Radiation Patterns from Periodic Straight-Wire Antennas

Figure 16. To obtain the radiation pattern in the vertical plane (in which the antenna lies) for a horizontal half-wave antenna λ/4 above the surface of the earth, multiply the free-space radiation (Fig. 13) at various angles by the ground-reflection factor (Fig. 15) at that angle. The areas of these figures should be disregarded.

factors (Fig. 15), or by equation 13 or 14, the theoretical radiation pattern of an antenna near the earth is obtained. In the following discussion half-wave antennas will again be considered, and the radiation in the most important planes will be depicted.

Horizontal Half-Wave Antennas, Vertical-Plane Radiation. The plane to be considered in this paragraph will be the vertical plane in which the antenna lies. As an example of the method of calculation, Fig. 16 has been included, which shows how the solutions are made for a half-wave antenna λ/4 above the earth. Thus, at 60°, the free-space radiation is 0.8, the reflection factor is about 2.0, and the theoretical

Figure 17. Radiation patterns for horizontal half-wave antennas at λ/4, λ/2, and λ above the earth. Figures at the left are for radiation in a vertical plane in which the antenna lies, and those at the right are for a vertical plane passing through the center of the antenna at right angles to the antenna.

radiation at 60° is 0.8 x 2.0 = 1.6., as shown in Fig. 16. The radiation at other angles is found in the same way. This same method is used to determine the radiation patterns (Fig. 17) for half-wave antennas at heights other than λ/4.

The radiation pattern to be considered in this paragraph is for a plane passing at right angles through the center of a horizontal half-wave straight-wire antenna. The pattern will have the same shape as the plot of the ground-reflection factors of Fig. 15 (a) for the antenna height under consideration (Fig. 17). This is because in free space the radiation as "viewed" from the end of a half-wave straight-wire antenna is a circle.

Horizontal Half-Wave Antennas, Horizontal-Plane Radiation. Theoretically, this antenna radiates no energy in a horizontal plane in which the antenna lies, because the reflection factors at a vertical angle of 0° are zero, as Fig. 15 (a) indicates. But, at many vertical angles there is radiation, and a plot can be made³⁷ of field intensities at these vertical angles in the various horizontal, or azimuth, directions from the antenna. Referring to Fig. 16, at 40° the radiation in the vertical plane in which the antenna lies is approximately 1.1 as determined from the "actual radiation pattern." At 40° at right angles to the

Figure 18. To obtain the radiation pattern in the vertical plane (in which the antenna lies) for a vertical half-wave antenna with its center λ/4 above the surface of the earth, multiply the free space radiation (Fig. 13) at the various angles by the ground-reflection factor (Fig. 15) at that angle. The areas of these figures should be disregarded.

center of the antenna the radiation is approximately 1.7 as determined from the "ground reflection factors," which, as mentioned in the preceding paragraph, is also the radiation pattern in a plane passing through the center of the antenna at right angles.

Vertical Half-Wave Antennas, Vertical-Plane Radiation. The plane to be considered is any vertical plane in which the antenna lies. As an example, Fig. 18 has been included, which shows how the calculations are made for a half-wave antenna the center of which is λ/4 above the earth. Thus at 30° the free-space radiation is about 0,8, the reflection factor is about 1.4, and the theoretical radiation at 30° is 0.8 x 1.4 = 1.12, as shown in Fig. 18. The radiation at other angles is found in the same manner. This same procedure is used to determine the radiation patterns for half-wave antennas at heights other than λ/4.

Vertical Half-Wave Antennas, Horizontal-Plane Radiation. The radiation pattern in a horizontal plane for a vertical half-wave straight-wire antenna at any height above the earth will be a circle. This can be visualized by "looking down" on Fig. 18. The radiation in all horizontal directions is uniform. However, the radiation at a vertical angle of 0° (the surface of the earth) is greater than the radiation at some vertical angle. It should be mentioned that losses in the surface of the earth and in objects on the surface of the earth reduce the radiation along the earth below the values indicated by Fig. 19.

Figure 19. Radiation patterns for vertical half-wave antennas with centers λ/4, λ/2, and λ above the earth. The antennas are located at the lower centers of the figures and are not shown for simplification.