At 01:21 PM 3/23/98 -0800, NM7M wrote:
>Right now all bets are still up for grabs. The original matter about
>long-path across a sunlit ionosphere is off the table. My suggestion of
>echoes from ducting is just another variation on Cary Oler's remark about
>multipathing. Maybe it was multiple F-hops but dipping down for ground
>reflections is a costly affair when it comes to signal strength. The
>real question is one of direction. So we'll keep working on that til
>there is something new to throw into the pot.
>
KL7Y: Long path is definitely not a consideration.
Some don't think regular earth/ionosphere bouncing with backscatter can
generate adequate signal strengths. Try this. One night when the Anchitka
OTH radar (HF) was opened up (slower pulses), we briefly held a target about
10,000 miles away. That means the radar signal left the transmitter, did
numerous bounces almost half way around the globe, until an extremely minute
fraction of that RF hit the body and wings of a likely 747. A even smaller
fraction reflected off of the plane and skipped all the way back to the RX
site. The signal arrived buried in the noise, but the DSPs pulled out the
signal and put the target on the screen. We could easily see each earth
reflection (backscatter) for every hop. Clearly there was no ducting. The
distant propagation had settled into narrow, illuminated arcs with high
signal levels interspresed with large areas of no or very weak propagation.
The target lay right on the peak of one of the narrow illuminated downrange
arcs. As the plane moved out of the narrow illuminated arc, the target faded.
I recently pointed out to ON4UN that most propagation programs assume a
fixed value of earth reflection loss, something like 2-3 dB per hop which is
a compromise between the land value (several dBs) and the sea water value
(which is a very small fraction of a dB on 160 meters for common angles).
For a long over water path (my radar example was mostly over water) the
actual signal levels may be as much as 15-20 dB or more stronger than what
the program predicts for such a path length. Most anecdotal ducting tales
are suspiciously on long, over-water paths.
In the case of NL7Z/K1ZM, it could be that there was a common backscattering
area somewhere around, say, the HC8 area. This would be a mostly all water
path for both parties. The amount of signal backscattered on the lower freqs
can be very dependent upon the ocean state - smooth as glass and there is
very little backscatter, but if the swells are just right in direction,
spacing, amplitude, length, etc., then the back- or sidescatter can be
appreciable. OTH radars can easily determine ocean wave velocity and some
kind of idea of the sea state thousands of miles away by the use of
backscattered signals. Given that the direct path between K1 and KL7 was
probably heavily absorbed, a backscatter path would certainly be feasible.
The multipathing could be from a direct ray combined with a backscattered
ray or even two backscattered signals from different hops.
Living under the auroral belt means dealing with these strange paths on a
regular basis. If there had been a strong direct path between KL7/K1 nobody
would have noticed another signal peaking from the wrong direction....
Dan KL7Y
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