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Daniel asks: Could you clarify what Kraus means by the energy being mostly stored at distances less than 1/(2 pi) and being mostly radiated at distances larger than that? I don’t think I’m getting the sense of how he’s referring to it as being stored.
The mathematical development showing this is too much to post here, but Kraus writes, “Many antennas behave like the dipole, with large energy storage close to the antenna. The region near the dipole is one of stored energy (reactive power) while regions remote from the dipole are ones of radiation. The situation is like that inside a resonator with high-density pulsating energy accompanied by leakage which is radiated.”
Kraus defines the radius of this imaginary, spherical resonator as 1/2*pi wavelengths from the radiator, as at that distance the magnitudes of the reactive and radiating fields have equal values.
Balanis wrote, “For an antenna, the radian sphere represents the volume occupied mainly by the stored energy of the antenna’s electric and magnetic fields.”