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What is the difference between the near-field and far-field radio frequency radiation?

Is it true that the near field radiation is caused by the the energy dissipated at the end of the antenna connected to the oscillator, but far-field radiation and propagation is caused by the repeated, energy-shifting interaction between the electric field and the magnetic field.

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I tend to think of it this way: Free space (i.e., "far field") has a certain inherent impedance that's determined by the relationship between its elecrical permittivity, ε0, and magnetic permeability, µ0. Together, they dictate an impedance of about 377 Ω, and this determines the magnitude relationship between the E-field and the M-field of an electromagnetic wave.

However, in the presence of conductors and dielectrics (including the antenna itself), which have very different values for either or both of these constants, the impedance changes, and the balance between E-field and M-field is different. The usual convention is to start considering these effects anywhere within about 1 wavelength of the objects.

With conductors in particular, you also need to take into account the current and voltage distributions in those conductors — whether driven by RF sources or not. The net effect at any given point will be the sum of all of the effects of each "quantum" of current. It's very difficult to get closed-form equations for anything but the simplest cases, so these problems are generally solved using numerical approximations.

Does this help, or have I misunderstood the question?

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  • \$\begingroup\$ @AndyzSmith Read the above, upvote it, and then consider this: the far field is basically the piece of the transmission that successfully hits that magic impedance number Dave is talking about. The parts that don't meet that fade away, unless you're close to the antenna. Anything meeting the criteria will achieve a self sustaining wave that can travel indefinitely through space until something (like a reciever or a planet) sucks the energy out of it. Transmission antenna design is complex, but focuses on maximizing that far field. \$\endgroup\$ – Sean Boddy May 11 '14 at 6:08
  • \$\begingroup\$ I guess the stupid question,is, ... You say " when conductors are present" .... Aren't.the con ductors 'present' miles from them?', in the far field area....imean....the conductors alter the impedence ofspace near them, but not far from them? so at one wavelength, the conductors effect on free space impedence stops. seems like a convenient simplification . Aren't most of those kinds of things inversely proportional with the square of distance? \$\endgroup\$ – Andyz Smith May 15 '14 at 22:19
  • \$\begingroup\$ Yes, of course these effects are a continuous function of distance -- some are inverse-square, some are inverse-higher powers -- but it's generally accepted that they become insignificant (relative to the far field) for most purposes at a distance of 1 wavelength. \$\endgroup\$ – Dave Tweed May 15 '14 at 22:37
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imagine your source a small point floating in space thus it radiates itself like a sphere,

simply stated if you make measurements on behavior of the source and it is spherical in attributes or its isotropic properties are apparent them you are within near field.

Now it becomes far field when frequency, environment and its effects, fading due to curvature of the earth and relative height/distance between source/observer have significant impact on the measurement of emission.

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  • \$\begingroup\$ That sounds a bit self-referential. Near field is when the far field effects are negligable and far field is where the far field effect take over, Right? \$\endgroup\$ – Andyz Smith Aug 7 '14 at 14:09

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