends waves away from Earth.

Highly Dense Center

Refraction happens slowly.

Refraction Process

Change in velocity causing a change in direction.

Lower Frequency

Refracts faster; covers shorter distances.

Higher Frequency

Refracts slower; covers longer distances.

100 MHz (VHF/UHF)

Passes straight through (too fast to bend).

Lower Radiation Angle

Creates a longer skip distance.

Higher Radiation Angle

Creates a shorter skip distance.

Skip Distance

The distance between the transmitter and the point where the sky wave first returns.

High Frequency + Low Angle

Maximum Distance

Low Frequency + High Angle

Minimum Distance

Skip Zone

A location (the empty "hole" on the map where you can't hear anything)

D-Layer

lowest, h=70km, thickness=10km

least important

D-Layer

ability to refract signals of low freq

high freq pass thru and attenuated

E-Layer

h=100km, thickness=25km

also known as KENELLY-HEAVISIDE LAYER

E-Layer

ability to refract signals as high as 20MHz

used for comm ranges up to 1500 miles

Es-Layer Sporadic

very high ionization density

occurs during the night

F2-Layer

most important for HF radiowaves

height=250km to 400km (daytime)

thickness=200km

F2-Layer

at night, height=300km and combines with F1 layer

F1-Layer

height=180km, thickness=20km (daytime)

combines with F2 layer at night

F1-Layer

more absorption for HF waves

ground waves are attenuated by:

ground conductivity

primary freq range for reliable ground wave

MF (300kHz-3MHz)

surface wave component of ground wave uses

earth’s curvature as waveguide

UHF frequencies primarily use:

space waves

when LOS antennas see each other, mode is

space/direct wave

sky wave is reliable during

nighttime (f-layer)

tropospheric ducting

rise in temp in the atm, instead of normal decrease (known as tem inversion) the higher refractive index of the atm there will cause the signal to be bent

tropospheric scattering

when a radio wave passing through the trophosphere meets a turbulence, it makes an abrupt change in velocity

virtual height

height of projected path of sky waves

virtual height

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

critical frequency

highest freq that will be returned down to earth by certain layer after having been beamed vertically upward

maximum usable frequency (MUF)

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

optimum working frequency (OWF)

freq chosen by practical to avoid irregularities of ionosphere

15% lower than MUF

frequency of optimum transmission (FOT)

other term for optimum working frequency (OWF)

optimum usable frequency

other term for optimum working frequency (OWF)

interference fading

mixing two or more signal components propagating along diff paths

absorption fading

caused by solar flare activities and particularly affects the lower freqs

polarization fading

caused by the so-called Faraday Effect or Faraday Rotation

selective fading

fading having diff effects on diff freq ranges

11-year sunspot cycle

sun’s magnetic field doesn’t flip

north pole becomes the south pole, and vice versa

27-day sunspot cycle

visible at 27 day intervals, approx period req for the sun to make one complete rotation

maximum usable frequency (MUF) defined as

highest frequency for given path

skip distance increases with

high frequency and lower take-off angle

why is the MUF generally higher during the day than at night?

ionization density in the f2-layer is higher during the day due to solar radiation

f2 layer provides longest

skip distances (300-4000 km)

tropospheric ducting occurs when:

warm air traps signals above cool layer (VHF/UHF)