Topband: Long Path Direction!

[John Kaufmann]

Don Andersen wrote:

> On occasions I hear hams talking about the direction of
> the long path.  It seems that many hams think that the
> "long path" is the inverse of the "short path.  In 
> reality it is the direction of the gray line.  Right now
> the long path is about 175 degrees in the morning.  A couple
> of months ago it was about 200 degrees.  This excludes
> skewed paths which occur here and there.  


What we call "long path" is a matter of semantics.  It can be whatever we define it to be.  The strictest interpretation would be the path that's 180 degress reversed from the great circle short path.  I think most of us use a looser interpretation than this, however, when referring to "long path" on the lowbands.

What Don describes is the gray line path, which sometimes coincides closely with the common notions of "long path", but sometimes not.  For example, using the VK6 example from my earlier posts, long path should be approximately SE according to the strict definition of long path.  In winter this also coincides with the gray line.  However, the path can also be E or NE on different days, which is most definitely not gray line.

The same VK6 path is very common on 80m and persists for almost the entire year.  Between the March and September equinoxes, the 80m "long path" to VK6 continues to be from the NE to SE direction, even though the gray line has shifted from S to SW!  There is clearly some other propagation mechanism at work besides gray line.  

Other 80m examples:  

The Singapore BC beacon on 3915 kHz, which I've found to correlate strongly with 160m propagation to that part of the world, is received daily from the SE direction around our sunset during winter.  However, the period when signals are strongest, on average, is late February to early March when our sunset beguns to coincide with 9V sunrise, but the direction is still SE even though the gray line is almost due south.  This beacon is audible on a daily the rest of the year, too.  By summer, the beacon is received from the NE direction, which begins to align again with gray line, but this time it is the summertime gray line over northern Europe!

9M2AX is also heard or worked fairly regularly on 80m on the SE gray line path in winter.  However, I have worked Ross as late as May on the same SE path, when it is no longer gray line.  

160m propagation is just too marginal, if not downright impossible, because of daylight conditions to observe these phenomena beyond the Northern Hemisphere winter.  However, based on the strong correlations I've seen between 160m and 80m long path during winter, I think the same "long path" propagation mechanisms must apply to both bands.

73, John W1FV