NASWA Journal Columns · Technical Topics, July 1996

Joe Buch, N2JB • P.O. Box 1552 • Ocean View, DE 19970-01552 joseph.buch◊

Technical Topics, July 1996

Does Weather Affect Shortwave Reception?

This question was posed by someone on the internet newsgroup last year. He never got an answer to my recollection. The question started me thinking and doing some research. Now I can report the results of that work. Now without the slightest doubt or hesitation I can safely answer that question. Possibly.

Sure we all know that lightning storms create static noise (QRN) which can obliterate weak SW signals. This month and next I’ll discuss some of the more subtle influences weather patterns may be having on shortwave propagation.

There are two possible ways that weather could influence SW propagation. The first way is by causing unusual ionized clouds in the lower levels of the ionosphere causing reflections that can either interrupt or enhance reception. I’ll discuss this possibility this month. Next month our topic will be how tropospheric refraction caused by temperature anomalies associated with weather fronts can bend radio waves to cause unusual propagation conditions.

The ionosphere consists of several layers of ionized gas molecules. These molecules absorb and reflect radio waves back to the ground enabling the DX reception we all enjoy.

The lowest layer is the D Layer which serves to absorb shortwave signals during daylight hours. The D-Layer is responsible for the poor performance of lower shortwave frequencies during the daytime. The D-Layer disappears at night allowing low frequency sky wave propagation.

At a height of about 50 miles there is the E-Layer which normally disappears at night but can provide short distance propagation of SW frequencies during daylight hours. This layer is important in many theories explaining unusual propagation conditions.

At a height of 150 to 250 miles is the F-Layer. Radio theory texts normally divide the F-Layer into F1, around 150 miles, and F2 around 250 miles. At night the two layers combine to form a single F layer around 250 miles high.

Shortwave propagation over intercontinental distances usually occurs via the F layer. The higher the reflecting point, the longer each hop will be. Normal F-Layer propagation requires multiple hops between the F-Layer and the ground for any path longer than about 2000 miles. At each ground reflection a significant fraction of the signal is absorbed or scattered in some other direction.

Sporadic E layer ionization has been correlated with the presence of strong thunderstorms underneath the point of maximum ionization. There have been several articles in QST magazine over the past years describing this phenomenon. Once the clouds of ionized molecules form, they tend to wander in position over a period of minutes to hours.

[Figure 1]

Normally these clouds are highly ionized and are very efficient reflectors of SW frequencies. In fact, the stronger clouds have actually been able to reflect signals above 150 Mhz. One way I use to detect the presence of Sporadic E clouds is to monitor TV channel 2 with a small 5″ TV near my operating position. Channel 2 in the USA and most of the Western Hemisphere has its video carrier near 56 Mhz. There is no channel 2 outlet within my normal receiving range. If I start to see TV signals from the Midwest or Texas, I can assume there is a Sporadic E cloud out there.

Sporadic E layer ionized clouds can block the transmission path to the F layer. Figure 1 shows how the E-Layer cloud can come between the F-layer reflection point and the receiver. The path from transmitter site 1 is obstructed by the E-Layer cloud while the path from transmitter site 2 remains normal. The cloud need not be near the receiver. A cloud near the transmitter site could also interrupt transmission.

[Figure 2]

Sporadic E ionization can also work to enhance received signal levels on long multi-hop paths. Figure 2 shows how this is possible. The signal bounces down from the ionosphere in the normal manner. Instead of reaching the ground, however, the signal bounces off the top of an E-Layer cloud. This process can continue indefinitely as long as clouds are in the right place at the right time. Because the cloud is a relatively good reflector compared to the ground, signals propagated via this mode are often extremely strong.

The geometry requires the clouds to remain in just the right location. But the clouds slowly move. So the enhancements caused by this mode of propagation are normally short lived. Enjoy them while you can and get a tape recording because nobody is going to believe you when you say you heard Bahrain on 6010 at noon.

Has the connection between Sporadic E clouds and the weather been proven? No, not to my satisfaction. Articles in QST Magazine have attempted to correlate the formation of E-Layer clouds with lightning storms. Once the clouds form, they can then drift in position.

Here is an alternative theory. Sporadic E-Layer effects usually are strongest from one month before to one month after the Summer and Winter solstice, roughly June 21 and December 21.

At these times the earth’s spin axis is tilted nearest to the line between the earth’s center and the sun. There is a theory that energetic particles flowing from the sun (referred to by scientists as the solar wind) are interacting with the earth’s magnetic field to create these clouds. The magnetic field dips down to the ground at the magnetic poles. The particles are thus able to penetrate further at the solstices. At other times of the year the particles encounter the magnetic field at higher altitudes where conditions do not permit the formation of ionized clouds.

But the fact remains that there is some evidence to correlate the formation of Sporadic E clouds with weather. Is it possible that there is no cause and effect relationship between the formation of thunderstorms and the formation of Sporadic E clouds? Could it be that the apparent correlation experimenters have observed is really the independent effects of some as yet unknown cause? Could the interaction of the solar wind and the earth’s magnetic field provide the energy needed for ionized cloud formation and thunderstorms? Or could thunderstorms be the trigger for Sporadic E cloud formation providing that the solar wind energy is also present? These are the kinds of questions scientists will no doubt be investigating as mankind strives to better understand the environment we all share.

In the meantime, enjoy the unusual radio conditions that the weather may very well be causing. Next time I’ll explain how weather fronts can influence shortwave reception. Stay tuned.

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