12.7 HF SKY WAVE ANTENNAS

Most HF sky-wave antennas are based on a horizontal dipole placed at an appropriate height above the ground to produce the required take-off angle for the radiation (see the patterns of Figure 12.13). Monopoles can be used for sky wave but are best suited for surface wave and direct wave. The following sections address HF sky-wave antennas based on the range required for the communication.

12.7.1 Short Range (0–300 Km)

For short ranges the take-off angle must be between 60˚ and 90˚, which can be achieved by a number of antenna designs including a horizontal half-wave dipole suspended above the ground at a height of λ/8 to λ/4. A three-quarter wave, end-fed antenna is a compromise between a sky-wave and a ground-wave antenna. Under best conditions it radiates approximately 2/3 of the energy as sky wave and 1/3 as ground wave. The quarter wave end-fed antenna can also be used but is not as efficient as a dipole or a three-quarter wave antenna.