07-25-1994 Council Packet - Laserfiche WebLink

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1 {toraiMfi Nfw). CM« lh« rttultant fi#W strtngth is equal to the sum of the tvw 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 m this case is the difference between the two. At still other angles the resultant field will have imermedlate 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 eiecirical characteristics of the ground have some slight affect.) If the earth is considered to be a perfect reflector, straightforward tngonometnc calculations can be made to determine the relative amount of radiation intensity at any vertical angle for any dipole height. Graphs from such calculatioos may be plotted as circular or polar diagrams, called radiation patterns. Fig 4 shows the vertical radiation pattern for a dipole antenna positioned one^ 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 broken lines, and appear as semncircles In the plots of Figs 4 and 5. the radiation angle Table 2—Electrical Heights For Amateur Bends below 30 MHa Frequency 1.8 MHz 35 7.0 101 140 18.1 21.0 24.1 28.3 35 bet phytKl tmgM 0 06 wavelength 012 025 0.36 050 0.64 0.75 0.89 1 00 70 feef physical height 0 13 wavelength 025 050 072 1 00 1 29 1 49 1.77 1.99 above the horizon is represented m the same tashion mat angles are measured on a protractor. The concentric aretes are calibrated to represent ratios of field strengms. referenced to the strength represented by the outer circle. The circles are calibrated in decibels. Diminishing strengths are plotled toward the center. Antenna heights are usually discussed in terms of wavelengths. The reason for this is that the length of a radio wave is inversely proportional to Its frequency. There'ore 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 ^/z wavelength ter a frequency of 7 MHz. Foi 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 increeae the intensity of radiation at some vertical angles and to d^rease it at othart. At a haiQht of Mi wavalanfllh (Fiq 4), tha radiated energy is strongest at a radiation angle of 30*. sn angle which was determined earlier to provide a maximum effective communications distance of about ■yean miles under the conditions assumed. The pattern of Fig 4 represents the radiation from a dipole ter 14 MHz at a height of 35 feet. As the horizontal antenna is raised to even greater heights, additional lobes are termed, 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. ter an antenna height of wavelengm. the energy in the lower lobes is strongest at 15* And Table l indicates that the optimum propagation distance per hop for 15* is 1200 Under the very same conditions as before, 5*hop propagation, one may see that the optimum communication no*' is 5 * 1200 or 5000 miles. The pattern of Fig 5 represents a i4-MHz * height of 70 feet. Thus, ter the conditions assumed, the optimum communications distance has been extend^ fr^ 3250 miles to 6000 miles, merely by raising the