Topband: 3B9C model

[Nm7m at aol.com]

-------------- next part --------------
     The 3B9C DXpedition is currently in progress but already
there is keen interest in the propagation of 3B9C's signals,
particularly to North America.  There, just to the East Coast,
distances are in excess of 16,000 km.  A number of stations have
reported strong signals, suggesting propagagtion to NA is via an
efficient ducted mode, just above the night-time E-region.

     The 3B9C operators have commented that at times propagation
to NA begins suddenly, "like someone flipped a switch in the
ionosphere."  They hypothesize "a small patch of the ionosphere
between Somalia and the Seychelles undergoes a structural change"
perhaps a tilt, that serves to illuminate all of North America at
once.  At the moment they are conducting an experiment to see if
stations in NA hear 3B9C all at once, confirming the existence of
such an ionospheric gate.

     A sudden tilt would be a transient, non-equilibrium set of
circumstances and require a considerable extent to cover the East
Coast of NA, from 300 to 320 degrees azimuth at 3B9C.  The site
suggested by the 3B9C operators is in the Indian Ocean and most
likely would involve storm conditions in that region.

     As indicated in an earlier e-mail, PropLab Pro calculations
showed the presence of ionospheric ducting between 3B9C and NA
at the times reported on the Topband Reflector.  Those calulations
involve an average, undisturbed ionosphere (CCIR model) and it is
of interest to examine what such an equilibrium model predicts and
to compare results with the suggestion put forward by the 3B9C
operators.

     In rough summary, the PropLab Pro calculations showed that
ducting occurred at low radiation angles, set in after sunset at
E-region altitudes near 3B9C and continued until sunrise at E-
region altitudes at the far end of the path.  The most interesting
aspect of the ducting was that it began at about 7,500 km from
3B9C, near the center of Africa, far removed from the region
between Somalia and the Seychelles.  Before the onset of ducting,
propagation was by F-hops, with signal loss about three times that
of the subsequent ducting.

     The onset of ducting at 7,500 km from 3B9C finds its
explanation in a double hump or peaks of the lower ionosphere
that straddles the magnetic equator: down-going 3B9C signals
incident on the first peak have increasing angles of incidence
that take them out of the ducting range.  After crossing the
magnetic equator, the rising second peak serves to decrease
angles of incidence of upgoing 3B9C sinals so they fall into the
range for ducting.

     The calculations with an equilibrium model do not admit
sudden changes in propagation, only gradual changes in ducting as
the sun sets and then slowly rises again.  Sudden changes are
possible, however, with ionospheric tilts from rising gravity
waves.  But the mechanism over desert sands would probably differ
from over the ocean, particularly when it comes to scale size.
In any event, it will be interesting to learn of the results from
the 3B9C experiment.