Sporadic E skip
(Es)
Es has a set of characteristics which,
when taken all together, set it apart from all other forms of DX propagation.
It builds up from low frequencies to a certain maximum usable frequency (MUF)
which may vary widely from minute to minute, and opening to opening. Es always
hits the lower frequencies first. It may or may not get above channel 2. Of
course, if the skip is coming from a sparsely populated area, there may be no
channel 2 transmitter --- so check channels 3 and 4 as well. A good opening
will not stop at channel 6, but may continue upward into the FM band which
begins immediately above channel 6. An extraordinary opening may even continue
into the aeronautical band above 108 MHz, through the "2 meter" (144
MHz) ham band, past the heavily-populated 2-way mobile bands, even up to
channel 7 --- 180 MHz! An Es opening reaching channel 7 is a rare treat;
high-band (TV channels 7-13) Es may even poke a channel or two above 7.
Usually, highband Es can be ruled out when the skip is not extending above
channel 6 into the FM band.
In general, as Es distances shorten on
the low band and FM, the opening is becoming more intense and the MUF goes up.
As experienced Dxers can attest, Es at 1000 miles is much more common than Es
at 600 miles. The usual minimum distance for Es is about 500 miles. In fact, a
study we made of hundreds of Es receptions revealed that 950 miles is the
optimum distance on channel 2, with distances lengthening slightly with each
higher channel, up into the FM band.
Es occurs when patches in the E layer of
the ionosphere, about 65 miles above ground, become ionized. This layer
normally refracts shortwave and mediumwave signals but is transparent to VHF
radiation. The cause of Es ionization is not precisely known; some researchers
connect it with low pressure areas and thunderstorms. However, it has no
direct connection with surface weather, and the novice's association of Es
with regular clouds in the sky is completely erroneous.
For once thing, the Es patch must be at
approximately the midpoint between the transmitter and receiver, far beyond
the visible horizon. Sometimes the patches remain fairly stationary, but
usually they move at speeds up to several hundred miles per hour, more or less
in a straight line. This means that one station after another will come in,
with quite a lot of interference as they overlap.
They will probably be in a rough
geographical progression, but not in a straight line. One can plot the
midpoints on a map, and by correlating observations with other DXers viewing
at the same time from other angles, pin down the Es patch with a degree of
accuracy. This can prove useful in determining probable target areas (PTAs).
Long single hops of Es can reach about
1500 miles. Double-hop or cloud-to-cloud hop Es often occurs during the summer
when more than one Es patch may be active simultaneously, in different parts
of the continent. The two patches, the station, and you must all be along the
same line. However, not too many stations are identified by double-hop Es for
several reasons:
(1) Interference: the TV and FM bands
are so congested in North America that there are usually stations on the air
near the double-hop path midpoint, severely interfering with further stations.
(2) The earth is a rather poor reflector
of VHF signals, but this it must do at the midpoint. Double-hop Es where the
midpoint is water (an ocean or Great Lake) is much more efficient.
(3) The patch with the lower MUF is the
controlling factor. For this reason, there's much more double-hop Es on
channel 2 than on channel 6 or FM.
Es is very unpredictable, but we do know
this much! Es is very much a summertime phenomenon in the temperate latitudes,
with peaks in June and July; very good openings also in May and August; and a
sprinkling in late April and early September. It can occur on any day of the
year; these are known as off-peak openings. The winter solstice also brings a
minor peak in December and January, as if some of the Es' fury were
"bleeding over" from the southern hemisphere where, of course, the
summer peak is in progress. The winter and off-season openings are most likely
in the early evening hours. During the main "season", Es may start
early in the morning and continue all day, into the night, but it likes to
take a breather around mid-morning and mid-afternoon, and seldom lasts much
past midnight. If you don't want to miss an Es opening, try checking once or
twice an hour just before ID time, or tune a VHF radio paging channel.
Es can be very strong with lots of
fading and interference. But strong signals may rival those of local stations
and even interfere with them. Es may build up rapidly, over the course of a
few minutes, but usually it decays more slowly. Weak openings in which the MUF
hovers around TV channel 2 may tantalize you as stations fade in and out.
Es is more likely in southern areas
during the off-season, but northerners should not assume that subzero
temperatures or snowstorms rule out any DX! Closer to the equator, Es becomes
more and more a year-round, daily phenomenon. "Diurnal Es" may
provide a weak, scattery signal virtually every day over an Es-distance path.
Other strange things happen, such as Es reception at double-hop distances but
with the signal bouncing from one path to another without touching ground in
between.
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