04-18-1994 Planning Packet - Laserfiche WebLink

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Antenna Height and Communications Effect veness Bv I Hai! KlTD AsstK aitf Technica' Ed»t4x The purpose of ‘ns paper is intoffTiation aDout cornmunicat^c'^s eh-^tueness as related to the physical heigh; o< antennas The pe'forrr.ancfe of honzcnta! antennas at heights 35 and 70 feet iS examined in detail Ven cai arrays are not considered her*’ oecaoso at shc^-Aave treg;^enoes over average terrain ana at Ica 'jo alien angies mev are less ehecti^e thar. ere nor ;:-:ai anterrnas Ionospheric Propagation Freouencies cetween 3 anc 30 nieganerte taP- previoted WHz) are chen called the ’ short-wave" bands In engineering terms tnis range of frecuencies is defined as the high-frequencv or HF portion ot the radio spectrum HF radio cemmun ’cations Petween two points that are separated Cy d star ’“»\ d* more I'.an about 15 to 25 miles depend almost solely upon propagation of radio signals through the ionosphere. The ionosphere IS a region of me earth’s upper atmosphere which is lOhired by ultraviolet rays received from the sun The ionosphere has the propedy that it will refract or bend radio vaves whicn pass through it However, the ionosphere is not one single "blanket" of ionization Instead, for reasons not fully understood, a few discrete layers are formed at different heights above the earth From the standpoint of radio propagation, each ionized layer has distinctive '•hi^ricteristics. related primarily to different amounts c! ration in the various layers The ionized layer wh - is most useful for HF radio com munications IS . iiied the F layer The F layer exists a! neights varying from approxi mately 130 to 260 miles aoove the eartn's surface Both the layer height and the amount c‘ ionization depend upon the latitude from the equator the time of day the season of the year, ana upon the level of sunspot activity Sunspot activity vanes generally m cycles tnat a^e approximately 11 years n duration although short-term bursts of activity may create cha.nges m propagation conditions that last for less than an rour The ionosphere is not homogeneous, and is undergoing continual change The F layer disappears at mght m periods of low and medium solar activity, as tne uhraviolet energy required to sustain ionization is no longer received from the sun The arriount of bending that will be imparled to a passing radio wave is related directly to the intensity of ionization in this layer, and to trie frequency of the radio wave. A triangle may thus be used to portray the cross- sectional path of ionospheric raoio-wave travel, as shown ih Fig 1. The base of the triangle is the surface of the f . A’f • •• A >. -X» ’ • t *♦ 4 k*’- Fig i_A Simplified cross-sectional representation of ionospheric propagation Typically the F layer exists at a height of 150 miles above the earth at mid-latitudes The distance between the transmitter and the receiver may range from a few miles to 2500 mi'es under normal conditions. earth between two distant points, and the apex of me triangle is the point which represents refraction ir. the ionosphere If all the necessary conditions are met. the radio wave wil! travel from the first point on the earth s surface to the ionosphere, where it will be bent suf ficiently to travel to the second point on the earth, many hundreds of miles away Of course the earth's surface is not a flat plane, but instead is curved High-frequency radio waves behave in essentially the same manner as light waves —they tend to travel in straight lines, but with a slight an.ount of downward bending caused by refraction m t.ce air For this reason it .s not possible to communicate b.* a direct path over distances greater than about 15 to 25 miles in this frequency range The curvature of the earth causes the surface to "fall away' from the path of the radio wave v/ith greater distances The'-efore it is the ionosphere that permits HF ra'^'o communication to be made between points separated by tnousands of miles. The range of frequencies from 3 to 30 MHz is unique in this respect, as ionospheric propagation is not consistently supported for any frequencies outside this range. One of the necessary conditions for 'onospneric communications is that the radio wave must encounter the ionosphere at the correct angle This ■$ illustrated in Fig 2. Radio waves which leave the earth at high angles above the horizon rnay receive only very slight bending, and are then lost to outer space For the same fixed frequency of ooera" as the radiation angle is lowered toward the horizon, a point is reached where the bending of the wave is sufficient to return the wave