08-22-1994 Council Packet - Laserfiche WebLink

Home Browse Search

small antennas is that they present less surtace area to passing radio waves, and are therefore less effective m picking up incoming signals. This effect is termed pickup efficiency. Pickup efficiency is reciprocative; it applies, to transmitted sig-als as well as to received signals. To obtain higher pickup efficiency and therefore greater communications effectiveness, many antenna elements are required at VHP 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 low pickup efficiency and because of the omnidirectional characteristic of the radiator. Stacked vertical radiators are sometimes used to obtain gam with omnidirectional coverage. Although ionospheric propagation does sometimes occur at the lower end of the VHP range, propagation at too MHz and above is almost always over a line-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 objects, 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 VHP signals to an even greater degree" creating ••shadows" behind them. At VHP and UHP. the lowest lobes for Yagi arrays at 35 and 70 feet are only a few degrees above the horizontal. However, for line-of-sight communications, the radiation angle should be zero, if possible., unless one wishes to communicate with airplanes or objects in outer space. Even more desirable are negative angles from high locations, so the antenna can ••look down on the te. iin At VHP and UHP the radiation angles for 35-fr 3t heights are twice those occurring for the 70-foot heights. Therefore at VHP it is also beneficial to place the antenna as high as possible, and m the clear from surrounding objects. Amateur Radio operators wishing to contact other amateurs at great distances use a time-worn but true expression, •‘"por 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 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 for all amateur frequencies, high horizontal antennas will outperform low antennas. Information has been developed to show a consistent power gam of the hiqher antennas over low ones, that is. at 70 feet and 35 feet. The 70-foot 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 loss of antenna ehectiveness. Por example, he might increase his transmitter power from 500 watts to 1 .5 kilowatts to compensate for a three-fold loss 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 tor 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 PP energy from an Amateur Radio station is its antenna. Ra sing that antenna to an increased height, far above *he level of existing antennas for television reception, will reduce the possibility of interference to television reception from fundamental overload.