Satgen524  Sporadic E is Coming  by GM4IHJ    (BID SGEN524) 1999-04-10

There are several types of signal propagation beyond line of sight at
VHF via the E Layer of the ionosphere roughly 90 kms above the earth.
Most occur at irregular intervals with no obvious seasonal pattern.
One exception is propagation via Auroral back Scatter, which has vague
ill defined peaks coincident with the Equinoxes, another is Sporadic E
SpE, which is clearly seasonal, with a small winter peak and a lengthy
sustained summer peak.

The broad summer peak starts with a few minor event days in April and
early May . Before becoming increasingly frequent in June, climaxing
in late June and early July, then declining slowly in August and early
September. Unfortunately , what is good news for those seeking
propagation of terrestrial signals beyond line of sight, is bad news
for those seeking to get their signals up through the E layer and
onward to satellites 600 or 800 kms higher. 

Not all is bad news however.Night time passes are rarely affected. SpE
is mostly a local daytime phenomenon. Starting in mid morning ,
peaking in the afternoon and , fading in the evening a little after
sunset. Better still some days have no Spe in your area at all (
though SpE is present most days in the season , somewhere ).

SpE can be present on several successive days before a break occurs. A
pattern which some observers suggest is correlated with presence of an
excess of meteor dust in the E layer , where it is pushed into dense
patches on the outside of Jetstream wind eddies. Indeed this
possibility is loosely supported by the repeated presence of SpE above
certain locations ( Eg as seen from UK - Over Nantes in France, and to
a lesser extent , over Denmark).

This patchiness of SpE can make or mar , successful uplinking of 2m
signals to satellites. 70 cm signals are attenuated passing through
SpE , but do not suffer to the same extent as 2m signals. With a
relatively small patch of SpE at 90 kms altitude making a signal
shadow across a much more extended region up with the satellite at say
1000 kms altitude. So that access to a satellite may be impossible for
several minutes of say a twelve minute in range pass. One factor
affecting where blocking occurs is Latitude. Signals to a satellite
overhead the latitude band 40 to 50 degrees are much more likely to be
affected than signals to satellites overhead higher latitudes. In a
typical case from Scotland , a satellite over Italy is not accessible
and does not become accessible until the satellite is over southern
Norway.

Satellites most at risk from SpE are the Mode J Fuji sats and the
various Pacsat, Uosats and Koreasats. But mode A 2m up links can also
be affected, whilst at times, mode A 29MHz downlinks are impossible to
read. Only mode K 21 MHz up and 29 MHz down seems to profit from SpE ,
and then only infrequently when combinations of the presence of
distant SpE and, a lack of local SpE, combine to give Sub horizon mode
K DX. Detailed analysis of mode K performance in the coming Sporadic E
season might make a very interesting experiment.