[RFI] Far Field, Near Field

Ian White, G3SEK G3SEK at ifwtech.co.uk
Tue Jan 25 04:29:23 EST 2005

Cortland Richmond wrote:
>An article in the IEEE Antennas and Propagation Magazine for October 
>2004 discusses the near field/far field distinction for dipoles of 
>various lengths up to one wavelength. It concludes that the lower 
>boundary of the far field is 9 wavelengths for 0.1 wavelength and 0.25 
>dipoles, 9.5 wavelengths for a half wave dipole,  and 12 wavelengths 
>for a one wavelength dipole.
>The authors define the very near field zone as that region inside which 
>the wave impedance is both different than  377 ohms and reactive power 
>is non-negligible with respect to the active power.   They conclude 
>that the upper boundary of the very-near-field zone is 1.6 wavelengths 
>from a 0.1 wavelength dipole, 1.75 wavelengths from a 0.25 wavelength 
>dipole, 2.05 wavelengths from a half wave dipole and 2.85 wavelengths 
>for a full-wave dipole.
>This may have implications for measurements taken at Amateur stations 
>and also of BPL emissions.
The actual physical E-field and H-field values around various lengths of 
dipoles have been known for a long time. Unless these authors are saying 
that existing calculations and measurements are wrong, all that can 
possibly be new is their analysis.

There is no sharp boundary between "near field" and "far field". Where 
you decide to draw the line will depend on your personal definitions of 
what words like "different than" and "non-negligible" (see above) 
actually mean in numerical terms. If these authors are setting very 
tight limits on the allowable differences, then of course that will push 
out the boundaries of what they choose to call the "near field".

Meanwhile, the physical fields around the dipoles remain exactly the 
same as they have always been.

Since health protection limits are ultimately based on SAR (Specific 
power Absorption Rate in W/kg) this paper should have no ultimate 
effect... though it may muddy the waters.

As for BPL, it only adds a little more mud to the superabundance that's 
already there.

73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)

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