07-25-1994 Council Packet - Laserfiche WebLink

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|htsvnaneeme*Kitwii )5 feet. I. thOM g over w Yagi h Yagi. yagi at hrough hrough mataur ibla 3. 70 feat for all It. This ness at I lower infield that the propa* tronger e same Kodtice luperior s being erence, re often [distant I unable I'i: V small antennas »that they nBessino radio waves, and are therefore less effwtiw mSSI?up incominO•fficieney. Pickup elliaency isto transmitted signals as wen asobtain higher pickup efficiency and therefore greaw oommumcations effectiveness, many antenna elemeritt SrjIowwmd^HF and UMF. The elements are usually SiSS^ multiple booms. even at ei.veied he«ht*. are ineffective beyond a few mHes at these frequency ranges. This is true because of low pickup efficiency and becau» of tfw omnidirectional characteristK; of ttw »wked Vertical ndiHors if# iCKt>tlifTi#s us0d to obtoin Qiin wit omnidirectional coverage. .Lim* Although iooospheric propagation does sometimes occur at the lower *id of the VHF range, prop^non at too MHi and above is almost always over« sight path. Undar usual conditions* signals at these frequencies encounter very little refraction in air. Ther^ fore It behooves a person using these frequencws to his antenna as high as possible, so it can see farther. Certainly the antenna must be above sur^ rounding objects, such as dense foliage and man-made structures. Such objects absorb radio-frequency energy in this portion of the spectrum, and therefore » block the signal from propagating. Objects such as hills, steel structure* and many commercial or industrial buildings wiH block VHF signals to an even greater degree, creating ••shadows ” behind them. At VHF and UHF. the lowest lobes for Yagi arrays at 35 and 70 feet are only a few degrees kb<)ve the nonzontal. However, for Hne-of-sight cortwnunicati^s. the radiation angle should be zero, if possible, wnles* one ¥wshes to communicate with airplanes or objects in outer space. Even more desirable are negative angles from high localions. so the antenna can "look down on the terrain. At VHF and UHF the radiation angles for 3S-foot heights are twice those occurring for the 70-foot heights. Therefore at VHF it is also beneficial to place the antenna as high as possible, and in the clear from surrounding objects. Amateur Radio operators w<shir>g to contact other amateurs at great distarices use a time-worn but true expression. "For the best results, put your antenna as high as you possibly can.*' Many amateurs in various parts of the world put their big antennas atop 120-foot towers, and enjoy communications with amateurs in foreign countries when rwighboring amateurs with much lower antennas cannot even hear the foreign signals. Concluaion The preceding information has dealt with antyi^ height and communicatioos effectiveness, it ha* been shown that for aM amateur frequencies, high horizontal antennas will outperform low antennas. Information has been developed to show a consistent power gain of the higher antennas over low ones, that is. at 70 feet and 35 feet. The 70-foot height was chosen only for the purpose of illustratior and is not inte..d*d to be construed as the optimum height for antennas. Heights above 100 feet will give significantly better performance than at 70 feet. I* is reasonable to assume that if an amateur operator is restricted to the use of low antennas, he will obtain transmitting equipment with a corresponding incrtiM in powtf output to cofiiptnoilo tor the loss o antenna effectiveness. For example, he might increase his transmitter power from 500 watts to l .5 kilow atts to coTTtoenaato for a three-loW toss in antenna effective^ because of height limitatioos. It should be realized that this increase in power, in turn. Increase* the posaibilitxw for his stronger signal to overload nearby television and radio receiver*, creating interference for viewer* and listeners. There is one further consideration concerning amateur antenna height with regard to television and radio interference. In essence, the sole source of radiated RF energy from an Amateur Radio »**J[^*'*• antenna. Raising that antenna to an »ncre^^ght. far above the level of existing antenna* reception, will reduce the possibility of intertemnce to television reception from fundamental overload. ler than iquency the ultra irs 30 to lOMHz. that HF degree. I to that tndUHF ntennas iptimum Hysically r— '