[SADXA] 40m Daytime US DX Propagation Corridor - from Bill W7EXG

[w7aqk]

Bill and all,

I read your postings with considerable interest.  I've never really been 
much of a student of propagation, except for the most rudimentary forms of 
prognostication.  To be honest, it's always a bit difficult for me to 
confirm or deny various predictions because my antennas are relatively 
simplistic.  There might be a good opening, but I'd still be apt to miss 
it!!!

I'm not sure I fully understood everything you posted, but I think I got the 
gist of it.  Admittedly, at first I thought you were going to conclude that 
I should possibly take up bowling, but apparently you anticipate some 
"better" conditions in the offing.  Which brings me to my question!

You are suggesting (I think) that towards mid December there may be some 
improvement in 40 meter propagation.  In the recent past, on another 
reflector, I commented on my difficulty working into the U.K. from here, 
although I was somewhat more successful working into southern Europe.   A 
friend on that reflector introduced me to the concept of the "auroral oval", 
and suggested that this was a large part of my problem.  Apparently this 
hangs over the northern part of the globe, gets larger and more problematic 
in winter, and tends to block propagation from here to those northern 
European latitudes.  In essence, it suggest that it might be easier to work 
northern Europe outside of the window you are suggesting.  Is that true, or 
am I misinterpreting things, mixing apples and oranges, etc.?

Cheers!

Dave W7AQK


-----Original Message----- 
From: W7EXG (Bill)
Sent: Sunday, November 18, 2018 6:35 PM
To: 'Southern AZ DX Association E-Mail Reflector'
Subject: Re: [SADXA] 40m Daytime US DX Propagation Corridor - from Bill 
W7EXG

FOOTNOTE: The northern terminator curvature referenced below is the apparent
terminator curvature on a Mercator world map projection, when the solar
terminator is also projected.

In reality the terminator is a linear circular boundary around the world,
but appears curved when mapped as noted above.

Bill W7EXG

-----Original Message-----
From: sadxa-bounces at mailman.qth.net [mailto:sadxa-bounces at mailman.qth.net]
On Behalf Of W7EXG (Bill)
Sent: Saturday, November 17, 2018 9:55 PM
To: 'Southern AZ DX Association E-Mail Reflector'
Subject: [SADXA] 40m Daytime US DX Propagation Corridor - from Bill W7EXG

11-17-18
TO: SADXA MEMBERS
SUBJECT: 40m Daytime US DX Propagation Corridor

On Dec. 18th, the northern solar terminator boundary is only about 1089
miles from the US northern border during solar noon.  This northern
terminator is also about 2200 miles from Tucson during solar noon.

During my past propagation presentations, I pointed out that the E-layer
Critical frequency (foE), and resulting EMUF (Max Usable Frequency for the
E-Layer) does NOT exceed 7 MHz for about 2800 Mi from and orthogonal to the
Sunrise terminator. This data (from Ionograms (foE vs Time)) is easily
obtainable from the 120+ vertical RF sounding digital Ionogram sites located
throughout the world at various latitudes.

Since on Dec. 18th, MOST of the US is less than 2500 Mi from the northern
terminator during solar noon, most any RF signal on 40m in the US with an
elevation angle of about 23 degrees (most 40m antennas) will NOT be
reflected by the E layer, and will ONLY be reflected by the F-Layer (DX
region -- 23 deg 745 mi / hop), provided the MUF of the F layer is greater
than 7 MHz.

This described behavior is even more pronounced during LOW SOLAR periods,
since the ionization of the E layer is diminished, even at solar noon,
extending the < 7 MHz EMUF boundary further below the US southern boundary.

NOTE:  This distance [orthogonal from Terminator to EMUF = 7 MHz boundary]
is latitude dependent, and decreases from northern latitude values to a
minimum value at the Equator [by about 10%].

In practice, there are two additional 40m effects that occur in the US:
1) At solar noon, the D layer can still be heavily ionized, absorbing
RF energy across this boundary.
2) The Western region BEFORE solar noon, and the Eastern region AFTER
solar noon experience the northern terminator curvature descending
southwards towards the US, further decreasing the EMUF values, decreasing
D-layer absorption and thus further enhancing the daytime F-Layer DX
propagation.

Please observe the DXmaps propagation archives for this date and timeframe
which clearly demonstrates this described daytime US 40m DX behavior.

Note that during the summer, this described effect does not occur, since the
solar  noon terminator is North of the North Pole, and the E layer is fully
ionized, with an EMUF > 7 MHz. This results in E layer propagation (23 deg -
250 mi / hop) throughout most of the day in the US on 40m during the summer.

Also, during HIGH SOLAR periods with the E layer significantly more ionized,
only the northern most portion of the US may experience this daytime DX
propagation effect during December. This is due to the reduced orthogonal
distance from the terminator to EMUF < 7 MHz boundary.

BOTTOM LINE:
40m DX in the US during the day is BEST during the winter months centered on
Dec. 18th, and BEST during LOW SOLAR.  We are currently approaching this
BEST/BEST scenario.

As a matter of interest, please post your 40m DX exploit's during this
upcoming winter season, especially very late morning and very early
afternoon 40m DX QSOs.

73's, and Happy DXing.

Bill W7EXG

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