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Radiation EfficiencyAuthor: Edmund A. Laport
In the absence of more precise information, one may assume that a welldesigned lowfrequency antenna and ground system will have a radiation efficiency of roughly 75 percent of the equivalent electrical length in degrees of the antenna system at the working frequency. In other words, an antenna system having a vertical height of 11 degrees, with top loading that makes it equivalent to 25 electrical degrees, should have a radiation efficiency of the order of 18 percent. excluding tuningcoil losses. While this estimate is necessarily a rough one, it will serve as a reasonable starting point for computations. The best guide for initial assumptions is information on systems that have already been built and measured. Such direct information can then be applied to a new problem by similitude. Some reference information of this nature is included later. The antennadesign problem will always assume one of three basic forms: the limitations on the ultimate design to be those of a reasonable investment; or a maximum height (where physical height is limited by conditions other than cost); or a fixed amount of money available. The design considerations are weighed differently for each of these three forms. Height is the most costly dimension for an antenna system, and so the determination of height is one of the most important considerations in any given problem. It is also the most important choice with respect to maximum realizable radiation efficiency, since radiation resistance is in general proportional to the square of the height. Since the radiation pattern is not under control in lowfrequencyantenna design, the radiation efficiency is purely a circuit problem. The object is to obtain the highest ratio of radiation resistance to total antennacircuit resistance. The total antenna resistance is the sum of five separate components:
The radiation efficiency of an antenna is therefore
Ground resistance often is larger than the radiation resistance even when large investments are made in the ground system. The tuningcoil resistance may be relatively large, even after careful engineering design, because of the large reactance usually necessary and because of practical limitations in minimizing the dissipation factor Q. The characteristically high operating potentials on the antenna, owing to its small resistance and high reactance, cause insulatorloss equivalent resistance to become appreciable and sometimes of great importance in relation to the radiation resistance. The conductor resistance can also become appreciable in systems where mechanical considerations dictate the use of highstrength alloy materials having effective resistivities two to three times greater than copper. Also, the great lengths of wire required for lowfrequency antennas add to conductorloss equivalent resistance. The absolute values of ground, insulation, and conductor resistance are unattainable by direct measurement and must in any event be estimated from final systemresistance measurements. In the design stage, unfortunately, no significant estimates can be made because of the great range of values encountered within the scope of lowfrequency design. From a knowledge of the current distribution in the antenna, the conductor resistance referred to the feed point can be computed by making a summation of PR losses over the complete system.


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