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[Music]
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the view from the edge of the exosphere
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480 kilometers above the surface of our
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planet
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is awe-inspiring
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380 kilometers below lies the e
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layer a 20 kilometer wide radio
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reflective ionospheric layer
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between 100 and 120 kilometers
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above the surface of our planet
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the ionization of this layer is
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fascinating
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as well as challenging for amateur radio
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operators around the world
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this is the layer where ionized e-clouds
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live
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[Music]
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e-clouds reflect vhf radio signals that
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would otherwise escape the earth's
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atmosphere
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back to earth
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our atmosphere has five layers
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the lowest layer is the troposphere
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which is where aircraft fly
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and where nearly all of our weather
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occurs
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the stratosphere is next with the
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mesosphere
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and the thermosphere above that
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sitting above all these layers is the
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exosphere
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a place for spacecraft and satellites
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[Music]
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earth's atmosphere contains a series of
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regions that have a relatively large
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number
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of electrically charged atoms and
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molecules
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as a group these regions are
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collectively called
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the ionosphere
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there are three main regions of the
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ionosphere
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the d layer around 75 kilometers up
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the e layer 110 kilometers up
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and the f layer that starts at about 150
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kilometers
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and extends upwards sometimes as high as
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500 kilometers
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[Music]
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these regions do not have sharp
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boundaries and the altitudes at which
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they occur
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vary during the course of the day and
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from season to season
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[Music]
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amateur radio operators take a keen
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interest in the tropospheric region of
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the atmosphere
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as well as the e-layer as these two
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regions can propagate vhf radio signals
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well beyond that of line of sight
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[Music]
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the troposphere is the lowest part of
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our atmosphere
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and what takes place within its
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boundaries largely determines our
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weather
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[Music]
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its upper boundary is the temperature
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inversion layer
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known as the tropopause which lies
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between 8 kilometers
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and 16 kilometers high depending on the
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latitude
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and prevailing conditions
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the low level of ionization is
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negligible as far as radio waves are
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concerned
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however small variations in the physical
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properties of the air
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give rise to variations in the
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refractive index
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which results in several distinct
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tropospheric propagation modes
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more about this in another video
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the d region lies between 60 and 90
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kilometers above the earth's surface
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it absorbs radio waves particularly at
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hf
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because the electrons are frequently
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colliding and dissipating the energy of
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the waves
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instead of propagating them
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[Music]
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although the d region is almost
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transparent at vhf and uhf
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it does contribute to ionospheric
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forward scatter
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especially when intense solar activity
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produces an excess of ultraviolet
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and high energy x-rays which can
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penetrate
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to this depth of the atmosphere
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the e region is defined as that between
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90
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and 120 kilometers above the earth's
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surface
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and it can be regarded as the transition
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zone from the earth's atmosphere
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into space
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there are discontinuities in pressure
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temperature
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and chemical composition at this height
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above 90 kilometers ions predominate
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over neutral atoms
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most of the shortwave ultraviolet rays
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and
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less energetic x-rays from the sun are
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absorbed by ionizing the e-layer
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although the ionization is normally
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insufficient to reflect vhf radio waves
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back to earth
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some thin and dense layers of ionization
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occasionally occur
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known as sporadic e
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this is also the region where auroras
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and meteors produce ionization
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capable of reflecting vhf and uhf
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signals
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above the e region lies the f region
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which in daytime usually subdivides into
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two distinct layers
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f1 and f2
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the lower f1 layer plays little part in
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vhf propagation
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except to contribute some refraction if
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vhf radio waves are returned back
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towards the earth
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it's from the f2 layer heights
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the f2 layer has an average effective
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height
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of between 300 and 350 kilometers
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which can increase to between 450 and
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kilometers the layer varies between 100
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and 200 kilometers deep during the peak
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of solar activity
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the f2 layer can provide worldwide
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contacts on the six meter band
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however the f2 layer does not reach the
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levels of ionization required to return
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higher frequency
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vhf and uhf signals to the earth except
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in the special case
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of trans equatorial propagation
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returning to the e-layer sporadic e is
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probably the most interesting and
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exciting form of signal enhancement for
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the amateur radio operator
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highly ionized patches or clouds
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occasionally form in the e region of the
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ionosphere
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known as sporadic e clouds they're
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usually fairly small in size
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but larger clouds or multiple clouds
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often form during substantial openings
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as the name indicates sporadically is
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not easy to predict
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after almost 80 years of study the true
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cause of sporadica
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is still unknown there are many
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different theories as to
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how and why sporadic e-clouds form
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there seems to be no correlation between
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the ionization level
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or formation of sporadic e-clouds and
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the 11-year sunspot cycle
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but there is evidence to suggest that
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the primary cause of spreader key cloud
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formation
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is wind shear a purely weather related
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phenomenon
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intense high altitude winds traveling in
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opposite directions at different
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altitudes
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produce wind shear it's believed that
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these wind shears
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in the presence of the earth's
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geomagnetic field
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cause ions to be collected and
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compressed into thin
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iron-rich layers approximately one
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to one and a half kilometers in
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thickness
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the area of these patches can vary from
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a few meters
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to hundreds or even thousands of square
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kilometers
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along the same line is the theory that
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sporadic e-clouds are formed in the
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vicinity of thunderstorms
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by the intense electrical activity
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associated with them
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there is often but not always a
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correlation between thunderstorm
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activity
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and the formation of sporadic e clouds
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enough to make the theory
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very feasible however strong
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thunderstorms often form along frontal
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boundaries an intense wind shear is
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usually found along the same frontal
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boundaries that produce these
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thunderstorms
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likewise strong sporadic e activity
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often appears
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when there is no apparent thunderstorm
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activity along or near the propagation
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path
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yet another emerging theory suggests
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that sporadic e-clouds are formed by
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concentrations of meteor debris
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again there seems to be a strong
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correlation between meteor shower
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activity
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and the number and intensity of sporadic
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e-clouds
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it's all speculation though as nobody
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has presented a definitive explanation
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for how and why sporadic e-clouds form
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[Music]
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the amount by which the path of a radio
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signal is refracted by sporadic e-clouds
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depends on the intensity of ionization
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and the frequency
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of the signal for a given level of
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ionization
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the signal refraction angle will
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decrease as the frequency is increased
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above a certain critical frequency
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refraction of the signal will be
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insufficient to return it to the surface
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of the earth
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this critical frequency is known as the
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maximum usable frequency
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or muf
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sporadic e is very common on the low vhf
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frequencies during the summer months
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it's not uncommon for the muf to rise up
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to
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and stop at a particular frequency
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within the fm broadcast band
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distant signals will be heard below the
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muf while only local
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or tropospherically enhanced signals
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will be heard above the muf
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over the years it's been observed that
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the signal strength of the received
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spreader key signals
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will be greatest just below the maximum
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usable frequency
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since the bending angle or angle of
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refraction decreases as the signal
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frequency is increased for a given
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ionization level
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we can surmise that the most distant
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receptions will occur
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as we approach the maximum usable
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frequency
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in other words an e-cloud will support
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longer signal paths at 100 megahertz
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than it will
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at 50 megahertz
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in this example ecloud 1 is more
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intensely ionized
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and thus capable of refracting signals
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at a sharper angle
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producing a shorter skip distance for a
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given frequency
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signals being refracted by ecloud number
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two are returned to earth at a lesser
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angle
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thus producing longer skip distances
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with clouds two and three in alignment
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along a signal path
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double hop skip can occur
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with this cloud alignment signals from
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both the transmitter and the signal hop
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zone
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would be heard at the receiving station
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the maximum distance for a single hop
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sporadically propagated signal
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is about two and a half thousand
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kilometers
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however if multiple sufficiently ionized
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patches exist
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in a line along a particular signal path
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this can extend the range of e-layer
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propagation signals
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out to around 5000 kilometers and beyond
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[Music]
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if the midpoint of a double hop happens
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to be on the water
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such as the ocean the signals will be
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stronger
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and there's less likely to be
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interference from midpoint stations
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sometimes we hear sporadic e-stations
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that don't seem to fit the normal model
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in terms of path distance
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it's not uncommon to receive signals
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beyond the range of what would be
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considered
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normal for a single hog but less than
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the range expected for a normal double
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hop
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there have been many theories to try and
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explain this phenomenon
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including paths along multiple sporadic
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e-clouds
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in this example neither e-cloud is
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sufficiently ionized to return a single
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hop to earth
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with two weak clouds working together
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the refraction
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angles of both es clouds are essentially
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added
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this would have the effect of raising
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the apparent maximum usable frequency
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and ultimately returning the signal to
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earth at a greater than normal
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e distance
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this would account for the variable path
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distances which fall between that of
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single and double hop sporadic e
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path distances although sporadic e may
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appear to give an improvement in some hf
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communications
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it can also have the effect of degrading
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some hf
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signals however the intermittent nature
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of the sporadically clouds
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and the fact that the clouds are very
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mobile means that any effects
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are likely to be relatively short-lived
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[Music]
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you
