Noontime AM Band DX - What your Elmer didn't tell you

Yes, there are certain advantages to the minimum of solar cycle 23. The Winter. What's that?

Winter in the high northern latitudes offers oblique solar radiation yielding lower ionization in the D layer than occuring during summer months - a direct function of the sun’s elevation above the horizon. Coupled with a minimum number of sunspots (thus minimal UV radiation) the percentage yield drops an additional ~50% as compared to solar maximum [1]. As a result, RF frequencies that normally are attenuated (600-1630 kHz) in the D layer during daylight hours now reach the E layer where they are refracted. In the high latitudes, sky wave propagation begins to compete with the more familiar ground wave. This is particularly evident near the Winter Solstice.

Of course, ionization in the E layer is also reduced, but the solar minimum affects the D layer relatively more severely. Most texts never deal with the specifics of the solar minimum when discussing the D and E layers. Perhaps surprising, most of the pioneering work conducted on the ionosphere occurred through investigation of E layer sky wave [1,2] not of the F layers utilized for most daytime amateur HF communication.

During lunch break at work, I've being going to my car to try some AM broadcast band DX listening. Some late model car AM receivers are particularly sensitive. In the past few days I heard stations from 300-1000 miles distance at 12:00 noon local time. At times they compete with and overcome local AM broadcasts.

This can be a bit disconcerting. As I left work at 4:00 pm here in Washington state, I was listening for a rush hour traffic report. The loudest station with a traffic report ( 59 +30db is my “ear” estimate) was 810 AM. But it was for the Bay Bridge in San Francisco! This was Talk Radio KGO.

I'd like to hear about other observations and reports of AM broadcast band DX, especially east-west propagation and from the Eastern seaboard of European AM broadcasters.

73,

---* Ken WI7B

[1] E.V. Appleton, “The Ionosphere”, Nobel Lecture (1947)

=> http://nobelprize.org/physics/laureates/1947/appleton-lecture.html

[2] J. A. Ratcliffe, “Early Ionosphere Investigations of Appleton and his Colleagues”, Phil. Trans. Of the Royal Soc., Ser. A - Mathematical, Physical, and Engineering Sciences , 280 (1293): 3-9 1975