The shortest length of wire that will resonate to a given frequency is one just long enough to permit an electric charge to travel from one end RF cycle. If the speed at which the charge travel is equal to the velocity of light,approximately 300,000,000 meters per second, the distance it will cover in one cycle or period will be equal to this velocity divided by the frequency in hertz, or = 300,000,000/f

note: ==> wavelength

f ==> frequency in meter

Since the charge traverses the wire twice , the length of wire needed to permit the charge to travel a distance in one cycle is / 2 , or one—half wavelength. Therefore the shortest resonant were will be a half wavelength long.

The reason for this length can be made clear by a simple example. Imagine a trough with barriers at each end. If an elastic ball is started along the trough from one end,it will strike the far barrier,bounce again,and continue until the energy imported to it originally is all dissipated. if however,whenever it returns to the near barrier it is given a new push just as it starts a way, its back-and-forth motion can be kept up indefinitely. The impulses, however must be timed properly ; in other words, the rate or frequency of the impulses and the speed of the ball are fixed, the length of the trough must be adjusted to “ Fit “.

In the case of the antenna, the speed is essentially constant, so we have the alternatives of adjusting the frequency to a given length of wire,or the length of wire to a given operating frequency. The latter is usually practical condition.

By changing the units in the equation just given and dividing by 2, the formula : l =492/f(MHz) is obtained. In this case “ l “ is the length in feet of a half wavelength for a frequency “ f “ , given in megahertz, when the wave travels with length in antenna work are developed. It represents the length of a half wavelength in space, When no factors that modify the speed of propagation exist.
To determine a half wavelength in meters.
The relationship is ; l =150/ f (Mhz)

© 2007-2010 Pin Details of IC A - TDA All rights reserved to AntennaFundamental. by Bee Buzz News
Template by: skebber