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

[Wes]

Thanks Bill.  A couple of things however, this is a directional event.  There 
won't be any enhanced propagation to the south for example.  This is best 
visualized using DX Atlas, a very useful tool.

Wes  N7WS




On 11/18/2018 6:35 PM, W7EXG (Bill) wrote:
> 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
>
> ______________________________________________________________