|MadSci Network: Engineering|
This deserves a better answer than I have time or space to give you. A faulty analogy would be to think of an light bulb. Current in and current out but energy loss thru electromagnetic radiation. The fault of the analogy is of course that the radiation is a function of the filament heating and not direct RF loss (though a new energy saving light does work thru microwave emissions). Any charge in motion will generate em radiation, something that caused folks to initially challenge the 'solar system' view of atoms. Since moving charges emit radiation, they argued, the electron should spiral into the nucleus. It turned out that the solar system model was much to simplistic and that electrons do not radiate just because they are associated with a nucleus to form an atom. But the principle that moving charges set up fields is still valid. An antenna is by design the most efficient way we know to 'leak' this em radiation into the environment. The shape of the antenna varies with the desired wavelength and physical constraints. So you will notice those which look like rakes, loops, circles, dishes, pizzas, pencils, pigtails, and more.
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