Topband: Long Path

[Bill Tippett]

Robin WA6CDR wrote:

>Just a quick reminder- especially with the hi performance effort at A61AJ 
>coming up -  "Long path" does NOT always mean the great circle reciprocal 
>of short path -  especially on top band and in the solar maxima.  Don't 
>forget to listen ( and transmit) on the southern non polar paths - 
>especially during the critical sunrise-sunset periods.

        What you say is generally true, although in this specific case
"long path" happens to be close to the true 180 degree reciprocal path because
of the alignment of A6 and W6/7.  However, as you point out, that is often
not the case (e.g. 3W5FM or XZ0A to the East Coast).  The most interesting
aspect of "long path" is the consistency of signal azimuths at ~210 degrees
before local sunrise and at ~150 degrees after local sunset.  This fact is
independent of true azimuths and is most likely influenced by the location
of the terminator which closely aligns with these two directions at sunrise
and sunset.  As I said in my 1991 Fine Tuning Proceedings article:

"The actual bearing is usually around 210 degrees for my location (in CO),
but it's important to note that this is relatively constant whether the 
target area is Europe or Asia, even though the "true" 180 degree bearing
for long path should be between 150 degrees and 210 degrees."

        K6UA, W6KW and K6SSS first described this path in their September 
1975 CQ Magazine article "The Gray Line Method of DXing".  They said:

"The best way to describe the "Grey Line" is to say that it is a long path
opening that exists between two points on the earth which are experiencing
simultaneous sunrise and sunset or vice versa".

(Note that they said "a long path opening"...not just the 180 degree academic
definition.  I would also add that this path exists not just at simultaneous 
sunrise/sunset. When I worked JJ1VKL/4S7 on 160 from Colorado, it was almost
1 hour before my sunrise and 1 hour after his sunset.)

>Signals were heard from east to west 
>somewhat following the terminator as it crossed the states... but- oddly 
>enough, this mode did not appear to continue to the west coast.  Most west 
>coast contacts were short path received at our end.

        This is also consistent with what we've seen for years...in the
same article I wrote:

"For reasons I do not understand, approximately 330 degrees azimuth seems
to be the transition between propagation by long path and direct path at
sunrise in Colorado.  For example JT (339 degrees), XW8 (331 degrees), 
VR2 (322 degrees) and UA0S (342 degrees) have all been worked on both long
and direct paths around local sunrise.  They typically are barely audible
on the direct path well before sunrise, then switch to long path (210 deg)
peaking about 20 minutes before local sunrise, and then switch back to
direct path after sunrise."

Stations South of 330 degrees (e.g. JA at 315 degrees) NEVER propagated to
CO via a long path.  From W6/7 it would be very unlikely for anything East
of 4S7 to propagate via long path.  Since XZ is about 315 degrees from W6,
I am not surprised the path was only direct.

        Most of this experience was on 80 meters but I've seen enough of
it on 160 now to say that it follows the same pattern.  In fact a few
mornings I noticed exactly the same pattern with XZ0A...the path began at
210 degrees and then gradually moved to the West and then Northwest as my
sunrise approached (XZ true azimuth ~5 degrees from here).

                                                   73,  Bill  W4ZV


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