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