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

[W7EXG (Bill)]

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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