Tropospheric Ducting

There are more than 10 accepted theories as to what causes E-skip. The strongest is that the windshear theory. I'll get to ducting in a second but this is important in trying to distinguish between the two. Theres one crazy idea implication magnesium particles from meteors but I dont lend it much creedence, but is fascinating to consider...

The wind shear theory says that gaseous ions can accumulate and be concentrated into small, thin, patchy sheets by the combined actions of high-altitude winds and the earth's magnetic field. when this occurrs in the E layer of the ionosphere. These resulting "sheets" can attain the required ion density to reflect radio waves. The big key here is that they are not stationary. So even if you remain still, which stations skip, changes in both frequency and distance.

It gets more complicated when you see the reports that indicate the "clouds" are not parallell with the earth. i.e. the angle of reflection may alter the point of reception by over 30 degrees!

Tropospheric Ducting is completely different. Signals don't bounce and are not reflected. It's more like they are "bent." Cold air is more dense than warm air, so it has a higher refractive index. Normally cold air normaly being on top and the warm air on the bottom. The opposite of that is called a temperature inversion.

This happens when a weather front approaches a high pressure area. Cold air is forced down and warm air is forced up. The air temperature increases with the altitude instead of decreasing. Radio signals are guided through thsi path like air through a "duct." Note: Tropospheric ducting affects all frequencies and can push your signal 800 miles.

That still leaves tropospheric scatter and tropospheric refraction.... to be explained tomorrow!

Graveyard Frequencies

Originally these were called "Local Channels." This was because the AM stations on these frequencies were intended to serve only small communities. On the engineering side they were only allowed to operate up to a maximum of 250 watts, non-directional . It was in November, 1928 that the FCC created this radio dial configuration. It was almost a dumping ground for the many very-low power stations that popped up in the mid-1920s. These Frequencies are:
1230 kHz
1240 kHz
1340 kHz
1400 kHz
1450 kHz
1490 kHz

The DXer's dubbed them "Graveyard channels." That was because each of those frequencies was home to 300 stations in the lower 48. This makes picking out any single station almost impossible. If this sort of challenging DX’ing appeals to you be sure to visit the Graveyard DX blog here. It's inactive, but very telling. In the US, Canada and Mexico stations on the AM dial are spaced at 10 kHz intervals.

In 1960 the FCC finally admitted that some of these small communities had grown larger than the coverage areas of their local stations. So, the suits allowed all stations on the six aforementioned frequencies to increase their daytime power to 1000 watts. This was modified in 1982 to permit the higher power 24/7. the assumption was that they will only interfere with each other. Which they do. Canadian and Mexican followed suit.The result? Congestion is very common, and DXers may hear dozens of stations on a single frequency. As Fybush once wrote :

"Interference... in AM broadcasting is cumulative; Just listen to any of the "graveyard" channels... to hear what it sounds like when 300 stations are each throwing a kilowatt into the ether. "

Now, most stations on the six "Grave yard" frequencies operate at the permitted max of 1000 watts, even at night. A small number of them have had to use directional patterns to suppress interference to each other. You still hear cross talk everywhere in American essentially until you're in their parking lot.