04-18-1994 Planning Packet - Laserfiche WebLink

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w lat modes ropagation However. 1 the writer as many as jossibility.* jnanimous tions at HF angle. ed for HF lipole. The into which Secause of bjected to he results For these :onvement r antenna Tna, when U of power r HF radio tenna are ath it. If a the earth, the dipole nbine with above the i height of character- tntenna. . the direct B—that is. waves are )int. In this er 1. Fig 12. der contract . NY A part Dartmouth, Table 2—Electrical Heights For Amateur Bands below 30 MHz F,Q 4-El.vat<on-ptan« pattern for a a height of wavelength (solid line) and m free space (brotien line). Fig 5-tlevation-ptane pattern for a f a height of 1 wavelength (solid line) and in free space (broken tine). 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 -s the d'Herence between the two At still other angles the resultant field will have intermediate values Thus, the effect of the around is to increase the intensity of radiation at some vertical angles and to decrease it at others The elevation angses at which the maxima and minima occur depend primarily on the antenna height above ground (The electrical characteristics of the ground have some slight 6tt6Ct.) If the earth is considered to be a perfect reflector, straiahttorward trigonometric calculations can be made 10 determine 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 diaqrams. called radiation patterns Fig 4 shows the vertical radiation pattern for a dipole antenna positioned one-half wavelength above the ground, viewed from one end and Fig 5 for a height of one wavelength The radiation from the d.pole if m free space is shown by the broken lines, and appear as semi-circles. !n the plots of Figs 4 and 5. the radiation angle frecfi/ency 18 MHz 35 35 ftper pny$<*' 0 06 wavefength 0 12 0 25 0 36 0 50 064 0 75 0 89 1 00 70 feet physical fwgKt 0 13 wav«i*ngth 025 0 50 0 72 1 00 1 29 1 49 1 77 1 99 above the horizon is represented in the same fasf.ion that angles are measured on a protractor. The concentric circles are calibrated to represent ratios of field stiengths, 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 'S inversely proportional to its frequency. Therefore a fixed physical height will represent different e ’ectrical heights at different radio frequencies For example, a height of 70 feet represents at a frequency of 14 MHz But the same 70- oo etg represents only Vz wavelength for a frequency of 7 MHz. For physical antenna heights of 35 and 70 feet, Table 2 shows the electrical heights m wavelengths for all the amateur bands below 30 MHz . e The lobes and nulls of the pattern cf Figs 4 and 5 Illustrate what was described earlier, that the ettect o the earth beneath the antenna is to increase the intensity of radiation at some vertical angles and ^ at others At a height of Vz wavelength (Fig 4 . the radiated energy is strongest at a radiation angle of 3 . an angle wmch was determined earl.er to provide a maximum ettective communications ^ ^ 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 adoitional lobes are formed, and those move closer to the horizon But yet the amplitude of the existing lobes is not ^'m-nished. As may be seen from Fig 5. for an antenna height o 1 wavelength, the energy in the lower lobes 's at 15®. And Table 1 indicates that the optimum propagation distance per hop for 15® is ^ Under the very same conditions as befor , P oropagation. one may see that the greatest distance for optimum communication now is 5 x 12^ or ^^00 mijs^ The pattern of Fig 5 represents a 14-MHz ip height of 70 feet. Thus, for the conditions ^ssumedjhe optimum communications distance has been extended from 3250 miles to 6000 miles, merely by raising the