Propagation

Report
Propagation of Waves
Ham Radio Class
Week 5
Wave propagation
1
Propagation of waves
Radio waves
spread out from
the antenna in
straight lines (like
light)
Wave propagation
2
• The farther it travels the weaker it
becomes, eventually becoming too
weak to be received.
The distance over which a
transmission can be received is
called range.
Wave propagation
3
Possible routes of wave propagation
The curvature of the earth
sets a range limit for many
signals; this is called a radio
horizon.
• The distance over which a transmission can be received
is called range.
The curvature of the earth sets a range limit for The
distance over which a transmission can be received is
called range.
The curvature of the earth sets a range limit for many
signals; this is called a radio horizon. (T3C10) many
signals; this is called a radio horizon. (T3C10)
Wave propagation
5
Communications at VHF and UHF
are generally “line of sight”
communications.
That is they travel directly from the
transmitting station to the
receiving
station.
Wave propagation
6
Direct (not via a repeater) UHF and VHF
signals are rarely heard from stations outside
your local
coverage area
because these
high
frequency
signals are
not reflected
by the
ionosphere.
Longer Radio waves
can travel along the
ground
Wave propagation
8
Possible routes of wave propagation
Radio waves can
be reflected or
refracted by
sudden changes
in the media
through
which they
travel.
Wave propagation
10
Obstructions can create radio shadows
Knife-edge propagation is the term used to describe when
signals are partially refracted around solid objects
exhibiting sharp edges.
Refraction also makes the earth seem less
curved to VHF and UHF signals making them
able to travel further that the line of sight.
Wave propagation
12
Shorter waves lengths can more
easily penetrate buildings, making
communication from inside a
structure more effective when
using UHF signals.
Wave propagation
13
Radio signals may take different paths
from the transmitter to the receiver. This
is called multipath.
Wave propagation
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Signals may interfere with each other
if they are out of phase.
Wave propagation
15
Problems/solutions with multipath propagation
The irregular fading of signals from distant
stations is caused by the random combining of
signals arriving via different paths.
Moving your antenna just a few feet may avoid
the random reflections causing the multipath
distortions.
Wave propagation
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Signals from mobile stations moving
through an area with multipath have
characteristic rapid variations in
strength known as picket-fencing.
Wave propagation
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What should you do if another operator
reports that your station’s signals were
strong just a moment ago, but now they
are weak or distorted?
Move a few feet, as random reflections
may be causing multipath distortions.
Wave propagation
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Why are UHF signals often
more effective from inside
buildings than VHF signals?
The shorter wave length
allows them to more easily
penetrate the structure of the
building.
Wave propagation
19
What term is commonly used to
describe the rapid fluttering sound
heard from mobile stations that are
moving while transmitting?
Picket fencing
Wave propagation
20
What is the cause of irregular fading of
signals from distant stations during times of
generally good reception?
A.absorption of signals by the D layer of the
ionosphere.
B.absorption of signals by the E layer of the
ionosphere.
C.random combining of signals arriving via
different path lengths
D.distortion in the local receiver
Wave propagation
21
What may occur if VHF or UHF data
signals propagate over multiple paths?
A. Transmission rates can be increased by
a factor equal to the number of paths
B. Transmission rates may decrease by a
factor equal to the number of paths
C. No significant changes will occur if the
signals are transmitting using FM
D. Error rates are likely to increase
Wave propagation
22
What does the term knife-edge propagation mean?
A. Signals are reflected back toward the originating
station at acute angles
B. Signals are sliced into several discrete beams and
arrive via different paths
C. Signals are partially refracted around solid objects
exhibiting sharp edges
D. Signals are propagated close to the band edge
exhibiting a sharp cutoff
Wave propagation
23
What is the radio horizon?
A. The distance at which radio signals
between two points are effectively
blocked by the curvature of the Earth
B. The distance from the ground to a
horizontally mounted antenna
C. The farthest point you can see when
standing at the base of your antenna
tower
D. The shortest distance between two
points on the Earth’s surface
Wave propagation
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Why do VHF and UHF radio signals usually
travel somewhat farther than the visual
line of sight distance between two
stations?
A. Radio signals move somewhat faster
than the speed of light
B. Radio waves are not blocked by dust
particles
C. The Earth seems less curved to radio
waves than to light
D. Radio waves are blocked by dust
particles
Wave propagation
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Wave propagation can be assisted
by atmospheric phenomena
Wave propagation
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Layers caused
by temperature
inversions form
ducts that can
carry signals
much farther
than typical line
of sight
communications
Wave propagation
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Layers with different properties refract
waves with slight variations.
This “tropospheric scattering often
allows contacts over 300 miles.
Wave propagation
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The ionosphere is an
area from 30 to 260
miles above the earth
that has some unique
electrical properties.
Wave propagation
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The ionosphere can completely reflect HF and VHF
waves back toward the earth. (sky wave or skip)
The earth’s
surface can
also reflect
radios
waves.
(hop)
Multiple reflections between the ionosphere/earth
allows radio waves to be received around the world!
Wave propagation
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The ability of the ionosphere to refract waves
depends on the frequency of the waves.
Wave propagation
31
High frequency waves usually pass right
through, this is
why VHF and
UHF signals are
rarely heard
beyond the
radio horizon
without being
relayed by a
repeater
Wave propagation
32
As the sun
illuminates the
ionosphere the
maximum usable
frequency
increases.
This is why upper
HF bands are more
likely to by usable
during the day.
Wave propagation
33
During solar cycles patches of the ionosphere
become sufficiently charged to reflect VHF and
UHF signals back to Earth.
This is called sporadic E
propagation.
Wave propagation
34
Sporadic E
Propagation is most
common during
early summer and
mid winter months
on the 10, 6 and 2
meter bands.
Wave propagation
35
Radio signals reflected off the aurora borealis
exhibit rapid fluctuations and are often distorted.
Wave propagation
36
It is possible to
bounce radio signals
of meteor trails.
The best band for
“meteor scatter” is
the 6 meter band.
Wave propagation
37
What mode is responsible for allowing overthe-horizon VHF and UHF communications
to ranges of approximately 300 miles on a
regular basis?
A. Tropospheric scatter
B. D layer refraction
C. F2 layer refraction
D. Faraday rotation
Wave propagation
38
What band is best suited to
communicating via meteor
scatter?
A. 10 meters
B. 6 meters
C. 2 meters
D. 70 cm
Wave propagation
39
What causes “tropospheric ducting”?
A. Discharges of lightning during electrical
storms
B. Sunspots and solar flares
C. Updrafts from hurricanes and tornadoes
D. Temperature inversions in the
atmosphere
Wave propagation
40
What is generally the best time for longdistance 10 meter band propagation?
A. During daylight hours
B. During nighttime hours
C. When there are coronal mass ejections
D. Whenever the solar flux is low
Wave propagation
41
Which part of the atmosphere
enables the propagation of
radio signals around the world?
A. The stratosphere
B. The troposphere
C. The ionosphere
D. The magnetosphere
Wave propagation
42
Why are “direct” (not via a repeater)
UHF signals rarely heard from stations
outside your local coverage area?
A. They are too weak to go very far
B. FCC regulations prohibit them from
going more than 50 miles
C. UHF signals are usually not
reflected by the ionosphere
D. They collide with trees and
shrubbery and fade out
Wave propagation
43
Which of the following might be
happening when VHF signals are being
received from long distances?
A. Signals are being reflected from
outer space
B. Signals are arriving by sub-surface
ducting
C. Signals are being reflected by
lightning storms in your area
D. Signals are being refracted from a
sporadic E layer
Wave propagation
44
What is a characteristic of VHF signals
received via auroral reflection?
A. Signals from distances of 10,000 or
more miles are common
B. The signals exhibit rapid fluctuations of
strength and often sound distorted
C. These types of signals occur only
during winter nighttime hours
D. These types of signals are generally
strongest when your antenna is aimed to
the south (for stations in the Northern
Hemisphere
Wave propagation
45
Which of the following propagation
types is most commonly associated
with occasional strong over-thehorizon signals on the 10, 6, and 2
meter bands?
A. Backscatter
B. Sporadic E
C. D layer absorption
D. Gray-line propagation
Wave propagation
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