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Power Losses

Author: Edmund A. Laport

A horizontal half-wave dipole depends, for its radiation pattern, upon reflection of waves from the surface of the ground in the vicinity of the antenna (image radiation).

The higher the antenna, the greater is the area directly concerned. In applications for short-distance operation, the angles of radiation are high, and the ground area involved in efficient wave reflection is rather small and almost directly beneath the antenna. For high dipoles used for low-angle radiation or reception, the ground should have maximum reflectivity out to a distance somewhat greater than that for the ray which will be reflected into space at the desired angle of radiation. This was illustrated in Fig. 3.13.

For the same reasons, the topography and characteristics of the terrain are important to achieving optimum results on point-to-point circuits. The area of dominant reflection should be as flat and of as high conductivity as possible. The locations of reflecting areas near the antenna can be determined by simple geometrical considerations.

Power losses at reflection from the ground are the most important encountered in ordinary high-frequency applications, even when the site is clear, flat, and of good conductivity. The effect of such losses is evident in Fig. 3.17.

In relation to the foregoing power losses, the loss in insulators and metal of the antenna can be quite negligible. Nevertheless, attention to design of insulation, especially for high-power use, is necessary to prevent mechanical failure due to heating, even though the amount of power lost is immaterial. In intermittent operation this is much less a matter of concern than in continuous service such as broadcasting or frequency-shift telegraph transmission. Insulation specifications should anticipate the most extreme weather conditions likely to be encountered and the possibility of constant reduction of surface resistivity with the accumulation of soot, water, ice, perhaps salt spray, and the deterioration of the glaze.

The gradual accumulation of corrosion on the surface of the wires can eventually increase the resistance of the antenna enough to become a cause of undesirable power loss. Corrosion, especially in salt-spray regions, can be retarded by painting the antenna and feeder conductors with glyptol when they are new and clean. There will be a small power loss in the glyptol but it will be less than with later heavy corrosion.

For a free-space half-wave dipole made of wire and having no end loading except its own natural end effect, the reactance at its center is zero when its physical length is 172 degrees instead of the theoretical 180 degrees. The attachment of insulators produces end loading in an amount that is empirical, so that further shortening is necessary to eliminate all reactance from its center impedance. When this dipole is near the ground, mutual impedance with its image again induces reactance into the center impedance, the amount depending upon its height above ground and the electrical constants of the ground. Some arbitrary length adjustment is then required to reduce the reactance component of center impedance to zero. The impedance is then restored to a pure resistance.

This resistance Ra can be matched with a balanced feeder of characteristic impedance Z0 by using a quarter-wave section of line of characteristic impedance Z00, which is the geometric mean of Ra and Z0.

The reason for eliminating the reactance by adjustment of antenna length is because it may have an appreciable value in relation to the antenna resistance and thus interfere with a correct match using the quarter-wave matching section. When the reactance is small with respect to the resistance, the resulting mismatch can usually be tolerated in practice, as in the case of the Y method of shunt feed. It is assumed that there is no coupling whatever by radiation between the antenna and the feeder.

Last Update: 2011-03-19