Propagation to Antipodal Regions and Other Considerations-
 
For Scarborough Reef, the most distant DX signals will be from
around Bolivia, where the antipodal point is located (15S, 72W).
In that regard, it is interesting to look at the times for sunrise
and sunset at Scarborough and see how the terminators fall in
South America.  For that purpose, one can use mapping programs
like GEOCLOCK or DXAID, the latter easily making printed copies of
maps while the former requires a screen capture program.
 
Of course, sunrise and sunset terminators passing through The Reef
are great-circles and define the limits of solar illumination.
For the period of the DXpedition, say April 30 to May 6, those
individual times vary only slightly, sunrise going from 2150 UTC
to 2147 UTC and then sunset going from 1023 UTC to 1024 UTC,
respectively.  Thus, the angular region between where the two
terminators intersect does not change much in that time.
 
To the south of the antipodal point, largely in Argentina and
Uraguay, paths from The Reef come up from the south and will have
passed through the dark, southern polar cap.  In the long polar
night, ionization in the polar F-region will decay to low levels
and thus propagation would be supported only on the lowest
frequencies, say for the 80 and 40 meter bands.  That would be
largely between sunset at The Reef and the local sunrise.
 
To the north of the antipodal point, largely in Venezuela and
Guyana, paths from The Reef come down from the north and will have
passed through the sunlit, northern polar cap.  In the long polar
day, F-region ionization will be at higher levels and propagation
would be supported on somewhat higher frequencies, say for the 30
and 20 meter bands.  At those frequencies, however, there is
still usual requirement that the MUF for the first and last hops
be greater than the operating frequency.  In any event, signals on
the 40 meter band would be strongly attenuated in the sunlit parts
of the path that pass through the D-region.
 
To the east and west of the antipodal point, signals reach South
America by going in opposite directions from The Reef, to the west
to reach Brazil and to the east to reach Columbia and Peru.  Going
to the great-circle map of paths from The Reef, the paths which
cross the east and west coastlines of South America pass mainly
though low latitude regions and cross the equator.  Those regions
are particularly rich in ionization at F-region altitudes and
support propagation at higher frequencies than noted above for the
more poleward paths, say to the 15 meter band.
 
Those paths, ending near the antipodal point, are quite long,
18,000-19,000 km.  As a result, any efforts at lower frequencies,
say the 40 meter band where absorption is a consideration, should
be carried out under favorable circumstances, largely darkness
over the entire path.
 
At the latitudes in question, there is little chance of serious
geomagnetic disturbance propagating down from the polar regions.
So there is a good possibility of long-path contacts across the
dark equatorial ionosphere, say on the 20 meter band, to Brazil
or Peru.  There would be sufficient ionization to support that
mode and the only real concern would be about sufficient activity
by amateurs at the times in question, 1100 UTC-1200 UTC to Brazil
and then 2300 UTC-0000 UTC to Peru.
 
However, there is still the hazard of noise from atmospheric
sources.  For the period of the DXpedition, April 30 to May 6,
long-term studies show that the seasonal thunderstorm activity
will be increasing in the China Sea area and decreasing in Brazil.
The localization of atmospheric noise sources near The Reef will
be particularly difficult to cope with on the lower bands.
 
Finally, paths from The Reef to South America will pass largely
over regions with small amateur populations - the mid-Pacific
Ocean area and Central Africa.  As a result, beam headings in
those directions at The Reef should be relatively free from any
QRM from mid-path locations.