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

Tue Nov 20 00:05:50 EST 2018

Hi Wes, 

Your intuition and observations are absolutely correct. No 40m DX
propagation to the South during the day this winter. 

The DX map propagation lines for Dec. 19 also shows no propagation to the
South, only from US to Europe region in the Easterly, and only US to similar
latitude regions in the Westerly. 

This event only occurs when the 40m DX path is always within the 40m daytime
propagation boundary. The 40m DX propagation boundary is defined by EMUF =
or > 7 MHz. This daylight EMUF=7 MHz boundary is currently about 2800 miles
from and orthogonal to the terminator all over the world. 

What is interesting is almost the entire US is sitting IN this 40m DX
propagation corridor during the day, peaking on Dec. 19th. (EMUF < 7 MHz). 

Thus this 40m daylight DX Corridor is not in any direction, as you pointed
out, as the signal path must stay within the corridor which is basically
parallel to the terminator and 2800 miles wide. The 40m signal must not
wander into the EMUF > 7 MHz region (Like South at Solar Noon), since the
40m signal quickly propagates into the E Layer, less distance per hop, more
signal loss per distance, and experiences additional D absorption.  

It should also be pointed out, that the 40m signal path may start in the
daylight F-layer region and enter into the nighttime F layer region, and in
some cases, re-emerge into the long path F-layer daylight region. There are
angular path metrics involved that are also relevant, but beyond this
discussion scope. 

You may recall working VK9EK (Heard Is) on 40m long path (our night) when
Heard Is was 2.5 hours past their SUNRISE - and still working the F layer. 

Much of the US to Europe signal path is more or less NW. Thus it is easy to
show that after Solar Noon these daytime 40m F-Layer signal paths cross
almost perpendicular to the northern terminator for nighttime F-Layer DX
to/from Europe. So these signals are headed directly toward the terminator
and not even close to being parallel to the terminator. 

A similar case can be shown for 40m propagation from the US to similar
western latitudes prior to Solar Noon. 

Dec. 19th is not that far away, and the 40m DX effects are apparent now, and
will get even better until Dec. 19th. 

Who is going to get the 40m DX Solar Noon Award??  Working Europe (or any
other DX) on 40m at or near solar noon! 

I have most of this information graphically defined in presentation
material. 

Thanks for your comments and clarification Wes.

Best 73's
Bill W7EXG

-----Original Message-----
From: sadxa-bounces at mailman.qth.net [mailto:sadxa-bounces at mailman.qth.net]
On Behalf Of Wes
Sent: Monday, November 19, 2018 7:00 AM
To: sadxa at mailman.qth.net
Subject: Re: [SADXA] 40m Daytime US DX Propagation Corridor - from Bill
W7EXG

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
>
> ______________________________________________________________

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