Practical_Antenna_Handbook_0071639586

56 p a r t I I : F u n d a m e n t a l s
higher power, lower transmission line losses, higher gain antennas at both ends of the
circuit, narrower receiver filter bandwidth, or some combination of all four.
Ionospheric Variations and Disturbances
The ionosphere is an extremely dynamic region of the atmosphere, especially from a
radio operator’s point of view, because it significantly alters radio propagation on a
minute-by-minute basis. For our purposes, it is convenient to divide the dynamics of
the ionosphere into two categories: regular variations and disturbances.
Regular Ionospheric Variations
There are several different forms of variation seen on a regular basis in the ionosphere:
• Diurnal (daily)
• 27-day (monthly)
• Seasonal
• 11-year and 22-year cycles
Diurnal (Daily) Variation
The sun rises and sets on a 24-hour cycle; because it is the principal source of ionization
of the upper atmosphere, you can thus expect diurnal variation. During daylight hours,
the E and D levels exist, but these disappear at night. The height of the F2 layer increases
until midday, and then it decreases until evening, when it disappears or merges
with other layers. As a result of higher absorption in the E and D layers, lower frequencies
are not as useful during daylight hours. At the same time, the F layers support reflection
at higher frequencies during the day. As a very rough rule of thumb, for
ionospheric skip communications purposes, the frequencies below 10 MHz can be
thought of as primarily “nighttime” bands and the frequencies above 10 MHz are primarily
“daylight” bands.
27-Day Cycle
Radiation from the sun also exhibits a 27-day cycle caused by the rotational period of
the sun’s surface. Sunspots form or come to the surface of the sun near its poles, then
gradually drift toward the equatorial region during their visible lifetime. During this
latitude migration they tend to rotate in synchronism with the surface of the sun as a
whole, so they will face the earth only during a portion of each month. As new sunspots
are formed, they are not visible on the earth until their region of the sun rotates earthside.
Sunspots are believed to be regions of swirling high-intensity magnetic fields and
plasmas that somehow stimulate greater overall solar radiation. Paradoxically, as seen
in Fig. 2.29B, sunspots are darker than the rest of the sun’s surface, indicating that they
are cooler. New aids, such as the STEREO satellite pair, coupled with widespread dissemination
of monitored data, have vastly improved our ability to observe sunspot
formation and movement as it happens.
Seasonal Variation
The earth’s tilt varies the angle of exposure of our planet to the sun on a seasonal basis.
In addition, the earth’s annual orbit is elliptical, not circular. As a result, the intensity of
the sun’s energy that ionizes the upper atmosphere varies with the seasons of the year.