Velocity factor

Should the folded dipole antenna design on the accompanying page incorporate a correction for the velocity factor of the twin lead? Electromagnetic waves traveling on transmission lines such as 300 ohm twin lead travel more slowly than they do in free space. Typical twin lead has a velocity factor of 0.83, meaning that electromagnetic waves travel at 83% the speed of light in a vacuum. However, this is the velocity factor for difference mode signals, with the current travelling in opposite directions on the two conductors. When used as an antenna, the current travels in the same direction on both conductors, the fields are not so concentrated in the insulator, and the velocity factor is much closer to 1.0, typically around 0.95.

The addition of a reflector and/or director turns the antenna into a directional antenna called a Yagi. Yagis typically have the reflector about 5 percent longer and the director about 5 percent shorter than the driven element (main antenna element).

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