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hello and welcome to this presentation
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understanding sporadic e in this short
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presentation we'll provide an overview
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of sporadic e and The Uncommon type of
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ionospheric propagation that primarily
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affects signals at VHF frequencies
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as you may already know the ionosphere a
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layer of charged particles surrounding
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the Earth consists of several layers
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one of these layers the F layer can
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refract or bend signals at HF
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frequencies back towards the Earth
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enabling very long distance
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Communications under the proper
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conditions
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this is often called skywave propagation
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however at VHF and higher frequencies
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signals reaching the ionosphere are
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generally not refracted back to Earth
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and simply pass through the ionosphere
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and into space
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below the F layer there's another layer
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of the ionosphere called the e-layer but
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under normal circumstances the e-layer
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is not capable of refracting either HF
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or VHF signals back towards the Earth
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however under special circumstances
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patches or regions of increased
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ionization in the e-layer can refract
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signals at lower VHF frequencies thus
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enabling limited skywave propagation
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over longer distances
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sporadic e sometimes also called e skip
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or ES refers to propagation by means of
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these highly ionized regions or clouds
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in the e-layer of the ionosphere
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this phenomenon takes place at altitudes
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of about 100 kilometers above the
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Earth's surface and can last for minutes
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or even up to an hour or more
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the signals propagated by sporadic e
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often have very low path loss and
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therefore lead to high received signal
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strengths at the receiver
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sporadic e can also propagate signals
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for long distances typically in the
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range of 700 to 2500 kilometers
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because of the lower altitude of the
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e-layer these distances are somewhat
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lower than the distances achievable with
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f layer propagation of HF signals
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this mode of propagation is called
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sporadic because it's difficult to
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predict but it's still common enough to
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cause problems
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for example European TV broadcasters and
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American FM radio broadcasters moved up
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from their original lower frequencies in
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part to avoid issues caused by sporadic
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e
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unlike F layer propagation which is
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usable across most of HF at different
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times of the day or different days of
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the year sporadic is only usable at
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higher HF frequencies and lower VHF
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frequencies
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it's most commonly seen around 50
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megahertz and on rare occasions it may
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extend above 150 megahertz
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another way of thinking about sporadic e
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is that it increases the maximum usable
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frequency or muff
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this is the highest frequency that can
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be used for Sky wave propagation and is
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usually no higher than 30 megahertz
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the dimensions of these clouds have
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increased e-layer ionization are
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difficult to measure they usually are
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tens of meters to a few hundred meters
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thick and from several hundred meters up
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to two kilometers wide
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it's important to remember that
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sporadici is a local or Regional
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phenomenon that is it only affects
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propagation over certain geographical
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regions we'll talk more about this on
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the next slide
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this is different from standard F layer
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propagation at HF frequencies which
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tends to be more uniform over
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hemisphere-sized regions
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we can map the location and dimensions
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of sporadic e clouds by looking at
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reception reports between Pairs of
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stations
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if we draw lines between transmitters
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and receivers the intersection of these
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reports will often show the rough size
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and Center position of the cloud
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sporadic e clouds are however not
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stationary and tend to move slowly over
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the course of their appearance
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in the northern hemisphere this motion
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is generally North to Northwest and is
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caused by both winds in the upper
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atmosphere as well as the Earth's
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rotation
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we've discussed barenaki in some detail
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but have yet to talk about what causes
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sporadic e
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there are in fact many different
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theories about the origins of sporadic e
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and as yet there is still no definitive
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universally accepted cause
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one of the more popular theories is that
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wind shear or other types of violent
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weather create thin layers of e-layer
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ionization
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experiments have shown that sporadic
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e-clouds contain high concentrations of
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metallic ions
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however the lack of a clear measurable
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cause makes sporadic e difficult to
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predict
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it's known that Peaks and sporadic
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activity occur between May and August
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and the Northern Hemisphere with some
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smaller peaks in December and January
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and while there are studies that suggest
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that spradiki may somehow be linked to
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meteor activity or solar activity there
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is still no conclusive correlation
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between these and the appearance of
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sporadic e
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before we end this presentation we need
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to briefly mention another way that VHF
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signals may be propagated over long
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distances
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sharp changes in the troposphere's
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refractive index can cause ducts that
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propagate VHF signals and this is
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therefore referred to as tropospheric
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ducting
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although they work in different ways
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both tropospheric ducting and sporadic e
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can produce strong signals over large
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Geographic areas so it's helpful to
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understand how to differentiate between
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them
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as we saw earlier sporadic e-clouds can
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connect many different locations whereas
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inducting signals usually only propagate
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between endpoints or sometimes along the
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path between them
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sporadic e tends to appear and disappear
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rather suddenly but tropospheric ducting
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normally builds up and Fades out more
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slowly and thus also tends to last
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longer than sporadic e-clouds
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let's end with a brief summary the
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ionosphere does not normally support
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skywave propagation at VHF frequencies
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in the same way that is commonly seen
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with lower frequency HF signals
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sporadic e refers to VHF Skyway
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propagation that's enabled by clouds of
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Highly ionized particles in the e-layer
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of the ionosphere
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generally speaking sporadic e is only
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seen at frequencies from about 50 to 150
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megahertz
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and achievable distances using sporadic
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e are on the order of 700 to 2500
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kilometers
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the size and location of sporadic e
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clouds can be approximately mapped by
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plotting lines connecting transmitters
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and receivers and then looking at their
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intersection
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the cause or causes of strategy are
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still not completely understood making
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sporadic e difficult to predict
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and finally both sporadic e and
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tropospheric ducting can propagate VHF
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signals over longer distances but these
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phenomena can be differentiated in a
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number of different ways
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this concludes our presentation
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understanding sporadic e
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if you'd like to learn more about other
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propagation modes or about rhodium short
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solutions for radio communications
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please see the links in the video
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description
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thanks for watching
