08-22-1994 Council Packet - Laserfiche WebLink

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Antenna Height and Communications Effectiveness B> Ge'ald L hall KlTD Associate Technical Editor The purpose of this paoer is to P’'o\ide aenerai information aPojt communications effectiveness as related to the physical height of antennas Tne performance of horizontal antennas at heights of 35 and 7'j f--‘et IS examined in oetail Vertical arrays are not consiaered here Decause at short-wave frequencies, over average terrain and at low rao ation angles, they are less effective than are honzo-tai antennas. ionospheric Propagation Frequencies oetween 3 ana 30 megahertz (ab breviated MHz) are often called tne short-wave ” bands. In engineering terms tnis range of frequencies is defined as the high-frequency or HF portion of the radio spectrum. HF radio communications between two points that are separated by distances of more than about 15 to 25 miles depend almost solely upon propagation of radio signals through the ionosphere. The ionosphere IS a region of the earth's upper atmosphere which is ionized Dy ultraviolet rays received from the sun The ionosphere has the property that it will refract or bend radio waves wmcn 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 propa'' * on. each ionized layer has distinctive characteristics, related primarily to diffeient amounts of ion>zation in the various layers. The ionized layer which is most useful for HF radio com munications IS called the F laver. Tne F layer exists at heights varying from approxi mately 130 to 260 miles above the eartn ’s surface Both the layer height and the amount of lonizaiion depend upon the latitude from the equator the time of day the season of the year, ana uoon the level of sunspot activity Sunspot activity varies generally m cycles that are approximately 11 years in duration although short-term bursts of activity may create changes in propagation conditions that last for less than an rour. The ionosphere IS not homogeneous, and is undergoing continual change Tne F layer disappears a: night in periods of low and medium solar activity, as tne ultraviolet energy required to sustain ionization is no longer received from the sun. The amount of bending that will be imparted to a passing radio wave is related directly to the intensity of ionization in this layer, and to the frequency of the radio wave. A triangle may thus be used to portray the cross- sectional path of ionospheric radio-wave travel, as shown in Fig 1. The base of the triangle is the surface of the ■ -i.'i .I' t ii«-- Fig 1—A simplified cross-sectional representation of ionospheric propagation. Typically the F layer exists at a height of ISO miles above the earth at mid-tatitudes. The distance between the transmitter and the receiver may range from a few miles to 2500 miles under normal conditions. earth between two distant points, and the apex of the triang'e is the point which represents refraction in the ionosphere. If all the necessary conditions are met. the radio 'wave will travel from the first poiht 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, pui irystead is c-.'ved High-frequency raaio waves behave in essentially the same manner as light waves —they tend to travel m straight lines, but with a slight amount of downward bending caused by refraction in the air. For this reason it is not possible to communicate bv a direct path over distances greater than about 15 to 25 miles in this frequency range The curvature of the earih causes the surface to "fall away ” from the path of tne radio v^ave v/ith greater distances Therefore it ts t.he ionosphere that permits HF radio communication to oe made between points separated by thousands of miles. Th} range of frequencies from 3 to 30 MHz is unique in this respect, as ionospheric propagation >s not consistently supported for any frequencies outside this range. One of the necessary conditions for lonospr.enc communications is that the ladio wave must encounter the ionosphere at the correct angle. This is illustrateo in Fig 2. Radio waves which leave the earth at high angles above the horizon may receive only very slight bending, and are then lost to outer space. For the same fixed frequency of operation, as the radiation angle is lowered toward the horizon, a point is reached where the bending of the wave is sufficient to return Ihe wave