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  • Stu WØSTU

Tech Bands Over-the-Horizon Propagation (T3C04)

Question pool item T3C04 (2018-2022) asks: Which of the following propagation types is most commonly associated with occasional strong over-the-horizon signals on the 10, 6, and 2 meter bands?

a. Backscatter

b. Sporadic E

c. D layer absorption

d. Gray-line propagation

This question on propagation provides four interesting response options, each genuinely related to matters of RF propagation. Let’s briefly cover the incorrect responses, and then we’ll dive a little deeper into the correct response.

Backscatter: This term is usually used to mean the RF waves are reflected approximately 180 degrees, back in the direction from which they came. Backscatter can occur in the atmosphere due to particles of dust, moisture, and other particulates. Related to backscatter is tropospheric scatter, the redirection of RF energy in many directions from small particles of matter high in the atmosphere that can provide very weak signal propagation over-the-horizon on a regular basis. Note that the question specifies occasional strong signals, not regular weak signals.

earth surrounded by ionospheric layers
The layers of the ionosphere.

D-Layer Absorption: The D-layer is the lowest of the four prominent layers of the ionosphere. It is comprised of charged atomic particles that result from the sun’s ionizing radiation stripping away electrons from atoms in the atmosphere. The D-layer is notable for its characteristic of RF absorption of the lower frequency bands, the 30m band and lower (40m, 60m, 75-80m, and 160m bands). Because the D-layer absorbs these low frequencies, these bands do not typically allow skip propagation during the daylight hours. However, the D-layer tends to disappear overnight, freeing these lower frequencies to be bent back to earth by the much higher altitude E and combined F layers. But D-layer absorption rarely has an impact on the higher bands mentioned in the question, 10m, 6m, and 2m bands.

Gray-line Propagation: The gray-line is the band around the earth where daylight is separated from darkness. Essentially, it is the line of dusk and dawn that runs roughly north-south around the planet. The D-layer ionosphere rapidly disappears on the sunset portion of gray-line and slowly builds on the sunrise portion. Further, the higher altitude F1 and F2 ionosphere layers are highly charged with ions since sunlight impinges on them earlier and later than the D-layer due to their high altitude. Thus, for a brief period of time along the gray line the lower frequency bands (30m to 160m) are not absorbed by the D-layer and are very effectively bent by the dense F-layers. During this period the lower bands may propagate very strongly north-south for great skip distances. However, gray-line has less of a contrasting effect on the higher bands mentioned in this question and it is a regular, not occasional, phenomenon.

Sporadic E: Between the low D-layer and the high F-layers of the ionosphere lies the E-layer. The E-layer produces unusual propagation effects for 10m, 6m, and sometimes even 2m band. The E-layer produces occasional clouds or patches of ionization that come and go sporadically, hence the name of this propagation phenomenon. The altitude and density of the sporadic clouds varies, and this has variable impact on the skip distances that are achieved and the lowest usable frequency (LUF) for which the E-layer is useable. The practical result on operations is an occasional strong over-the-horizon signal propagation capability on these higher bands. It is a lot of fun when suddenly your local 6m signal is answered by a station hundreds or thousands of miles distant!

Although unpredictable and possible almost anytime, sporadic E does have some seasonal peaks of occurrence. In North America the peak sporadic E occurs in mid- to late-June, and may persist through July or August. A less significant peak occurs from mid-December into January.

Sporadic E can open up the 6m band with single skips of up to approximately 1200 miles, and rarer multiple skips of much greater distances. This effect, combined with the inconsistent and random nature of the ionization patches, has led to the 6m band’s moniker as the “magic band.” However, sporadic E propagation can also effect 10m band and, in particularly strong instances, the 2m band.

The answer to Technician question T3C04, “Which of the following propagation types is most commonly associated with occasional strong over-the-horizon signals on the 10, 6, and 2 meter bands?” is B: Sporadic E.

-- Stu WØSTU


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