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A Long Balanced Feedline is a Balun

April 23rd Fact-of-the-Day

A current balun will make the radiation pattern of most center-fed dipole antennas more symmetrical, because it will tend to force currents in the two dipole legs to be equal. That is true even where a dipole is center-fed with a short balanced line, because even though two feedline leads may be perfectly balanced with respect to earth, few antennas are similarly balanced. One side nearly always is closer to trees, power lines, other antennas, buildings, earth, or something else that will cause leg-current imbalance. However, a straight balanced feedline functions as a current balun to an extent that depends on conductor spacing and line length. The reason is due to Ampere's Law, which involves integral calculus. However, the 'bottom-line' for two parallel conductors that are tightly coupled magnetically is that current flowing in one line induces an equal and opposite current in the other, which forces the two current magnitudes to be equal. Consequently, if conductor spacing is close enough and a feedline is long enough, a balanced feedline functions as an antenna-current-balancing balun. ©2005 Martek International All rights reserved.


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