.




Sporadic E skip

Irregular  scattered  patches of relatively  dense ionization that develop seasonally within the E region (pic 01) and that reflect and scatter frequencies up to 150 MHz.

Note 1: The sporadic E is a regular daytime occurrence over the equatorial  regions  and is common  in the temperate latitudes in late spring, early summer and, to a lesser degree, in early winter.
Note 2: At high, i.e., polar, latitudes, sporadic E can accompany auroras and associated disturbed magnetic conditions.
Note 3: The  sporadic E  can sometimes  support  reflections  for distances up to 2,400 km at frequencies up to 150 MHz.



 

In the E region, that is medium on the 100 km of quota, they are formed every so often, species in  the  months that go from May to  October, of  the Ionian  heaps of and electrons, in which NOT +prevail,  Or+ and nuclei of Fe and Mg that raise a lot the density of onizzazione in  limited  areas.  From the point of  view  of  the propagation, this  complex  phenomenon of  the high atmosphere produces  an  elevation  of  the  maximum  frequency  that  can  endure reflection. The increase is always considerable: while the E layer normal school, to the summery midday, in the moments of greater ionization Mhz can have a m.u.f. for 15/20 shaving beam of,  the  Es  is of  rule in  a  position  to  sending  back to  Earth  beacons  to the frequencies of  30/80MHz.  In particular  cases,  whose percentage  decreases to  growing of the  frequency, the density  is such to  interest  the  beacons of  80,  100  and  also 200MHz, This last frequency seems to be the limit: but in 1% of the cases, regarding the 30MHz, the communications in range 2 meters through Es can happen.  The Es openings  in two meters are therefore insufficient, but therefore rare like it would not be capacities to think: since, usually, they are of short duration, and however the reflection area is narrow to a zone from 50 to 100 Kmq, the stations that can avvalersi of the phenomenon, in the range two meters, they would not have to trovarsi not more than 1000Km from the area of reflections (or point of scatter) and can communicate with other placed stations to similar distances and single in determined directions. It would seem also that the formation of such phenomenon is concentrated over an area of low pressure, comprised between two cold fronts; that is over a cyclonic area, faced from a cold front. Usually such phenomenon happens with greater frequency between April to October with its maximum (duration, stability and m.u.f.) between high May and August. Usually the better timetables are those from the eight to the 19 (that is of day) can in any case happen such phenomenon also after the 20 and that often coincides with the more interesting openings, in other words the distance between the correspondents is greater that not during the day.

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).

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.

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.

References:

- Dyson, P. L., and J. A. Bennett, "A model of the electron concentration in the ionosphere and its application to oblique propagation studies", J. Atmos. Terr. Phys., 50, 251-262, 1988.
- Houminer, Z., Russell, C. J., Dyson, P. L., and J. A. Bennett, "Study of sporadic-E clouds by backscatter ionograms", Ann. Geophysicae 14, 1060-1065, 1996.
- Norman, R. J., Dyson, P. L., and J. A. Bennett, "Modeling and mapping sporadic E using backscatter ionograms", Report to the Technical Steering Group, Jindalee Project, Telstra Corporation pp. 21, 1998

   
 
 
 
 

Last modified:
25 avg 2001

millennium
1999 - 2002
http://www.qsl.net / s57onw /