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 approximately 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
