Horizontal Antennas near EarthApril 20th Fact-of-the-Day
The feed-impedance of a horizontal antenna cycles above and below its free-space value as an antenna is lowered from great height toward earth. The impedance is typically near its free-space value at a height of 1/4 wavelength and drops to lower values as the height is further reduced. Earth conductivity and permittivity have little effect upon the impedance at heights above 1/2 wavelength, because strong magnetic and electric fields from a high antenna span a comparatively wide area on earth, but both have increasing effects below that height as fields become concentrated over comparatively-smaller areas of soil. Not only does earth proximity change an antenna's impedance, but an increasing amount of power is wasted in ground resistance as a transmitting antenna is lowered. Ground losses can be significantly reduced where an antenna must be installed near earth by laying numerous spaced, parallel conductors on the ground below. ©2005 Martek International All rights reserved.
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