<19990205.005019.4487.1.k6se@juno.com>
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Sender: owner-topband@contesting.com
Precedence: bulk
X-List-Info: http://www.contesting.com/topband.html
X-Sponsor: W4AN, KM3T, N5KO & AD1C
Earl W Cunningham wrote:
>
> Without skew, the long path from N7DD to A45XR would reach a south
> latitude of 80.4 degrees. The terminator at that point is at about 72
> degrees south, so on direct long path at that point the sun would be
> about 8 degrees above the horizon. This is equivalent to you QSOing a DX
> station 32 minutes before/after your sunset/sunrise, which is not too out
> of the ordinary on 160m.
>
Not quite! While I agree the the sun would be around 8 degrees above
the horizon, the 32 minute figure is inaccurate except at the subsolar
point in the equatorial areas. The sun, as viewed in the polar regions,
sets at a very shallow angle with respect to the horizon. The sun still
moves across the sky at 15 degrees per hour, but in this case most of
the movement is horizontal with a smaller fraction vertical. Imagine
you were right at the pole during the equinox. The sun would still move
15 degrees per hour, but the movement would be entirely horizontal. The
sun would just move along the horizon. If you were at the equator, the
sun would move only vertically. In between is a mixture of both.
In this case it would be more like 2 or 3 hours before sunset on a time
basis. (The same would be true at 2 or 3 hours after sunrise, but for
ease of explanation, I'll stick with pre-sunset.) Most pre-sunset DX on
160 is in one direction - to that point where the terminator is closest.
To open the band on 160 you beam towards the darkness. You can still be
in daylight while the ionospheric reflection point is already in
darkness. East Coast to EU in winter before East Coast sunset is a
classic example of this. But the geometry for these early openings means
that the signal approaches roughly perpendicular to the terminator.
Propagation tangential (gray-line) must wait until the terminator
actually is overhead. A true long path signal between N7DD and A45XR
would pass well into the area of 24 hour daylight in the southern polar
regions. Furthermore, this signal would be nearly tangential to the
terminator in Anarctica which negates the ability of pre-sunset DX. If
you look at the path, you will see that a true long path would never get
too far from the darkness, but a considerable fraction of the path is
indeed in daylight. Even if the D layer is relatively weak, lingering
in it is death to 160m signals.
All in all, a skew path seems the most logical. First, it wouldn't take
a lot of skew. Second, a skew path avoids the daylight dilemma of the
long path. Third, a skew path would avoid the auroral attenuator.
Finally, a skew path would be consistent with the pattern of the
beverage, unlike the short path. I heard the A45 on 160 in the CQWW CW
and my estimation then was that the signal was arriving skewed out of
the west rather than true short path which would have been due north.
The difference was that Larry worked him!
Dan KL7Y
--
FAQ on WWW: http://www.contesting.com/topband.html
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Problems: owner-topband@contesting.com
|