Practical_Antenna_Handbook_0071639586
C h a p t e r 2 : r a d i o - W a v e P r o p a g a t i o n 49 Figure 2.29A Solar event affecting radio propagation on the earth. At some frequencies, solar and galactic noise establish a practical lower limit on the reception of weak signals. Solar noise can also affect radio propagation and act as a harbinger of changes in propagation patterns. Solar noise can be demonstrated by using an ordinary radio receiver and a directional antenna, preferably operating in the VHF/ UHF regions of the spectrum (150 to 152 MHz frequently is used). Simply aim the antenna at the sun on the horizon at either sunset or sunrise—a dramatic change in background noise will be noted as the sun rises or sets across the horizon. Sunspots Sunspots are arguably the most well known contributor to the sun’s effect on our ionosphere. Sunspots (Fig. 2.29B) can be as large as 70,000 to 80,000 mi in diameter and often appear in clusters. The number of sunspots typically visible on the face of the sun on any given day is quite variable in the short term, but as shown in Fig. 2.29C exhibits a fairly consistent and pronounced long-term cycle over a period of approximately 11 years. This is a very rough number; actual cycles since 1750 (when records were first kept) have varied from 9 to 14 years. The sunspot number is reported daily as the statistically massaged Zurich smoothed sunspot number, or Wolf number. The monthly smoothed sunspot number (SSN) is based on a weighted average of a 12-month period centered on the month being reported. Therefore, it is not possible to know what the SSN is for a given month until a half year later! Nonetheless, a century of observations has confirmed the strong relationship between the SSN and general levels of ionospheric radio propagation. The lowest monthly SSNs calculated during the entire radio era hovered around 1.5 in mid-1913 and did not go that low again until mid-2008. The highest recorded monthly SSN was 201 (in March of 1958). Daily sunspot numbers vary
50 p a r t I I : F u n d a m e n t a l s “all over the map” and can remain at zero for weeks on end (as they did throughout much of 2008 and 2009) or rise to values of 150 percent or more of the corresponding monthly SSNs. NOAA’s National Geophysical Data Center Web site (www.ngdc.noaa .gov/stp/SOLAR) is an excellent source of background information on the methodology involved. For many decades, the smoothed sunspot number was the best (and perhaps the only) available indicator of ionospheric propagation potential. With advances in science and technology, and especially with the availability of satellites, balloons, and other extraterrestrial probes, other measures now augment SSNs. One such indicator is the solar flux index (SFI). This measure is taken in the microwave region (at a wavelength of 10.2 cm, or 2.8 GHz), at 1700 UTC at Ottawa, Canada. The SFI is reported in an over-theair announcement hourly by the National Institute for Standards and Technology (NIST) radio stations WWV (Fort Collins, Colorado) and WWVH (Maui, Hawaii) and is also available from many additional sources via the Internet. The calculated SSN is roughly proportional to the measured SFI, but the two use different scales (the SFI has a minimum value of around 60 when the SSN is at zero) and the relationship is not perfect, as can be seen by examining a scatter diagram of hundreds or thousands of data points collected over at least two consecutive 11-year cycles. Two short-term propagation forecasting tools, the A-index and the K-index, relate to how “unsettled” the ionosphere is and are included in the NIST announcements. These, and many other monitoring and analysis tools are available; readers are encouraged to Figure 2.29B Sunspots.
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50 p a r t I I : F u n d a m e n t a l s<br />
“all over the map” and can remain at zero for weeks on end (as they did throughout<br />
much of 2008 and 2009) or rise to values of 150 percent or more of the corresponding<br />
monthly SSNs. NOAA’s National Geophysical Data Center Web site (www.ngdc.noaa<br />
.gov/stp/SOLAR) is an excellent source of background information on the methodology<br />
involved.<br />
For many decades, the smoothed sunspot number was the best (and perhaps the<br />
only) available indicator of ionospheric propagation potential. With advances in science<br />
and technology, and especially with the availability of satellites, balloons, and other<br />
extraterrestrial probes, other measures now augment SSNs. One such indicator is the<br />
solar flux index (SFI). This measure is taken in the microwave region (at a wavelength of<br />
10.2 cm, or 2.8 GHz), at 1700 UTC at Ottawa, Canada. The SFI is reported in an over-theair<br />
announcement hourly by the National Institute for Standards and Technology<br />
(NIST) radio stations WWV (Fort Collins, Colorado) and WWVH (Maui, Hawaii) and is<br />
also available from many additional sources via the Internet. The calculated SSN is<br />
roughly proportional to the measured SFI, but the two use different scales (the SFI has<br />
a minimum value of around 60 when the SSN is at zero) and the relationship is not<br />
perfect, as can be seen by examining a scatter diagram of hundreds or thousands of data<br />
points collected over at least two consecutive 11-year cycles.<br />
Two short-term propagation forecasting tools, the A-index and the K-index, relate to<br />
how “unsettled” the ionosphere is and are included in the NIST announcements. These,<br />
and many other monitoring and analysis tools are available; readers are encouraged to<br />
Figure 2.29B Sunspots.
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