Sky-Wave Propagation

Absorption by the ionosphere becomes important, particularly in daytime, in the medium-frequency band (300 to 3000 kilocycles) and the upper part of the low-frequency band. At the standard amplitude-modulation frequencies of 550 to 1600 kilocycles, only the ground wave need be considered in the daytime. During daytime, absorption occurs in the E layer. During the night, and particularly in the winter, the decrease in density of ionization and the resulting reduction in absorption by the E layer cause reflection to occur. It is difficult to calculate with accuracy the strength of the sky-wave signal, but it can be estimated,⁶ assuming a reflection coefficient of about 0.25. Curves are given in reference 16 by which sky-wave field intensities can be determined for the standard broadcast band.

If a radio station is receiving a signal by both the ground-wave path and the sky-wave path, then the received signal is the vector sum of the two components. Sky-wave signals are erratic, and, whenever they are involved, variations in signal strength occur. This is called fading and is defined¹ as "variation of radio field intensity caused by changes in the transmission medium." Sometimes, selective fading occurs,²⁴ defined¹ as "fading which is different at different frequencies in a frequency band occupied by a modulated wave."

In the high-frequency band (3 to 30 megacycles) the ground wave is absorbed so rapidly that usually only sky-wave transmission is important. An exception is for such purposes as city police radio systems and installations where the antennas are at a distance above the earth. In such instances space-wave transmission is important. But, for strictly sky-wave propagation, the transmission is so erratic that calculations are unreliable, and curves and maps with field-strength contours are used,^6,8,22 an example being Fig. 10.

Figure 10. Lines of equal average received intensity for a particular frequency, time of day, season, epoch of solar cycle, and location of transmitter. The numbers are field intensities expressed in decibels above 1 microvolt per meter, for 1 kilowatt radiated. The dotted line is the locus of sunrise (L) and sunset (C). The shaded areas are regions of essentially zero reception. The transmitting antenna is located in the upper center in the small shaded circle. The frequency is 18.8 megacycles, and the time is noon at the transmitter on a winter day. (From reference 8.)

In fact, as Dellinger has stated,⁸ "The more one views the complexities of radio transmission via the ionosphere, the more he marvels that it provides any intelligible communication."⁽¹⁾ Only because of exhaustive studies^25,26,27 over long periods has sky-wave communication been made useful.

Depending on conditions, above about 30 megacycles reflection from the ionosphere becomes uncertain, and above about 60 megacycles reflection from the ionosphere rarely occurs. Thus, at very high frequencies (30-300 megacycles) transmission is normally by the space wave.

(1)	* Quoted by permission, courtesy J. H. Dellinger, and the American Institute of Electrical Engineers.