[IRCA] Fwd: Noon-time MF propagation

[Lynn Hollerman <lynn@xxxxxxxxx>]

For those interested:

> -
> - - - - - - - -
>
> Hello, I'm new IRCA member Steve Johnston, Interim GM and Director of
> Engineering/Operations at Boise State Radio.
>
> During my talk on Synchronous Detectors and HD Radio at the recent Boise
> IRCA Conference, I mentioned in passing my observation that signals AM
> broadcast stations and other MF communications can experience a
> remarkable change at local high-noon.  Distant stations will
> sometimes jump up out of the noise right around noon.
>
> This has been a puzzle to me, but I've experienced it several times over
> the years, and an old-timer once mentioned the same thing to me without
> knowing I'd already encountered it.  It seems counter-intuitive
> since the D-layer should be at maximum absorbtion at noon.  Several
> IRCA members were interested in this phenomenon, but I did not have any
> further info at the time.  
>
> A search on the web on this topic this evening revealed the following
> info from the "KN4LF 7 Day Propagation Outlook #2003-17". 
> In this report Thomas Giella, KN4LF, of Plant City, Florida
> wrote:
>
> A Propagation Observation-
>
> As an Amateur Radio Operator and 160 meter enthusiast I regularly monitor
> MF propagation conditions. During daylight hours while I toil in my
> website design business, I monitor a 10 kw expanded band broadcast
> station, WCNZ 1660 kc from Marco Island, FL. The station is approximately
> 140 miles SSE of my QTH here in Plant City, east of Tampa. I use my 160
> meter 1/4 wave inverted L as the listening antenna and the station puts
> in a decent signal of at least S9. But throughout the daytime hours
> including at local noon time the station signal arrives via skywave, with
> peaks at +20 over S9, fading down to S3 with phase distortion. 
>
> Conventional propagation wisdom holds that sky wave propagation on medium
> frequencies does not occur at daytime due to total D layer absorption.
> The D-layer, which is at an approximate height of 30-60miles in the
> mesosphere, totally absorbs daytime medium frequency RF signals
> "most of the time". I say most of the time because at high
> latitudes, during the winter season and especially at the low part of a
> sunspot cycle, penetration of RF signals through the weakened daytime
> D-layer and then refraction via the E-layer does occur. 
>
> True to form during the Winter season of 2002-2003 I regularly observed
> sky wave propagation on 160 meters at local noon time on 1845 kc and
> occasionally WSM 650 kc in Nashville, TN. A group of stations here in
> Florida meet on this frequency, the Old Florida Amateur RadioTransmitting
> Society! 
>
> However one might expect a ground wave signal only in late Spring with a
> high Sun angle here at 27-28 deg north latitude, especially during
> ahigher part of Cycle 23. But as the daily solar flux level drops under
> approximately 110, the skywave propagation continues to occur.  It
> will be interesting to see what happens later this summer.
>
> This confirms my observations and provides a very interesting hypothesis
> for this noon-time enhancement - the E-layer reflection/refraction peaks
> at noon and provides skywave propagation in spite of the D-layer
> absorbtion.  Verty interesting...
>
> Steve 
>
> sbjohnston@xxxxxxx

Lynn.
Lafayette, LA
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