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Name: Soaliman M.
Status: Educator
Age: 20s
Location: N/A
Country: N/A
Date: February 2004


Question:
What is the reason of selecting the length of antenna 1/2 or 1/4 of wavelength?



Replies:
Resonance. If you were to inject an electrical pulse into one end of a wire, the pulse would travel along the wire until it reached the other end. Then it would be reflected and travel back to your end of the wire. When it reached your end, it would again be reflected. If you were to inject a second pulse at the instant of this second reflection, the two pulses would add constructively -- both pushing electrons in the same direction. But if you injected your second pulse when the first pulse was at the far end, the two pulses would tend to cancel each other out -- one pushing electrons away from you, the other pushing them toward you. (An electron pushed in both directions at once will mostly just sit still.)

So, a length of wire will become strongly excited when pulses are periodically injected if the pulse period matches the time that a pulse takes to travel twice the length of the wire. This explains a 1/2 wavelength antenna.

A 1/4 wave antenna placed near a perfectly reflecting surface has an image in that surface. (Place a ruler perpendicular to a mirror. You will see a two-foot ruler.) The ground is a pretty good reflector at the frequency of radio waves, so a 1/4 wave antenna placed near the ground, and its image, act together as a 1/2 wavelength antenna.

Tim Mooney



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