Practical_Antenna_Handbook_0071639586

52 p a r t I I : F u n d a m e n t a l s
search the Internet using terms such as “sunspot numbers” and “solar cycle 24”. (Appendix
B includes a bountiful list of propagation-related Web sites.) As this is written,
the current data for today’s date, partway into solar cycle 24, are: SFI = 88; SSN = 51;
A = 12; K = 2.
The ionosphere offers properties that affect radio propagation at different times.
Variations occur not only over the 11-year sunspot cycle, but also diurnally and seasonally.
Obviously, if the sun affects propagation in a significant way, then differences between
nighttime and daytime, as well as between summer and winter, must cause
variations in the propagation phenomena observed.
Ionospheric Layers
The ionosphere is divided, for purposes of radio propagation studies, into multiple layers.
Like the boundary between the ionosphere and the stratosphere, the boundaries
between these layers are well defined only in textbooks. However, even there you will
find varying heights given for the altitudes of the layers above the earth’s surface. In
reality, they don’t have sharply defined boundaries but, rather, blend one into another.
Thus, the division of the ionosphere into layers is quite arbitrary. These layers (Fig. 2.26)
are designated D, E, and F (with F being further subdivided into F1 and F2 sublayers).
D Layer
The D layer is the lowest layer in the ionosphere, at approximately 30 to 50 mi above the
surface. This layer is not ionized as much as the higher layers because all forms of solar
energy that cause ionization are severely attenuated by the higher layers before the
energy reaches down into the D layer. Also, the D layer is much denser than the E and
F layers, causing any positive and negative ions to recombine and form electrically
neutral atoms more quickly than in the higher layers.
The extent of D-layer ionization is proportional to the elevation of the sun, so it
achieves maximum intensity at midday or very shortly thereafter. The D layer exists
mostly during the warmer months of the year because of both the greater height of the
sun above the horizon and the longer hours of daylight. As might be expected, therefore,
the D layer almost completely disappears after local sunset, although some observers
have reported sporadic incidents of D-layer activity for a considerable time past
sunset. The D layer typically is a strong absorber of medium-wave signals (to such an
extent that signals below 3 or 4 MHz are completely absorbed by the D layer). Because
of this, most AM broadcast stations enjoy only local ground-wave coverage during daylight
hours.
E Layer
The E layer exists from approximately 50 to 70 mi above the earth’s surface and is considered
the lowest region of the ionosphere that supports, rather than absorbs, ionospheric
radio communications. Like the D layer, this region is ionized only during the
daylight hours, with ionization levels peaking at midday. The ionization level drops off
sharply in the late afternoon and almost completely disappears after local sunset.
For most of each year, the E layer is absorptive and does not reflect radio signals.
During the summer months, however, E-layer propagation occurs frequently. A phenomenon
called short skip (i.e., less than 100 mi for medium-wave and 1000 mi for shortwave
signals) occurs in the E layer during the summer months and in equatorial regions
at other times, as well.