Long-distance
communication
is
possible
at
frequencies as high as 30 MHz. Waves at frequencies
above this range pass through the D layer but are
attenuated. After sunset. the D layer disappears
because of the rapid recombination of ions. Low-
frequency and medium-frequency long-distance
communication becomes possible. This is why AM
behaves so differently at night. Signals passing
through the D layer normally are not absorbed but
are propagated by the E and F layers.
E LAYER.— The E layer ranges from approxi-
mately 55 to 90 miles above the earth. The rate of
ionospheric recombination in this layer is rather
rapid after sunset, causing it to nearly disappear by
midnight. The E layer permits medium-range
communications on the low-frequency through very-
high-frequency bands. At frequencies above about 150
MHz, radio waves pass through the E layer.
Sometimes a solar flare will cause this layer to
ionize at night over specific areas. Propagation in this
layer during this time is called SPORADIC-E. The
range of communication in sporadic-E often exceeds
1000 miles, but the range is not as great as with F
layer propagation.
F LAYER.— The F layer exists from about 90 to
240 miles above the earth. During daylight hours, the
F layer separates into two layers, F1 and F2. During
the night, the F1 layer usually disappears, The F
layer produces maximum ionization during the
afternoon hours, but the effects of the daily cycle are
not as pronounced as in the D and E layers. Atoms in
the F layer stay ionized for a longer time after sunset,
and during maximum sunspot activity, they can stay
ionized all night long.
Since the F layer is the highest of the
ionospheric layers, it also has the longest propagation
capability. For horizontal waves, the single-hop F2
distance can reach 3000 miles. For signals to
propagate over greater distances, multiple hops are
required.
The F layer is responsible for most high-
frequency,
long-distance
communications.
The
maximum frequency that the F layer will return
depends on the degree of sunspot activity. During
maximum sunspot activity, the F layer can return
signals at frequencies as high as 100 MHz. During
minimum sunspot activity, the maximum usable
frequency can drop to as low as 10 MHz.
ATMOSPHERIC PROPAGATION
Within the atmosphere, radio waves can be
refracted, reflected, and diffracted. In the following
paragraphs, we will discuss these propagation
characteristics.
REFRACTION
A radio wave transmitted into ionized layers is
always refracted, or bent. This bending of radio
waves is called refraction. Notice the radio wave
shown in figure 1-3, traveling through the earth’s
atmosphere at a constant speed. As the wave enters
the denser layer of charged ions, its upper portion
moves faster than its lower portion. The abrupt speed
increase of the upper part of the wave causes it to
bend back toward the earth. This bending is always
toward the propagation medium where the radio
wave’s velocity is the least.
Figure 1-3.—Radio-wave refraction.
The amount of refraction a radio wave undergoes
depends on three main factors.
1. The ionization density of the layer
2. The frequency of the radio wave
3. The angle at which the radio wave enters the
layer
1-4
