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Fig 1-3—At A, evolution of
Sunspot Cycle 23, showing
smoothed (dark line),
monthly (jagged dark line)
and daily (light line). At B,
Sunspot Cycles 21, 23 and
23 superimposed with
monthly and smoothed
data. (Courtesy of Solar
Influences Data analysis
Center, Royal Observatory
of Belgium.)

Fig 1-4—Conversion from 2800-MHz solar flux to
Smoothed Solar Number (SSN). (Courtesy The ARRL
Antenna Book, 20th Edition.)

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CH1.PMD

Fig 1-5—Ionospheric propagation: At A, we see
refraction of the vertically transmitted wave—this
means that the frequency is below the critical fre­
quency for that refracting layer. At B, the angle is too
high and the refraction is insufficient to return the
wave to Earth. At C, we have the highest angle at
which the refracted wave will return to Earth. The
higher the frequency the lower the angle will become.
Note the skip zone, where there is no signal.

Chapter 1

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2/17/2005, 2:39 PM