08-22-1994 Council Packet - Laserfiche WebLink

Home Browse Search

9C Table 2—Electrical Heights For Amateur Bands below 30 MHz Fig 4—Elevation-plarre pattern for a horizontal dipole at a height of Vi wavelength (solid line) and in free space (broken line). Fig 5—Elevation-plane pattern tor a hoiizontal dipole at a height of 1 wavelength (solid Une) and in free space (broken line). case the resultant field strength is equal to the sum of the two components. At other vertical angles the two waves may be completely out of phase at some distant point—that is. the fields are maximum at the same instant but the phase directions are opposite. The resultant field strength in this case is the difference between the two At still other angles the resultant field will have intermediate values Thus, the effect of the ground is to increase the intensity of radiation at some vertical angles and to decrease it at others. The elevation angles at which the maxima and minima occur depend primarily on the antenna height above ground. (The electrical characteristics of the ground have some slight effect.) If the earth is considered to be a perfect reflector, straightforward trigonometric calculations can be made to oetermine the relative amount of radiation intensity at any vertical angle for any dipole height. Graphs from such calculations may be plotted as circular or polar diagrams, called radiation patterns Fig 4 shows the vertical radiation pattern for a dipole antenna positioned one-naif wavelength above the ground, viewed from one end. and Fig 5 for a height of one wavelength. The radiation from the dipole if in free space is shown by the broVin lines, and appear as semi-circles. In the plots of Figs 4 and 5. the radiation angle 35 reef TO reef pnysical physcai Frequency heigr.:netgdt 1.8 MHz 0 06 wavelength 0 13 wavelength 3 5 0 12 0 25 70 0 25 0 50 10 1 036 0 72 14 0 0 50 1 00 18 1 0 64 1 29 21.0 0.75 1 49 24 9 0 89 1 77 28 0 1 00 1 99 above the horizon is represented m the same fashion that angles are measured on a protractor. The concentric circles are calibrated to represent ratios of field strengths, referenced to the strength represented by the outer circle. The circles are calibrated m decibels. Diminishing strengths are plotted toward the center. Antenna heights are usually discussed m terms of wavelengths. The reason for this is that the length of a radio wave is inversely proportional to its frequency. Therefore a fixed physical height will represent different electrical heights at different radio frequencies. For example, a height of 70 feet represents one wavelength at a frequency of 14 MHz. But the same 70-foot height represents only V? wavelength for a frequency of 7 MHz. For physical antenna heights of 35 and 70 feet. Table 2 shows the electrical heights in wavelengths for all the amateur bands below 30 MHz. The lobes and nulls of the pattern of Figs 4 and 5 illustrate what was described earlier, that the effect of the earth beneath the antenna is to increase the intensity of radiation at some vertical angles and to decrease it at others. At a height of v? wavelength (Fig 4). the radiated energy is strongest at a radiation angle of 30®. an angle which was determined earlier to provide a maximum effective communications distance of about 3250 miles under the conditions assumed The pattern of Fig 4 represents the radiation from a dipole for 14 MHz at a height of 35 feet. As The horizontal antenna is raised to even greater heights, additional lobes are formed, and those that exist move closer to the horizon. But yet the maximum amplitude of the existing lobes is not diminished. As may be seen from Fig 5. for an antenna height of 1 wavelength, the energy in the lower lobes is strongest at 15®. And Table 1 indicates that the optimum propagation distance per hop for 15® is 1200 miles. Under the very same conditions as before. 5-hop propagation, one may see that the greatest distance for optimum communication now IS 5 x 1200 or 6000 miles. The pattern of Fig 5 represents a 14-MHz dipole at a height of 70 feet. Thus, for the conditions assumed, the optimum communications distance has been extended from 3250 miles to 6000 miles, merely by raising the