Spring 2008 Issue

PROPAGATION
The Science of Predicting VHF-and-Above Radio Conditions

A Switch is Thrown

By Tomas Hood,  NW7US

As we move into May, short-distance (only short when compared to long-haul DX of thousands of miles often experienced in the high-frequency spectrum) propagation opens up in the VHF, and sometimes UHF, spectrum. These openings provide propagation of radio signals for hundreds of miles and occur almost as if a switch has been turned on. This is a mostly summer-time phenomenon called sporadic-E.

Sporadic-E (Es) is the term given to the mode of propagation in which clouds of highly dense ionization develop in the E layer of the ionosphere. These clouds might be very small, but regardless of their size, they seem to drift and move about, making the propagation off these clouds short and unpredictable. It is well documented that Es occurs most often in the summer, with a secondary peak in the winter. These peaks are centered very close to the solstices. The winter peak can be characterized as being five to eight times less than the summer Es peak.

Ten-meter operators have known Es propagation as the summertime “short skip.” These “clouds” appear unpredictably, but they are most common over North America during the daylight hours of late spring and summer. Es events may last for just a few minutes up to several hours and usually provide an opening to a very small area of the country at any one particular time.

During periods of intense and widespread Es ionization, two-hop openings considerably beyond 1400 miles should be possible on 6 meters. Short-skip openings between about 1200 and 1400 miles may also be possible on 2 meters.

Scientists are still pursuing the multiple causes of sporadic-E. As far back as 1959, ten distinct types of sporadic-E and at least nine different theories of causation were offered. The classification of distinct types has been retained, but since the 1960s the wind-shear theory has become one of the most accepted theories.

Wind shearing occurs when the wind blows at different directions and speeds as you increase with height. Simply, the wind-shear theory holds that gaseous ions in the E layer accumulate and are concentrated into small, thin, patchy sheets by the combined actions of high-altitude winds and the Earth’s magnetic field. The resulting clouds may attain the required ion density to serve as a reflecting medium for VHF radio waves. Although most research has confirmed a close association between wind shear and sporadic-E, not all aspects of the sporadic-E phenomenon can be explained, including its diurnal and seasonal variations.

How can we know when a sporadic-E opening is occurring? Several e-mail reflectors have been created to provide an alerting service. One is found at <http://www.gooddx.net/> and another at <http:/www.vhfdx.net/sendspots/>. These sporadic-E alerting services rely on live reports of current activity on VHF. When you begin hearing an opening, you send out details so that everyone on the distribution list will be alerted that something is happening. They, in turn, join in on the opening, making for a high level of participation. Of course, the greater the number of operators on the air, the more we learn the extent and intensity of the opening. The bottom line is that you cannot work sporadic-E if you are not on the air when it occurs.
 

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