06-16-1997 Planning Packet - Laserfiche WebLink

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Fig 13 —At A, vertical-profile pattern of a 24.9-MHz YagI beam at 35 feet, and at B. 70 feet. Fig 14—At A, vertical-profile pattern of a 28-MHz Yagi beam at 35 feet, and at B, 70 feet. Fig 8 shows the vertical profile for 7-MHz Yagi beams. For a height of 35 feet, the energy is maximum at a vertical angle of 34° above the horizon. At a height of 70 feet, maximum radiation occurs at an angle of 24°. Table 3 —Comparison of Yagi Beams at Heights of 35 and 70 Feet Frequency ( At 35 Feet-) Etev. angle of max. radiation (-At 70 Feet-) Elev. angle of max. radiation Performance difference at 5* 7.0 34 degrees 24 degrees 5.7 decibels 10.1 29 18 5.7 14.0 24 14 5.6 18.1 20 11 5.4 21.0 18 9 5.2 24.9 15 8 4.9 28.0 14 7 4.6 This is 10° lower than for the same antenna at 35 feet. In addition, in the 2- to 10-degree elevation range, those angles that are necessary for communicating over thousands of miles, the performance of the tow Yagi beam is nominally 5.6 dB below that of the high Yagi. At a wave angle of 5.0°, the response of the Yagi at 70 feet is superior by 5.7 dB. Similar data can be obtained from Figs 9 through 14 for the amateur frequencies from 10.1 through 28 MHz. The differences in performance for the amateur bands from 7 to 28 MHz are summarized in Table 3. As Table 3 indicates, a Yagi antenna at 70 feet provides far greater radiation at low angles for all frequencies than the same antenna at 35 feet. This translates to greater communications effectiveness at long distances, and is especially true for the lower frequencies. At a 5° wave angle, the differences in field strengths range from 4.6 to 5.7 dB. This means that the power at a distant receiving antenna for most propa gation conditions will be from 2.9 to 3.7 times stronger from a Yagi antenna at 70 feel, compared to the same Yagi at 35 feet. Heights greater than 70 feet will produce even higher power ratios. In summary, a high Yagi antenna provides superior performance over a low Yagi, all other factors being equal. Depending on the level of noise and interference, the performance differences related to height are often enough to mean the difference between making distant radio contact with fairly reliable signals, and being unable to make contact at all. Very High and Ultra High Frequencies Commonly used amateur frequencies higher than those discussed earlier are the very high frequency portion of the spectrum, abbreviated VHF, and the ultra high frequencies, or UHF. The VHF range covers 30 to 300 MHz, and the UHF range from 300 to 3000 MHz. In an earlier section it was mentioned that HF antennas must be large to be effective. To some degree, however, those considerations are not unique to that frequency range. They apply in general to VHF and UHF as well. A difference at VHF and UHF is that antennas may be physically small in order to realize optimum electrical lengths. However, a disadvantage of physically I