Sky wave propagation

As already mentioned, sky-wave propagation makes use of the ionosphere to refract and reflect radio waves so that they can be sent over long distances (Figure 7.7.a). Whenever radio waves enter the ionosphere, both reflection and refraction take place. How much of each occurs depends on the charge density of the ionosphere and on the frequency and angle of incidence of the radio wave.

Several important terms are used when describing this kind of propagation.

(i) Critical frequency ($f_c$)

For a given ionospheric layer, the critical frequency is the highest frequency of a beamed radio wave that will still be returned to the earth by that layer. For the $F_2$ layer, for example, $f_c$ lies in the range of about 5 MHz to 12 MHz.

(ii) Critical angle ($\theta_c$)

The critical angle is the maximum angle of incidence (within the usable frequency) at which a radio wave sent into the ionosphere is still returned to the earth. Signals beamed in above the critical angle simply penetrate the layers and escape into outer space without being returned.

(iii) Skip distance ($\Delta$)

The skip distance is the shortest (minimum) distance, measured along the surface of the earth, between the transmitting antenna and the first receiving antenna that picks up the wave after ionospheric propagation (Figure 7.7.b).

(iv) Skip zone or silent zone

The skip zone, also called the silent zone, is the region around the receiving side — lying within the skip distance — where neither the ground wave nor the sky wave is received, so no signal is heard there (Figure 7.7.b).

(v) Single hop distance

The transmission path is limited both by the skip distance and by the curvature of the earth. The single hop distance is the maximum distance, measured along the earth's surface, between the transmitting antenna and the first receiving antenna after one ionospheric bounce (Figure 7.7.c). For the $F_2$ layer this can be as much as about 4000 km. Since the average semi-circumference of the earth is about 20000 km, a signal must undergo several hops to achieve round-the-globe communication.

(vi) Multiple hop transmission

Multiple hop transmission uses repeater stations that receive the ionospheric signal, reflect and re-transmit it, so the wave makes several successive hops (Figure 7.7.d). This technique is used extensively for long-distance communication.

(vii) Fading …