Radio waves propagation is a term used to explain how radio waves behave when they are transmitted, or are propagated from one point on the Earth to another.
Radio Waves Propagation
See also Solar Cycle
See also Multipath & Fading
Radio propagation is a term used to explain how radio waves behave when they are transmitted, or are propagated from one point on the Earth to another.
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 "r" (where "r" is the
distance [radius] from the source) or:
Doubling the distance 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 far-field magnitudes of the electric and magnetic field
components of electromagnetic radiation are equal, and their field
strengths are inversely proportional to distance. The power density per
surface unit is proportional to the product of the two field strengths,
which are expressed in linear units. Thus, doubling the propagation
path distance from the transmitter reduces their received field
strengths by one-half.
Electromagnetic wave propagation is also affected by several other
factors determined by its path from point to point. This path can be a
direct line of sight path or an over-the-horizon path aided by refraction in the ionosphere.
Lower frequencies (between 30 and 3,000 kHz) have the property of following the curvature of the earth via groundwave
propagation in the majority of occurrences. The interaction of radio
waves with the ionized regions of the atmosphere makes radio
propagation more complex to predict and analyze than in free space.
Ionospheric radio propagation has a strong connection to space weather.
Since radio propagation is somewhat unpredictable, such services as
emergency locator transmitters, in-flight communication with
ocean-crossing aircraft, and some television broadcasting have been moved to satellite
transmitters. A satellite link, though expensive, can offer highly
predictable and stable line of sight coverage of a given area.
A sudden ionospheric disturbance is often the result of large solar flares directed at Earth. These solar flares can disrupt HF radio propagation and affect GPS accuracy.
Radio waves at different frequencies propagate in different ways.
The beginning and end of a communication circuit is the antenna. The antenna can provide gain and directivity on both transmit and receive.
The take-off angle of the antenna is based on the type of antenna, the
height of the antenna above ground, and the terrain below and in front
of the antenna. The take-off angle will determine the angle of
incidence on the ionosphere, which will affect where the signal will be
refracted by the ionosphere.
Radio frequencies and their primary mode of propagation
||Very Low Frequency
||3 – 30 kHz
||100 – 10 km
||Guided between the earth and the ionosphere.
||30 – 300 kHz
||10 – 1 km
||Guided between the earth and the D layer of the ionosphere.
||300 – 3000 kHz
||1000 – 100 m
E, F layer ionospheric refraction at night, when D layer absorption weakens.
||High Frequency (Short Wave)
||3 – 30 MHz
||100 – 10 m
||E layer ionospheric refraction.
F1, F2 layer ionospheric refraction.
||Very High Frequency
||30 – 300 MHz
||10 – 1 m
||Infrequent E ionospheric refraction. Extremely rare F1,F2 layer ionospheric refraction during high sunspot activity up to 80 MHz. Generally direct wave. Sometimes tropospheric ducting.
||Ultra High Frequency
||300 – 3000 MHz
||100 – 10 cm
||Direct wave. Sometimes tropospheric ducting.
||Super High Frequency
||3 – 30 GHz
||10 – 1 cm
||Extremely High Frequency
||30 – 300 GHz
||10 – 1 mm
||Direct wave limited by absorption.
The above table listed as radio spectrum is incorrect in its
identification of frequency bands for ULF, ELF and AF (audio frequency)
ULF = 3 to 30 Hz, ELF = 30 to 300 Hz, Audio Frequency = 300 to 3000 Hz,
VLF = 3000 to 30000 Hz, LF = 30 kHz to 300 kHz, MF = 300 kHz to 3 MHz,
HF = 3 to 30 MHz, VLF = 30 to 300 MHz, UHF = 300 to 3000 MHz SHF = 3 to
30 GHz EHF = 30 to 300 HGz
The mode is commonly called the "Ground wave". This can cause confusion since the Direct mode is also sometimes called the "Ground wave".
In this mode the radio wave propagates by interacting with the
semi-conductive surface of the earth. The wave "clings" to the surface
and thus follows the curvature of the earth. Vertical polarization
is used to alleviate short circuiting the electric field through the
conductivity of the ground. Since the ground is not a perfect
electrical conductor, ground waves are attenuated rapidly as they follow the
earth’s surface. Attenuation is proportional to the frequency making
this mode mainly useful for LF and VLF frequencies.
Today LF and VLF are mostly used for time signals, and for military communications, especially with ships and submarines. Early commercial and professional radio services relied exclusively on long wave,
low frequencies and ground-wave propagation. To prevent interference
with these services, amateur and experimental transmitters were
restricted to the higher (HF) frequencies, felt to be useless since
their ground-wave range was limited. Upon discovery of the other
propagation modes possible at medium wave and short wave
frequencies, the advantages of HF for commercial and military purposes
became apparent. Amateur experimentation was then confined only to
authorized frequency segments in the range.
Direct Modes (Line-of-sight)
is the direct propagation of radio waves between antennas that are
visible to each other. This is probably the most common of the radio
propagation modes at VHF
and higher frequencies. Because radio signals can travel through many
non-metallic objects, radio can be picked up through walls. This is
still line-of-sight propagation. Examples would include propagation
between a satellite and a ground antenna or reception of television
signals from a local TV transmitter.
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 law for
ground-plane limited radiation.
Ionospheric Modes (Sky-Wave)
Skywave propagation, also referred to as skip, is any of the modes that rely on refraction of radio waves in the ionosphere, which is made up of one or more ionized layers in the upper atmosphere.
F2-layer is the most important ionospheric layer for HF propagation,
though F1, E, and D-layers also play some role. These layers are
directly affected by the sun on a daily cycle, the seasons and the
11-year sunspot cycle
determines the utility of these modes. During solar maxima, the whole
HF range up to 30 MHz can be used and F2 propagation up to 50 MHz are
observed frequently depending upon daily solar flux values. During solar minima, propagation of higher frequencies is generally worse.
Forecasting of skywave modes is of considerable interest to amateur radio operators and commercial marine and aircraft communications, and also to shortwave broadcasters.
Meteor scattering relies on reflecting radio waves off the intensely ionized columns of air generated by meteors. While this mode is very short duration, often only from a fraction of second to couple of seconds per event, digital Meteor burst communications
allows remote stations to communicate to a station that may be hundreds
of miles up to over 1,000 miles (1,600 km) away, without the
expense required for a satellite link. This mode is most generally
useful on VHF frequencies between 30 and 250 MHz.
Intense columns of Auroral
ionization at 100 km altitudes within the auroral oval reflect radio
waves, perhaps most notably on HF and VHF. The reflection is
angle-sensitive - incident ray vs. magnetic field line of the column
must be very close to right-angle. Random motions of electrons
spiraling around the field lines create a Doppler-spread that broadens
the spectra of the emission to more or less noise-like - depending on
how high radio frequency is used. The radio-aurora is observed mostly
at high latitudes and rarely extend down to middle latitudes. The
occurrences of radio-auroras depends on solar activity (flares, coronal holes, CMEs)
and annually the events are more numerous during solar cycle maximas.
Radio aurora includes the so-called afternoon radio aurora which
produces stronger but more distorted signals and after the
Harang-minima, the late-night radio aurora (sub-storming phase) returns
with variable signal strength and lesser doppler spread. The
propagation range for this predominantly back-scatter mode extends up
to about 2000 km in east-west plane, but strongest signals are observed
most frequently from north at nearby sites on same latitudes.
Rarely, a strong radio-aurora is followed by Auroral-E, which resembles both propagation types in some ways.
(Es) propagation can be observed on HF and VHF bands. It must not be
confused with ordinary HF E-layer propagation. Sporadic-E at
mid-latitudes occurs mostly during summer season, from May to August in
the northern hemisphere and from November to February in the southern
hemisphere. There is no single cause for this mysterious propagation
mode. The reflection takes place in a thin sheet of ionisation around
90 km height. The ionisation patches drift westwards at speeds of few
hundred km per hour. There is a weak periodicity noted during the
season and typically Es is observed on 1 to 3 successive days and
remains absent for a few days to reoccur again. Es do not occur during
small hours, the events usually begin at dawn, there is a peak in the
afternoon and a second peak in the evening. Es propagation is usually
gone by local midnight.
Maximum observed frequency (MOF) for Es is found to be lurking
around 30 MHz on most days during the summer season, but sometimes MOF
may shoot up to 100 MHz or even more in ten minutes to decline slowly
during the next few hours. The peak-phase includes oscillation of MOF
with periodicity of approximately 5...10 minutes, possibly related to gravity waves.
The propagation range for Es single-hop is typically 1000...2000 km,
but with multi-hop, double range is observed. The signals are very
strong but also with slow deep fading.
Thomas F. Giella, a noted retired Meteorologist, Space Plasma Physicist and Amateur Radio Operator, KN4LF cites the following from his professional research.
Just as the E layer is the main refraction medium for medium
frequency (300-3000 kc) signal propagation within approximately 5000
km/3100 mi, so is a Sporadic-E (Es) cloud. Sporadic-E (Es) clouds occur
at approximately 100 km/60 miles in altitude and generally move from
ESE to WNW. Like Stratosphere level warming and Troposphere
level temperature and moisture discontinuities, Sporadic-E (Es) clouds
can depending on the circumstances absorb, block or refract medium,
high and very high frequency RF signals in an unpredictable manner.
The main source for "high latitude" Sporadic E (Es) clouds is geomagnetic storming induced radio aurora activity.
The main source for "mid latitude" Sporadic-E (Es) clouds is wind shear
produced by internal buoyancy/gravity waves (IBGW's), that create
traveling ionosphere disturbances (TID's), most of which are produced
by severe thunderstorm
cell complexes with overshooting tops that penetrate into the
Stratosphere. Another tie in between Sporadic-E (Es) and a severe
thunderstorm is the Elve.
The main sources for "low latitude" Sporadic-E (Es) clouds is wind
shear produced by internal buoyancy/gravity waves (IBGW's), that create
traveling ionosphere disturbances, most of which are produced by severe
thunderstorm cell complexes tied to tropical cyclones. High electron
content in the Equatorial Ring Current also plays a role.
The forecasting of Sporadic-E (Es) clouds has long been considered
to be impossible. However it is possible to identify certain
troposphere level meteorological conditions that can lead to the
formation of Sporadic E (Es) clouds. One is as mentioned above the
severe thunderstorm cell complex.
Sporadic-E (Es) clouds have been observed to initially occur within
approximately 150 km/90 mi to the right of a severe thunderstorm cell
complex in the northern hemisphere, with the opposite being observed in
the southern hemisphere. To complicate matters is the fact that
Sporadic-E (Es) clouds that initially form to the right of a severe
thunderstorm complex in the northern hemisphere, then move from ESE-WNW
and end up to the left of the severe thunderstorm complex in the
northern hemisphere. So one has to look for Sporadic-E (Es) clouds on
either side of a severe thunderstorm cell complex. Things get even more
complicated when two severe thunderstorm cell complexes exist
approximately 1000- 2000 miles apart.
Not all thunderstorm cell complexes reach severe levels and not all
severe thunderstorm cell complexes produce Sporadic-E (Es). This is
where knowledge in tropospheric physics and weather
analyses/forecasting is necessary.
Some of the key elements in identifying which severe thunderstorm
cell complexes have the potential to produce Sporadic-E (Es) via wind
shear, from internal buoyancy/gravity waves, that produce traveling
ionosphere disturbances include:
1.) Negative tilted mid and upper level long wave troughs.
2.) Approximate 150 knot/170 mph jet stream
jet maxes that produce divergence and therefore create a sucking vacuum
effect above thunderstorm cells, that assist thunderstorm cells in
reaching and penetrating the Tropopause into the Stratosphere.
3.) 500 mb temperatures of -20 deg. C or colder, which produce numerous positive and negative lightning bolts and inter-related Sprites and Elves.
4.) Approximate 150-175 knot/172-200 mph updrafts within
thunderstorm cells complexes that create overshooting tops that
penetrate the Tropopause into the Stratosphere, launching upwardly propagating internal
buoyancy/gravity waves, which create traveling ionosphere disturbances
and then wind shear.
and higher frequencies, small variation (turbulence) in the density of
the atmosphere at a height of around 6 miles (10 km) can scatter
some of the normally line-of-sight beam of radio frequency energy back
toward the ground, allowing over-the-horizon communication between
stations as far as 500 miles (800 km) apart.
Tropospheric ducting and enhancement or refraction via inversion layer
Sudden changes in the atmosphere's vertical moisture content and temperature profiles can on random occasions make microwave and UHF & VHF
signals propagate hundreds of kilometers up to about 2,000 kilometers
(1,300 mi)—and for ducting mode even farther—beyond the normal
radio-horizon. The inversion layer
is mostly observed over high pressure regions, but there are several
tropospheric weather conditions which create these randomly occurring
propagation modes. Inversion layer's altitude for non-ducting is
typically found between 100 meters (300 ft) to about 1 kilometer
(3,000 ft) and for ducting about 500 meters (1,600 ft) to 3
kilometers (2 mi), and the duration of the events are typically from
several hours up to several days. Higher frequencies experience the
most dramatic increase of signal strengths, while on low-VHF and HF the
effect is negligible. Propagation path attenuation may be below
free-space loss. Some of the lesser inversion types related to warm
ground and cooler air moisture content occur regularly at certain times
of the year and time of day.
Rain scattering is purely a microwave propagation mode and is best observed around 10 GHz, but extends down to few GHz - the limit being the size of the scattering particle size vs. wavelength. This mode scatters signals mostly forwards and backwards when using horizontal polarization and side-scattering with vertical polarization.
Forward-scattering typically yields propagation ranges of 800 km.
Scattering from snowflakes and ice pellets also occurs, but scattering
from ice without watery surface is less effective. The most common
application for this phenomenon is microwave rain radar, but rain
scatter propagation can be a nuisance causing unwanted signals to
intermittently propagate where they are not anticipated or desired.
Similar reflections may also occur from insects though at lower
altitudes and shorter range. Rain also causes attenuation of
point-to-point and satellite microwave links. Attenuation values up to
30 dB have been observed on 30 GHz during heavy tropical rain.
Aeroplane scattering (or most often reflection) is observed on VHF
through microwaves and besides back-scattering, yields momentary
propagation up to 500 km even in a mountain-type terrain. The most
common back-scatter application is air-traffic radar and bistatic
forward-scatter guided-missile and aeroplane detecting trip-wire radar
and the US space radar.
Lightning scattering has sometimes been observed on VHF and UHF over
distance of about 500 km. The hot lightning channel scatters radiowaves
for a fraction of a second. The RF noise burst from the lightning makes
the initial part of the open channel unusable and the ionisation
disappears soon because of combination at low latitude high atmospheric
pressure. This mode has no practical use.
is the propagation mode where radio waves are bent around sharp edges.
For example, this mode is used to send radio signals over a mountain
range when a line-of-sight
path is not available. However, the angle cannot be too sharp or the
signal will not diffract. The diffraction mode requires increased
signal strength, so higher power or better antennas will be needed than
for an equivalent line-of-sight path.
Diffraction depends on the relationship between the wavelength and
the size of the obstacle. In other words, the size of the obstacle in
wavelengths. Lower frequencies diffract around large smooth obstacles
such as hills more easily. For example, in many cases where VHF (or
higher frequency) communication is not possible due to shadowing by a
hill, one finds that it is still possible to communicate using the
upper part of the HF band where the surface wave is of little use.
Diffraction phenomena by small obstacles are also important at high frequencies. Signals for urban cellular telephony
tend to be dominated by ground-plane effects as they travel over the
rooftops of the urban environment. They then diffract over roof edges
into the street, where multipath propagation, absorption and diffraction phenomena dominate.
Low-frequency radio waves travel easily through brick and stone and
VLF even penetrates sea-water. As the frequency rises, absorption
effects become more important. At microwave or higher frequencies, absorption by molecular resonance in the atmosphere (mostly water, H2O and oxygen, O2)
is a major factor in radio propagation. For example, in the 58 - 60 GHz
band, there is a major absorption peak which makes this band useless
for long-distance use. This phenomenon was first discovered during radar research during World War II.
Beyond around 400 GHz, the Earth's atmosphere blocks some segments of
spectra while still passes some - this is true up to UV light, which is
blocked by ozone, but visible light and some of the NIR is transmitted.
Heavy rain and snow also affect microwave reception.
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)