Transcript Chapter 3 - Radio Phenomina

Chapter 3 – Radio Phenomena
• Propagation: How Signals Travel
• Propagation On The HF Bands
– Ground-wave Propagation
– Sky-wave Propagation
– HF Scatter Propagation
• VHF/UHF Propagation Characteristics
– Line-of-sight Propagation
– Tropospheric Bending and Ducting
– VHF/UHF Signals Through The Ionosphere
T3-1
Radio Wave Propagation
Radio waves travel to their destination in four ways:
1. Line of Sight

Directly from one point to another.
2. Ground-Wave

Along the ground, bending slightly to follow the Earth’s
curvature.
3. Tropospheric Bending and Ducting

In the lower layer of the Earth’s atmosphere.
4. Sky-Wave

Refracted or bent back to the Earth’s surface by ionized layers
in the ionosphere.
T3-2
Atmospheric Regions
Region
Height
Troposphere 7 miles
Notes
Region where all weather occurs
Stratosphere 6 to 30
miles
Region where atmospheric gases
“spread out” horizontally. The high
speed jet stream travels in the
stratosphere.
Ionosphere
30 to 400 Region where solar radiation from
miles
the sun creates ions. Major
influence on HF radio wave
propagation.
T3-3
Atmospheric Regions
T3-4
HF Band Propagation
• Ground-Wave Propagation
• Sky-wave Propagation
• HF Scatter Mode
T3-5
Ground-Wave Propagation
• Results from a radio wave diffraction along the Earth’s
surface.
• Primarily affects longer wavelength radio waves that have
vertical polarization (electric field is oriented vertically).
• Most noticeable on AM broadcast band and the 160 meter
and 80 meter amateur bands.
• Communication distances often extend to 120 miles or
more.
• Most useful during the day at 1.8 MHz and 3.5 MHz when
the D-Region absorption makes sky-wave propagation
impossible.
T3-6
Ground-Wave Propagation
The curved surface of the Earth horizon can diffract long-wavelength
(low frequency) radio waves. The waves can follow the curvature of the
Earth for as much as several hundred miles.
T3-7
Sky-wave Propagation
• Ionization levels in the Earth’s ionosphere can
refract (bend) radio waves to return to the surface.
– Ions in the Earth’s upper atmosphere are formed when
ultraviolet (UV) radiation and other radiation from the
sun knocks electrons from gas atoms.
– The ionization regions in the Earth’s ionosphere is
affected the sunspots on the sun’s surface. The
sunspots vary in number and size over a 11 year cycle.
• Sky-wave propagation is determined by radio
wave frequency and level of ionization in the
ionosphere.
T3-8
Sky-wave Propagation (Cont’d)
• Communication distances of 2500 miles are
possible with one skip off the ionosphere.
– Skip propagation has both minimum and maximum
ranges.
– The area between the maximum ground wave distance
and the minimum skip distance is called the skip zone.
– World-wide communications is possible using several
skips (or multi-hops)
• The highest frequency that a radio wave
transmitted straight up is reflected back to Earth is
called the critical frequency.
T3-9
Sky-wave Propagation (Cont’d)
• The maximum usable frequency (MUF) is the
highest frequency at which the ionosphere bends
radio waves back to a desired location on earth.
– MUF is dependant on level of solar radiation strength
and time of day.
– The maximum usable frequency (MUF) tends to be
higher during periods of high sunspots.
T3-10
Sky Wave Propagation
T3-11
Regions In The Ionosphere
The Earth’s ionosphere
contains several regions of
charged particles which
affect radio signal
propagation.
The ionization regions
change from day to night
periods.
Region
Height Above Surface
D Region
30-60 miles
E Region
60-70 miles
F Region
100-310 miles
T3-12
Regions In The Ionosphere
• D Region
–
–
–
–
Height about 35 to 60 miles above Earth.
Maximum ionization at or near noon.
Ionization disappears by sunset.
Absorbs energy from radio waves. Absorption on
lower frequencies is greater than higher frequencies.
– Radio wave absorption is most pronounced at mid-day.
– Responsible for short daytime communication ranges
on lower-frequency HF bands (160, 80 and 40 meters).
T3-13
Regions In The Ionosphere (Cont’d)
• E Region
– Height about 50 to 70 miles above Earth.
– Ionization useful for bending radio waves when in
sunlight.
– Reaches maximum ionization level around mid-day.
– Ionization reaches a minimum level just prior to
sunrise.
– Radio wave propagation up to about 1250 miles in a
single skip hop.
T3-14
Regions In The Ionosphere (Cont’d)
• F Region
– Height ranges from 100 to 310 miles above Earth.
– Ionization reaches a maximum about noon and tapers
off gradually toward sunset. Minimum ionization is
reached just prior to sunrise.
– F region splits into two parts (F1 and F2) during the day
and recombine at night.
• F1 region forms about 140 miles above Earth
• F2 region forms about 200 miles above Earth
– F2 region is responsible for long distance HF band
communication with distances of about 2500 miles.
T3-15
HF Scatter Modes
• All electromagnetic wave propagation is subject to
scattering influences from the Earth’s atmosphere,
ionospheric regions and objects in radio path.
• Scattered signals may be received in sky-wave
propagation skip zone.
• Scatter signals are generally weak and subject to
echoes and distortion.
• Most common when operating near the MUF.
• Under ideal conditions, scatter propagation is
possible over 3000 miles or more.
T3-16
The Scatter Modes
Back Scatter Propagation
T3-17
VHF/UHF Propagation
•
•
•
•
Line Of Sight (LOS)
Tropospheric Bending
Tropospheric Ducting
VHF/UHF Signals Through The Ionosphere
– Sporadic “E”
T3-18
Line-Of-Sight Propagation
• Radio signals travel in a straight line from a
transmitting antenna to the receiving antenna.
• Provides VHF/UHF communications within a 100
miles or so.
• Signals can be reflected by buildings, hills,
airplanes, etc.
• Reflections vary the propagation path causing
signal cancellation and reinforcement. This results
in a rapid fluttering sound called picket fencing.
T3-19
Line-Of-Sight (LOS) Propagation
T3-20
Tropospheric Bending
• Slight bending of radio waves occur in the
troposphere close to the Earth’s surface.
– There is always a radio signal loss as radio waves travel
through the troposphere
• Radio signal loss increases as the frequency increases
– The radio path horizon is generally 15 percent farther
away than the visible horizon (typically 8 to 9 miles).
• Communication distances can be increased by increasing the
antenna height above the terrain
– Tropospheric bending propagation is most useful at 144
Mhz and higher frequencies
T3-21
Radio Path Horizon
• The farthest point to which radio waves will travel
directly.
• The structure of the atmosphere near the Earth’s
surface causes the radio waves to bend in a curved
path.
• The radio horizon exceeds the geometric horizon
by approximately 15%.
T3-22
Radio Path Horizon
The distance D to the radio horizon is greater from a higher
antenna. The maximum distance over which two stations may
communicate by space wave is equal to the sum of their
distances to the horizon.
T3-23
Radio Path Horizon
Distance(miles)
Chart shows theoretical communication distance
(in miles) to the radio horizon for various
transmitter antenna heights above terrain (in feet).
50.0
45.0
40.0
35.0
30.0
25.0
20.0
15.0
10.0
5.0
0.0
1.0
D  1.415x H
10.0
100.0
1000.0
Height(ft)
T3-24
Tropospheric Ducting
• Radio signals can also be trapped in the troposphere,
traveling a long distance before returning to the Earth’s
surface.
• Results when a “duct” is formed by a temperature
inversion level (warm air over cold air) over land or water.
– Adjacent tropospheric regions having different densities will bend
radio waves passing through the regions
• Most useful at VHF/UHF frequencies.
• Most frequent during spring, summer and fall.
• Can provide contacts of 950 miles or more over land and
up to 2500 miles over ocean
T3-25
Tropospheric Ducting
When a cool air mass is overrun by a mass of warmer air, a “duct”
may be formed, allowing VHF and UHF radio signals to travel great
distances with little attenuation or signal loss.
T3-26
VHF/UHF Signals Through Ionosphere
• Sporadic E
– A type of sky-wave propagation that allows long
distance communication on the VHF bands (6 meters, 2
meters and 220 Mhz) through the E region of the
atmosphere.
– Occurs only sporadically during certain times of the
year.
– Most common type of VHF atmospheric propagation.
– The 6 meter band is most likely experience sporadic-E
propagation during the summer months ... even during
periods of low sunspot activity.
T3-27