TopBand: Ray Tracing...Indonesia to Georgia

[Bill Tippett]

Here is Cary Oler's response to W9VNE's query about 160 propagation 
from YB0ARA/9:


Jim, these are good questions you raised.

First, the equator can have a substantial impact on 160 meter signals, but the
relationship is very complicated and depends on the time of day, the azimuth of
the signal you're transmitting or receiving, etc.  This is because the
electron density gradients vary over wide ranges across the equator - primarily
at geomagnetic latitudes of about +/- 20 degrees. The phenomenon is called the
equatorial (or Appleton) anomaly and transequatorial propagation on higher
frequencies is directly attributed to this feature of the equatorial anomaly.

However, in this particular case, I don't think the equator has much of an
impact at all because the signal would be reflected from the lower E-region
until about 2,000 kilometers downrange.  The E-region is less of an influence
the further north the signal travels because the signal is gradually departing
the terminator.  But more importantly, the signal encounters an area of
stronger horizontal gradients about 2,000 to 3,000 km downrange that pulls the
signal out of the great-circle path and more towards the east.

The true bearing from Indonesia to Georgia along the great-circle path is
roughly 18 to 19 degrees, but the horizontal refraction that occurs changes
the heading of this signal to about 25 degrees.  It is interesting to note that
this change of bearing occurs for a signal transmitted with a takeoff angle of
about 20 degrees (a fairly realistic figure for many antennas).  But more
importantly, this change of bearing is enough to cause the signal to miss the
northern auroral zone on its way to Georgia.

The other very important aspect in this particular case is the geometry of the
path from Indonesia to Georgia.  If you look closely, you'll notice that
Georgia and Indonesia are almost antipodal. For this reason, you can expect
some focusing of the signals at each point, which explains a lot.

For those who are unconvinced, I invite you to grab an image of the raypath
that I analyzed for this case that shows the locations along the great-circle
path where the ordinary rays (extraordinary rays are heavily absorbed) reach
the ground.  Anonymously FTP to: solar.uleth.ca and grab the file "raypath.gif"
from the directory "pub/solar/Topband".  Or, using your web browser, go to the
URL: http://solar.uleth.ca/solar/Topband/raypath.gif

The light-gray line that curves northward from Indonesia along the northern
region of Alaska is the great-circle path.  It is 16,891 km and follows an
azimuth of about 19 degrees.  I don't claim this to be the precise path length
or azimuth of the two locations we're talking about, but it's certainly close
enough in this case.

Each cyan-colored "X" marks the location where a ray hits the ground and
bounces back into the ionosphere.  There are three sets of results here: one
for a 5-degree transmission takeoff angle, one for a 10-degree takeoff angle,
and one for a 20-degree takeoff angle.

The signal with the 5-degree takeoff angle fairly precisely follows the
great-circle path.  It also penetrates through the auroral oval and is likely
degraded considerably or absorbed.

The signal with the 10-degree takeoff angle is the path where I havemanually
painted in a white-line between each of the locations where the ray hits the
ground.  Notice how it has been deviated from the great-circle path and begins
following an altered trajectory toward the SW U.S.

The signal with the 20-degree takeoff angle is the path where I joined the
ground reflections with a magenta-colored line.  This is the path that ended up
following an azimuth of about 25 degrees.  It veered off to the east and
therefore travelled a more southern route to the U.S.  A close examination of
this path (and even the 10-degree circuit) with a map of the auroral ovals for
11:00 UTC on 27 March shows that they would have missed the auroral oval and
therefore should have retained much of their signal integrity.

MOST IMPORTANTLY, because these two points (Indonesia and Georgia) are close
to the antipodes, paths on similar but differing azimuths should begin to
converge, resulting in some signal focusing.  The map I produced illustrates
this convergence very nicely.  Notice how the raypaths begin to converge back
toward the great-circle path.

Refering back to your first questions, you can see that the equator in this
case has no effect on the path the signal takes.  The divergence begins near 
Taiwan where horizontal electron density gradients begin to refract the signal
out of the great-circle path (for the skeptics, I can illustrate this with a
map if necessary).

>From the foregoing, it's clear that a viable circuit exists between the U.S.
and Indonesia at this particular time of day.  Extending the analysis suggests
that signal strengths should have gradually increased the further south you
went along the great-circle path, or slightly west of the great-circle path.
Unfortunately, this probably can't be confirmed, since the greatest signal
strengths due to focusing may have occcured in the Gulf of Mexico and extreme
northern South America.

Please bear in mind that this analysis assumes that everything (the
ionosphere, magnetic field, etc) are in a quiet "normal" state with no
sporadic-E or other disturbing ionospheric anomalies present.  Everyone knows
conditions can change rapidly and so the picture I've painted obviously will
not always hold true.  But for this particular case, it's probably a fairly
good representation.

And finally, for the individual(s) who are skeptical about the value of
computer software, I've just presented a case that provides more information
about this particular signal path than anything you've probably seen before.
A computer program (Proplab-Pro) performed the entire analysis.  All I did was
play dot-to-dot with a paint program to join the "X"'s with lines so everyone
would be able to see what I was talking about. Yes, almost every propagation
program in existence is unable to glean information of value about 160 meter
propagation.  And that is simply because almost every computer program in
existence is based on empirical algorithms that tend to ignore (for obvious
reasons) the nature of propagation on 160 meters.  To tackle 160 meters, you
need software that will do REAL (not simulated) ray-tracing to answer
the tougher questions.  The software I used in this case does exactly that.

I hope this discussion answers your questions.

Best regards.

        -Cary Oler
         Oler@Holly.CC.Uleth.CA, Oler@Uleth.CA,
         Oler@Solar.Uleth.CA, or
         COler@Solar.Stanford.Edu


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