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Folded Dipole Applications

May 20th Fact-of-the-Day

Folded dipoles usually are presented in textbooks and handbooks as providing a way to match 300-ohm twin-lead. However, there are many other applications. Folding a dipole once doubles its terminal voltage and halves its terminal current. Because R=E / I, doubling E and halving I increases dipole radiation resistance four times. The radiation resistance of an unfolded wire dipole in free-space is approximately 72 ohms, so the radiation resistance of a folded wire dipole in free-space is approximately 4 x 72 = 288 ohms. However, the radiation resistance of an unfolded dipole near earth is typically much lower than 72 ohms. For example, the radiation resistance of a 1.9 MHz wire dipole 40-feet (12.2 meters) above ground is approximately 12.5 ohms. Folding the dipole once increases its radiation resistance to 4 x 12.5 = 50 ohms and eliminates the need for an impedance matching line transformer. ©2005 Martek International All rights reserved.


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