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F <br />Tabit 3 —Comparison of Vagi B^ams at Heights <br />of 35 and 70 Feet <br />Fig 13 —At A. vertical-profile pattern Of a 24 9-MHz Vagi <br />beam al 35 feel, and at B. 70 feet. <br />^ ao <br />X, <br />,,>c \ <br />lA) <br />A <br />'0. <br />0.' <br />i ■ <br />Fig 14—Al A, vertical-profile pattern of a 28-MHz Vagi <br />beam at 3S feet, and at B, 70 feet. <br />Fig 8 shows the vertical profile for 7-MHz Yagi <br />beams. For a height of 35 feet, the energy is maximum <br />at a vertical angle of 34® above the horizon At a height <br />of 70 feet, maximum radiation occurs at an angle of 24®. <br />( At 35 Feet-) <br />£ ev angm ot <br />ms» radtsbon <br />(•^At 70 Feet-) <br />£ e» enge of <br />mat radition <br />Pvfform^ca <br />d'-Yenence <br />5* <br />70 34 degrees 24 degrees 5 7 drcibels <br />to 1 29 18 57 <br />14 0 24 14 56 <br />18 1 20 11 5 4 <br />21 0 13 9 52 <br />24 9 15 e 4 9 <br />28 0 14 7 4 6 <br />This IS 10® lower than for the same antenna at 35 feet <br />In addition, ir tne 2- to i0-degree elevation range those <br />angles that ait necessary tor communicating over <br />thousands cf miles, the performance of the low Yagi <br />beam is nominally 5 6 dB below that of the high Yagi <br />At a wive angle of 5 0®. the response of the Yagi at <br />70 feel IS superior by 5.7 dB <br />S milar data can be obtained from Figs 9 through <br />14 for the amateur frequencies from 10 i througn <br />20 MHz. The differences m perlormance for the amateur <br />bands from 7 to 28 MHz are summarized m Table 3 <br />As Table 3 indicates, a Yagi antenna at 70 feet <br />provides far greater radiation at low angles for all <br />frequencies than the same antenna at 35 feet. This <br />translates to greater communications effectiveness at <br />long distances, and is especially true for the lower <br />frequencies At a 5® wave angle, the differences m field <br />strengths range from 4 6 to 5 7 dB This means that tne <br />power at a distant receiving antenna for most propa ­ <br />gation conditions will be from 2 9 to 3 7 times stronger <br />from a Yagi antenna at 70 feet, compared to the same <br />Yagi at 35 feet Heights greater than 70 feet will produce <br />even higher power ratios <br />In summary, a high Yagi antenna provides ajpenor <br />performance over a low Yagi, ail other factors being <br />equal. Depending on the level of noise and interference, <br />the performance differences related to height are often <br />enough to mean the difference between making distant <br />radio contact with fairly reliable signals, and being unable <br />to make contact at all. <br />Very High and Ultra High Frequencies <br />Commonly used amateur frequencies higher than <br />those discussed earlier are the very high frequency <br />portion of the spectrum, abbreviated VHF. and the ultra <br />high frequencies, or UHF The VHF range covers 3C to <br />300 MHz and the UHF range from 300 to 3000 MHz. <br />In an earlier section it was mentioned that HF <br />antennas must be large to be effective. To some degree, <br />however, those considerations are not unique to that <br />frequency range They apply in general to VHF and UHF <br />as well. A difference at VHF and UHF is that antennas <br />may be physically small in order to realize optimum <br />electrical lengths. However, a disadvantage of physically