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Straight, Uniform Vertical RadiatorAuthor: Edmund A. Laport
In practice, an antenna that is a quarter wavelength electrically is somewhat shorter than a quarter wave long physically. The difference is due to end effect, which results from the finite ratio of antenna length to antenna diameter. Table 1.2 shows the effect of height (length)todiameter ratio h/d on physical length G for a cylindrical vertical radiator for the condition of zero reactance with no artificial end loading.
From this table, the fundamental frequency of a vertical radiator can be determined from its geometry. For example, a cylindrical conductor with h/d = 200 has a physical height of 85 degrees. Its fundamental wavelength is λ_{0} = 360h/85.0, and its fundamental frequency is
For an idealized uniform vertical antenna without end effect, the fundamental wavelength in meters is
These relations are useful in deriving a simple equivalent vertical antenna that has the same fundamental frequency as an antenna of any arbitrary configuration. The current distribution along the vertical portion can thus be determined and from that the radiation resistance, and eventually the field strength per ampere at a unit distance can be calculated.


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