04-18-1994 Planning Packet - Laserfiche WebLink

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smalt antennas is that they present less surface area to passing radio waves, and are therefore less effective m picking up incoming sigr^ais This effect is terrried picKup efficiency Pickup efficiency is reciprocative, it applies, to transmitted signals as well as to received signals To obtain higher pickup efficiency and therefore greater communications effectiveness, many antenna ele.me.nts are required at VHF and UHF The elements are usually installed on multiple booms Single, vertical radiators, even at elevated heights, are ineffective for distances beyond a few miles at these frequency ranges This is true because of tow pickup efficiency and because of tie orr iidireciional characteristic of the radiator Stacked vertical radiators are sometimes used to obtain gam witfi omnidirectionat cove'age Although ionospheric propagation does sometim.es occur at the lower eno of the VHF range, propagation at too MHz and above is almost always over a iine-of- sight path Under usual conditions, signals at these frequencies encounter very little refraction in air There fore It behooves a person using these frequencies to place his antenna as high as possible, so it can "see" farther Certainly the antenna must be above sur rounding objer’s. such as dense foliage and man-made structures Such objects absorb radio-frequency energy in this portion of the spectrum, and therefore tend to block the signal from propagating Objects such as hills, steel structures and many commercial or industrial buildings will 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 above the horizontal. However, tor Ime-of-sight communications, the radiation angle should be zeto. if possible, unless one wishes to commumcale with airplanes or objects in outer space Even more desirable are negative angles from high locations, so the antenna can "took down on" the terrain At VHF and UHF the radiation angles for 35-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 wishing to contact other amateurs at great distances use a hme-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 1 2lv)-foot towers, anc enjoy communications with amateurs in foreign countries when neighboring amateurs with much lower antennas cannot even hear the foreign signals. Conclusion The preceding information has dealt with antenna height and communications effectiveness It has been shown that tor ail amateur frequencies, high horizontal antennas will outperform tow antennas Information has been developed to show a consistent power gam of the higher antennas over low ones, that is. at 70 feet and 35 feet The 70-fooi height was chosen only for the purpose of illustration, and is not intended to be construed as the optimum height for antennas Heights above 100 feet will give significantly better performance than at 70 feet. It 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 increase in power output to compensate for the toss of antenna effectiveness For example, he might increase his transmitter power from 500 watts to 1 5 kilowatts to compensate for a three-fold toss in antenna effectiveness because of height limitations. It should be realized that this increase in power, in turn, increases the possibilities for his stronger signal to overload nearby television and radio receivers, creating interference for viewers 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 station is its antenna Raising that antenna to an increased height, far above the level of exrtmg antennas for television reception, will reduce the possibility of interference to television reception from funoamenial overload