Radio propagation
is the behavior of
radio waves
as they travel, or are
propagated
, from one point to another in
vacuum
, or into various parts of the
atmosphere
.
[1]
:?26?1?
As a form of
electromagnetic radiation
, like light waves, radio waves are affected by the phenomena of
reflection
,
refraction
,
diffraction
,
absorption
,
polarization
, and
scattering
.
[2]
Understanding the effects of varying conditions on radio propagation has many practical applications, from choosing frequencies for
amateur radio
communications, international
shortwave
broadcasters
, to designing reliable
mobile telephone
systems, to
radio navigation
, to operation of
radar
systems.
Several different types of propagation are used in practical radio transmission systems.
Line-of-sight propagation
means radio waves which travel in a straight line from the transmitting antenna to the receiving antenna. Line of sight transmission is used for medium-distance radio transmission, such as
cell phones
,
cordless phones
,
walkie-talkies
,
wireless networks
,
FM radio
,
television broadcasting
,
radar
, and
satellite communication
(such as
satellite television
). Line-of-sight transmission on the surface of the Earth is limited to the distance to the visual horizon, which depends on the height of transmitting and receiving antennas. It is the only propagation method possible at
microwave
frequencies and above.
[a]
At lower frequencies in the
MF
,
LF
, and
VLF
bands,
diffraction
allows radio waves to bend over hills and other obstacles, and travel beyond the horizon, following the contour of the Earth. These are called
surface waves
or
ground wave
propagation
.
AM broadcast
and amateur radio stations use ground waves to cover their listening areas. As the frequency gets lower, the
attenuation
with distance decreases, so
very low frequency
(VLF) to
extremely low frequency
(ELF) ground waves can be used to communicate worldwide. VLF to ELF waves can penetrate significant distances through water and earth, and these frequencies are used for mine communication and military
communication with submerged submarines
.
At
medium wave
and
shortwave
frequencies (
MF
and
HF
bands), radio waves can refract from the
ionosphere
, a layer of
charged particles
(
ions
) high in the atmosphere. This means that medium and short radio waves transmitted at an angle into the sky can be refracted back to Earth at great distances beyond the horizon ? even transcontinental distances. This is called
skywave
propagation
. It is used by
amateur radio
operators to communicate with operators in distant countries, and by
shortwave broadcast stations
to transmit internationally.
[b]
In addition, there are several less common radio propagation mechanisms, such as
tropospheric scattering
(troposcatter),
tropospheric ducting
(ducting) at VHF frequencies and
near vertical incidence skywave
(NVIS) which are used when HF communications are desired within a few hundred miles.
Frequency dependence
edit
At different frequencies, radio waves travel through the atmosphere by different mechanisms or modes:
[3]
Radio frequencies and their primary mode of propagation
Band
|
Frequency
|
Wavelength
|
Propagation via
|
ELF
|
Extremely Low Frequency
|
3?30
Hz
|
100,000?10,000?km
|
Guided between the Earth and the
D layer
of the ionosphere.
|
SLF
|
Super Low Frequency
|
30?300
Hz
|
10,000?1,000?km
|
Guided between the Earth and the
ionosphere
.
|
ULF
|
Ultra Low Frequency
|
0.3?3?
kHz
(300?3,000?Hz)
|
1,000?100?km
|
Guided between the Earth and the
ionosphere
.
|
VLF
|
Very Low Frequency
|
3?30?
kHz
(3,000?30,000?Hz)
|
100?10?km
|
Guided between the Earth and the
ionosphere
.
Ground waves
.
|
LF
|
Low Frequency
|
30?300?
kHz
(30,000?300,000?Hz)
|
10?1?km
|
Guided between the Earth and the ionosphere.
Ground waves
.
|
MF
|
Medium Frequency
|
300?3,000?
kHz
(300,000?3,000,000?Hz)
|
1000?100?m
|
Ground waves
.
E,
F layer
ionospheric refraction at night, when D layer absorption weakens.
|
HF
|
High Frequency (
Short Wave
)
|
3?30?
MHz
(3,000,000?30,000,000?Hz)
|
100?10?m
|
E layer
ionospheric refraction.
F1,
F2
layer ionospheric refraction.
|
VHF
|
Very High Frequency
|
30?300?
MHz
(30,000,000?
? ? 300,000,000?Hz)
|
10?1 m
|
Line-of-sight propagation
.
Infrequent
E ionospheric (E
s
) refraction
. Uncommonly
F2
layer ionospheric refraction during high sunspot activity up to 50?MHz and rarely to 80?MHz. Sometimes
tropospheric ducting
or
meteor scatter
|
UHF
|
Ultra High Frequency
|
300?3,000?
MHz
(300,000,000?
? ? 3,000,000,000?Hz)
|
100?10?cm
|
Line-of-sight propagation
. Sometimes
tropospheric ducting
.
|
SHF
|
Super High Frequency
|
3?30
GHz
(3,000,000,000?
? ? 30,000,000,000?Hz)
|
10?1?cm
|
Line-of-sight propagation
. Sometimes
rain scatter
.
|
EHF
|
Extremely High Frequency
|
30?300?
GHz
(30,000,000,000?
? ? 300,000,000,000?Hz)
|
10?1?mm
|
Line-of-sight propagation
, limited by atmospheric absorption to a few kilometers (miles)
|
THF
|
Tremendously High frequency
|
0.3?3?
THz
(300,000,000,000?
? ? 3,000,000,000,000?Hz)
|
1?0.1?mm
|
Line-of-sight propagation
, limited by atmospheric absorption to a few meters.
[4]
[5]
|
FIR
|
Far infrared light
(overlaps radio)
|
0.3?20?
THz
(300,000,000,000?
? ? 20,000,000,000,000?Hz)
|
1,000?150?μm
[6]
[7]
[8]
|
Line-of-sight propagation
, mostly limited by atmospheric absorption to a few meters.
[6]
[8]
|
Free space propagation
edit
In
free space
, all
electromagnetic waves
(radio, light, X-rays, etc.) obey the
inverse-square law
which states that the power density
of an electromagnetic wave is proportional to the inverse of the square of the distance
from a
point source
[1]
:?26?19?
or:
-
At typical communication distances from a transmitter, the transmitting antenna usually can be approximated by a point source. Doubling the distance of a receiver from a transmitter means that the power density of the radiated wave at that new location is reduced to one-quarter of its previous value.
The power density per surface unit is proportional to the product of the electric and magnetic field strengths. Thus, doubling the propagation path distance from the transmitter reduces each of these received field strengths over a free-space path by one-half.
Radio waves in vacuum travel at the
speed of light
. The Earth's atmosphere is thin enough that radio waves in the atmosphere travel very close to the speed of light, but variations in density and temperature can cause some slight
refraction
(bending) of waves over distances.
Direct modes (line-of-sight)
edit
Line-of-sight
refers to radio waves which travel directly in a line from the transmitting antenna to the receiving antenna, often also called direct-wave. It does not necessarily require a cleared sight path; at lower frequencies radio waves can pass through buildings, foliage and other obstructions. This is the most common propagation mode at
VHF
and above, and the only possible mode at
microwave
frequencies and above. On the surface of the Earth, line of sight propagation is limited by the
visual horizon
to about 40 miles (64?km). This is the method used by
cell phones
,
[c]
cordless phones
,
walkie-talkies
,
wireless networks
, point-to-point
microwave radio relay
links,
FM
and
television broadcasting
and
radar
.
Satellite communication
uses longer line-of-sight paths; for example home
satellite dishes
receive signals from communication satellites 22,000 miles (35,000?km) above the Earth, and
ground stations
can communicate with
spacecraft
billions of miles from Earth.
Ground plane
reflection
effects are an important factor in VHF line-of-sight propagation. The interference between the direct beam line-of-sight and the ground reflected beam often leads to an effective inverse-fourth-power
(
1
?
distance
4
)
law for ground-plane limited radiation.
[
citation needed
]
Surface modes (groundwave)
edit
Ground wave propagation
Lower frequency (between 30 and 3,000?kHz)
vertically polarized
radio waves can travel as
surface waves
following the contour of the Earth; this is called
ground wave
propagation.
In this mode the radio wave propagates by interacting with the conductive surface of the Earth. The wave "clings" to the surface and thus follows the curvature of the Earth, so ground waves can travel over mountains and beyond the horizon. Ground waves propagate in
vertical polarization
so vertical antennas (
monopoles
) are required. Since the ground is not a perfect electrical conductor, ground waves are
attenuated
as they follow the Earth's surface. Attenuation is proportional to frequency, so ground waves are the main mode of propagation at lower frequencies, in the
MF
,
LF
and
VLF
bands. Ground waves are used by
radio broadcasting
stations in the MF and LF bands, and for
time signals
and
radio navigation
systems.
At even lower frequencies, in the
VLF
to
ELF
bands, an
Earth-ionosphere waveguide
mechanism allows even longer range transmission. These frequencies are used for secure
military communications
. They can also penetrate to a significant depth into seawater, and so are used for one-way military communication to submerged submarines.
Early long-distance radio communication (
wireless telegraphy
) before the mid-1920s used low frequencies in the
longwave
bands and relied exclusively on ground-wave propagation. Frequencies above 3?MHz were regarded as useless and were given to hobbyists (
radio amateurs
). The discovery around 1920 of the ionospheric reflection or
skywave
mechanism made the
medium wave
and
short wave
frequencies useful for long-distance communication and they were allocated to commercial and military users.
[9]
Non-line-of-sight modes
edit
Non-line-of-sight
(NLOS) radio propagation occurs outside of the typical
line-of-sight
(LOS) between the transmitter and receiver, such as in
ground reflections
.
Near-line-of-sight (also NLOS) conditions refer to partial obstruction by a physical object present in the innermost
Fresnel zone
.
Obstacles that commonly cause NLOS propagation include buildings, trees, hills, mountains, and, in some cases, high voltage
electric power
lines. Some of these obstructions reflect certain radio frequencies, while some simply absorb or garble the signals; but, in either case, they limit the use of many types of radio transmissions, especially when low on power budget.
Lower power levels at a receiver reduce the chance of successfully receiving a transmission. Low levels can be caused by at least three basic reasons: low transmit level, for example
Wi-Fi
power levels; far-away transmitter, such as
3G
more than 5 miles (8.0?km) away or
TV
more than 31 miles (50?km) away; and obstruction between the transmitter and the receiver, leaving no clear path.
NLOS lowers the effective received power. Near Line Of Sight can usually be dealt with using better antennas, but Non Line Of Sight usually requires alternative paths or multipath propagation methods.
How to achieve effective NLOS networking has become one of the major questions of modern computer networking. Currently, the most common method for dealing with NLOS conditions on wireless computer networks is simply to circumvent the NLOS condition and place
relays
at additional locations, sending the content of the radio transmission around the obstructions. Some more advanced NLOS transmission schemes now use
multipath
signal propagation, bouncing the radio signal off other nearby objects to get to the receiver.
Non-Line-of-Sight (NLOS) is a term often used in
radio communications
to describe a radio channel or link where there is no
visual
line of sight
(LOS) between the
transmitting
antenna
and the
receiving antenna
. In this context LOS is taken
- Either as a straight line free of any form of visual obstruction, even if it is actually too distant to see with the unaided
human eye
- As a virtual LOS i.e., as a straight line through visually obstructing material, thus leaving sufficient transmission for radio waves to be detected
There are many electrical characteristics of the transmission media that affect the radio
wave propagation
and therefore the quality of operation of a radio channel, if it is possible at all, over an NLOS path.
The acronym NLOS has become more popular in the context of
wireless local area networks
(WLANs) and wireless metropolitan area networks such as
WiMAX
because the capability of such links to provide a reasonable level of NLOS coverage greatly improves their marketability and versatility in the typical
urban
environments where they are most frequently used. However NLOS contains many other subsets of radio communications.
The influence of a visual obstruction on a NLOS link may be anything from negligible to complete suppression. An example might apply to a LOS path between a television broadcast antenna and a roof mounted receiving antenna. If a cloud passed between the antennas the link could actually become NLOS but the quality of the radio channel could be virtually unaffected. If, instead, a large building was constructed in the path making it NLOS, the channel may be impossible to receive.
Beyond line-of-sight (BLOS) is a related term often used in the military to describe radio communications capabilities that link personnel or systems too distant or too fully obscured by terrain for LOS communications. These radios utilize active
repeaters
,
groundwave propagation
,
tropospheric scatter links
, and
ionospheric propagation
to extend communication ranges from a few kilometers to a few thousand kilometers.
Measuring HF propagation
edit
HF propagation conditions can be simulated using
radio propagation models
, such as
the Voice of America Coverage Analysis Program
, and realtime measurements can be done using
chirp transmitters
. For radio amateurs the
WSPR mode
provides maps with real time propagation conditions between a network of transmitters and receivers.
[10]
Even without special beacons the realtime propagation conditions can be measured: A worldwide network of receivers decodes morse code signals on amateur radio frequencies in realtime and provides sophisticated search functions and propagation maps for every station received.
[11]
The average person can notice the effects of changes in radio propagation in several ways.
In
AM broadcasting
, the dramatic ionospheric changes that occur overnight in the mediumwave band drive a unique
broadcast license
scheme in the United States, with entirely different
transmitter power output
levels and
directional antenna
patterns to cope with skywave propagation at night. Very few stations are allowed to run without modifications during dark hours, typically only those on
clear channels
in
North America
.
[12]
Many stations have no authorization to run at all outside of daylight hours.
For
FM broadcasting
(and the few remaining low-band
TV stations
), weather is the primary cause for changes in VHF propagation, along with some diurnal changes when the sky is mostly without
cloud cover
.
[13]
These changes are most obvious during temperature inversions, such as in the late-night and early-morning hours when it is clear, allowing the ground and the air near it to cool more rapidly. This not only causes
dew
,
frost
, or
fog
, but also causes a slight "drag" on the bottom of the radio waves, bending the signals down such that they can follow the Earth's curvature over the normal radio horizon. The result is typically several stations being heard from another
media market
? usually a neighboring one, but sometimes ones from a few hundred kilometers (miles) away.
Ice storms
are also the result of inversions, but these normally cause more scattered omnidirection propagation, resulting mainly in interference, often among
weather radio
stations. In late spring and early summer, a combination of other atmospheric factors can occasionally cause skips that duct high-power signals to places well over 1000?km (600 miles) away.
Non-broadcast signals are also affected.
Mobile phone signals
are in the UHF band, ranging from 700 to over 2600?MHz, a range which makes them even more prone to weather-induced propagation changes. In
urban
(and to some extent
suburban
) areas with a high
population density
, this is partly offset by the use of smaller cells, which use lower
effective radiated power
and
beam tilt
to reduce interference, and therefore increase
frequency reuse
and user capacity. However, since this would not be very cost-effective in more
rural
areas, these cells are larger and so more likely to cause interference over longer distances when propagation conditions allow.
While this is generally transparent to the user thanks to the way that
cellular networks
handle cell-to-cell
handoffs
, when
cross-border
signals are involved, unexpected charges for international
roaming
may occur despite not having left the country at all. This often occurs between southern
San Diego
and northern
Tijuana
at the western end of the
U.S./Mexico border
, and between eastern
Detroit
and western
Windsor
along the
U.S./Canada border
. Since signals can travel unobstructed over a
body of water
far larger than the
Detroit River
, and cool water temperatures also cause inversions in surface air, this "fringe roaming" sometimes occurs across the
Great Lakes
, and between islands in the
Caribbean
. Signals can skip from the
Dominican Republic
to a mountainside in
Puerto Rico
and vice versa, or between the U.S. and British
Virgin Islands
, among others. While unintended cross-border roaming is often automatically removed by
mobile phone company
billing systems, inter-island roaming is typically not.
A
radio propagation model
, also known as the
radio wave propagation model
or the
radio frequency propagation model
, is an
empirical
mathematical
formulation
for the characterization of
radio wave
propagation as a
function
of
frequency
,
distance
and other conditions. A single model is usually developed to predict the behavior of propagation for all similar links under similar constraints. Created with the goal of formalizing the way radio waves are propagated from one place to another, such models typically predict the
path loss
along a link or the effective coverage area of a
transmitter
.
The inventor of radio communication,
Guglielmo Marconi
, before 1900 formulated the first crude empirical rule of radio propagation: the maximum transmission distance varied as the square of the height of the antenna.
As the path loss encountered along any radio link serves as the dominant factor for characterization of propagation for the link, radio propagation models typically focus on realization of the path loss with the auxiliary task of predicting the area of coverage for a transmitter or modeling the distribution of signals over different regions.
Because each individual telecommunication link has to encounter different terrain, path, obstructions, atmospheric conditions and other phenomena, it is intractable to formulate the exact loss for all telecommunication systems in a single mathematical equation. As a result, different models exist for different types of radio links under different conditions. The models rely on
computing the median path loss
for a link under a certain probability that the considered conditions will occur.
Radio propagation models are empirical in nature, which means, they are developed based on large collections of data collected for the specific scenario. For any model, the collection of data has to be sufficiently large to provide enough likeliness (or enough scope) to all kind of situations that can happen in that specific scenario. Like all empirical models, radio propagation models do not point out the exact behavior of a link, rather, they predict the most likely behavior the link may exhibit under the specified conditions.
Different models have been developed to meet the needs of realizing the propagation behavior in different conditions. Types of models for radio propagation include:
- Models for free space attenuation
- Models for outdoor attenuation
- Terrain models
- City models
- Models for indoor attenuation
- ^
At microwave frequencies, moisture in the atmosphere (
rain fade
) can degrade transmission.
- ^
Skywave communication is variable: It depends on conditions in the
ionosphere
. Long distance shortwave transmission is most reliable at night and during the winter. Since the advent of
communication satellites
in the 1960s, many long range communication needs that previously used skywaves now use satellites and
submerged cables
, to avoid dependence on the erratic performance of skywave communications.
- ^
Cellular networks function even without a single clear line-of-sight by relaying signals along multiple line-of-sight paths through cell towers.
- ^
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b
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- ^
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{{
cite magazine
}}
: CS1 maint: multiple names: authors list (
link
) CS1 maint: numeric names: authors list (
link
)
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