Topband: Near Field/Far Field

[Tom W8JI]

> According to antenna engineering textbooks (Kraus, Balanis. Johnson & 
> Jasik etc), the free space, far-field radiation pattern is not a function 
> of the distance from the radiator, as it is in the near field.  The 
> near-field/far-field boundary conventionally is defined as equal to 
> 2L^2/lambda, where L = the greatest physical dimension of the radiator. 
> So for a 1/4-wave monopole on 160 meters, that boundary would be located 
> about 20.25 meters from the radiator.

The boundary is around 1 wavelength minimum even for a very small antenna, 
like a "magnetic" loop. As a matter of fact a small magnetic loop is 
electric field response dominant at a distance of about 1/8th wave from the 
antenna outward to about 1 wavelength.

http://www.w8ji.com/images/emfield.gif

Page 341 of Jordon and Balmain ( as with other EM radiator texts) define 
farfield boundary as a distance "large with respect to a wavelength and also 
large with respect to the largest dimension of the source". A wavelength is 
safe for a small antenna, but not a large system. My 4 square on 160 still 
behaves slightly like nearfield at a distance of two wavelengths, but my 200 
foot omni vertical is settled down about 1 wavelength away.

73 Tom