Z 4 6 a 10 12 14 16 16 20 22 £4 26 25 30 FREOUfNCV-MC
Fig. 3. A typical field-intensity graph for incident skywaves. The appropriate adsorption-factor K is used to predict the received field strength in JUv/meter (microvolts excited per meter of antenna length). The curves show the variations in received field strength for a kilowatt of rf power radiated from a dipole antenna. Corrections are required for different power outputs and antenna gains.
Sunspot Cycles and Ionospheric Variations
Ultraviolet radiation from the sun is believed to be the source of energy which ionizes the ionospheres numerous layers. This belief is constituted on the basis of what is known as the 11-year sun spot-cycle. Over a period of a few years, the number of sunspots increases to a peak and then gradually declines. The duration from one sunspot peak to another usually covers about 11 years. During its peak, communicating conditions are excellent especially for 20, 15 and are even better on 10 meters. We are approaching a sunspot peak now and therefore world-wide contacts should be at their best before very long. At a sunspot minimum or "sunspot dip," communicating ranges are poor with very few band openings. There also exist what is known as ionospheric storms, and when they occur, the critical frequencies drop considerably and absorption of radio signals in the ionosphere is substantially increased. These ionospheric storms are resultant of certain sunspot activity. They usually last from a few hours to several days.
The critical frequencies vary with the seasons as with the days. During the summer, the E layer maintains a higher critical frequency as does the F1 layer. Conversely, the F2 layer's critical frequency peaks occur during the winter months and are at a minimum during the summer.
vhf Propagation Characteristics
Many amateurs are inclined to believe that the several types of low-band propagation effects are equally present on the vhf frequencies. This is not so. The vhf bands have rather unusual propagation phenomena which is most certainly very effective at times. The first of these is known as tropospheric bending and it's brought about by changes in the humidity and temperature in the lower atmosphere at altitudes of about 4500 feet. This type of effect is occasionally present on 28 mc but in the majority of cases is more prevalent and potent on 50 and 144 mc, usually more pronounced on the latter frequency. Another phenomena is the aurora effect. Frequencies below 30 mc are severely attenuated while those above are favorably propagated when an aurora occurs. However, the auroras have a tendency toward sustaining a flutter or rapid fading on signals and for this reason, phone operation during these effects is usually unreadable. Many amateurs resort to cw when there is an aurora opening while most phone operators find themselves struggling. Best results can be obtained by directing your beam toward the aurora display itself found in the northern latitudes. Sporadic-E skip, although used to some extent on the lower bands for short distances, is most often responsible for ranges in some instances up to 1,400 miles on either 50 or 144 mc, although the effect is much more pronounced on the former frequency. F2 layer skip is best at the peak of the 11-year sunspot cycle and can easily propagate a 50mc signal to distances exceeding 2,000 miles. Thus, worldwide communications are possible. F2 layer skip was responsible for the first transatlantic QSO on 50 mc on November 24, 1946.
Both forward and back scatter are also effective means of propagation. The former type is more widely found in the vhf and uhf regions, and back scatter usually only on 50 mc (and the lower-bands). These two types of scatter are present in both the troposphere and ionosphere. Tropospheric forward scatter usually produces distances up to 400 or so miles on 50 mc; ionospheric forward scatter has ranges of 500 to 1300 miles attributed to it. Back scatter most often occurs when either sporadic-E or F2 layer skip is taking place. At the bottom of the totem pole we have the most intriguing and challenging of all vhf propagation methods, known as moonbounce, irregularly referred to as "lunar communication." In order to have a successful QSO via moon-bounce, one must possess the absolute ultimate in receiver performance, mass arrays of high gain beams or a large parabolic reflector of helical type antenna in addition to a large amount of transmitter power usually on the order of one kilowatt or just a little less, not to mention the technically correct type of antenna-rotation polarization, tremendous receiver and transmitter
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stability amongst other factors. Some amateurs who have these prerequisites for effective moonbouncing have had their signals reflected back to earth at frequencies exceeding 1,200 mc! However, only the amateur who can afford such immense requirements and who is technically competent of undertaking such an effort should try any moonbounce work.
If the amateur has in his shack a wealth of propagation charts Or just one or two of them, he will be much better off than the other amateur who doesn't have any. By subscribing to the institute for Telecommunication Sciences and Astronomy, you can be put on the mailing list to regularly receive their monthly prediction charts if you are interested in predicting muf's for the upcoming months. The subscription rate is $2.75 annually or you can purchase these charts separately for 25 cents apiece.
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