Slot aerials and their relation to complementary wire aerials (Babinet's principle)
A half-wave slot in a metal sheet may be used as a resonant aerial in a manner similar to a half-wave dipole. The polar diagrams are the same, but the directions of vibration of the electric and magnetic fields are interchanged. If the slot is driven by means of a transmission line connected between opposite edges at the centre, the input impedance at resonance is about 485 ohms. The relation between resonant dipoles and resonant slots is an example of what in optics is known as Babinet's principle. This principle is established in the form required for electromagnetism and quantitative relations are deduced from it.Arrays of slots may be fed by transmission lines or wave guides to form linear or broadside aerials. Resonant gratings of slots may be used to polarize incident waves or act as band-pass filters. Single resonant slots may be used in wave guides in the same way as paralleltuned circuits are shunted across transmission lines. All devices adopted in connection with wire aerials have complements for slot aerials. Slots and dipoles may be combined to produce aerials with special polarization properties. Slots (with dielectric plugs) may be used in the skin of aircraft to provide dragless aerials.In addition, Babinet's principle may be used to reduce new problems to old ones whose solution is already known. These include problems connected with discontinuities in transmission lines, wave guides with corrugated sides, magnetron cavities, and the leakage of electromagnetic radiation through holes in metal walls.There is a wide variety of problems, both practical and theoretical for which it is always wise to consider the possibility of applying Bubinet's principle.
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