Topband: Near Field/Far Field

[Richard Fry]

Guy Olinger wrote:
>We will run NEC4 near field calculations on a 1/4 wave radiator with 120 
>buried 0.4 wavelength radials at 1.825 MHz, soil char of (5, .13).  Even at 
>30 (thirty) km the depth of the notch near ground is still increasing. ... 
>At 50 km out the minimum at 100m height is -28.69 dB  below the max value 
>at 17 km height,  with the pattern of values up to elevation 50 km looking 
>very much like the familiar FAR field process pattern plot which 
>coincidentally has a value of -28.93 dB below max at 0.2 degrees elevation. 
>... If given enough room to work, the NEAR field generation will show the 
>same notch.  The problem all along has been 3 km is nowhere near far enough 
>away to complete whatever accounts for the notching.

There is little point in dissecting the far field tens of kilometers from a 
vertical monopole to find the field remaining there at low elevation angles, 
because that does not account for ALL of the fields radiated by the 
monopole. In fact, that approach misses the existence of the greatest 
contributor to low-angle radiation -- the fields of the elevation pattern 
within 1 km of a 160m monopole radiator.

The reality being overlooked is that radiation already has been launched 
from a 1/4WL monopole toward an elevation angle of 0.2 degrees from the 
fields that are present within the first kilometer of the monopole site. 
This is shown in the plot at 
http://i62.photobucket.com/albums/h85/rfry-100/160m_Monopole_ElPat_at_1km.jpg .

Note in this plot that the field at 0.2-degree elevation is greater than the 
field at the peak of the assumed "take-off angle" lobe commonly attributed 
to a NEC far-field plot.  That low-angle radiation from a monopole existing 
within 1 km of the site proceeds to the ionosphere, and on 160m is capable 
of providing skywave service whose first hop return to the earth can provide 
the greatest range.

>Far field is the point of increasing distance where the shape of a pattern 
>calculated by a near field process quits changing at elevations and 
>azimuths of concern.

This belief includes the effects of the propagation path, whereas the 
classic definition in antenna engineering texts considers only the fields 
radiated by the antenna, itself.  The distance to the near-field/far-field 
boundary for the antenna alone is, by textbook definition (e.g. John Kraus' 
ANTENNAS, 3rd edition, page 39):   2L^2/lambda, where L is the greatest 
physical dimension of the antenna.

Here is a link with more detail: 
http://www.phys.hawaii.edu/~anita/web/paperwork/currently%20organizing/Military%20EW%20%20Handbook%20Excerpt/antnrfld.pdf