Re: NEC-LIST: cfa near field and far field

Yes, the commonly-applied "far field criterion" for aperture antennas
(such as a dish) is R>2DD/L, as you state. Frequently not recognized
or ignored, this criterion applies for 0.99 (-0.04 dB) of the far
field gain at infinity, being developed at the stated 2DD/Lrange. For
less accuracy, and frequently very useful, even "handy", is the fact
that 0.94 (-0.27 dB) of the gain at infinity is developed at but 1DD/L
range, 1/2 of of the range that is usually quoted. For electrically
big dishes, this frequently has important implications.

It is indeed difficult to "draw a distinct line in the sand".

(Silver, MIT Rad Lab Series #12, p 199 and elsewhere)

Dan Bathker

At 08:49 AM 5/26/1999 -0700, you wrote:
>Chuck W1HIS wrote:
>
>>In free space, the near field is usually regarded as extending to a
>>radius of roughly lambda over two pi, but bear in mind that there is
>>no sharp cutoff. The strength of the reactive component of the total
>>E or H field decreases as the inverse cube of the radius as the radius
>>goes to infinity, whereas the radiation field strength decreases as
>>the inverse first power of the radius.
>
>Of course the nearfield/farfield transition point depends on how much
>accuracy one needs from the farfield model. At lambda over two pi the
>farfield term only contributes 30% of the overall field (for a
>dipole). The standard of farfield transition is given by three
>criteria which all must be met. Given a radiating structure of size
>D, a wavelength of L, and a distance from the structure of R. Here
>are the three criteria.
>
>R>>L
>R>>D
>R>2DD/L
>
>For the CFA the R>>L is the criteria to meet.
>
>Alexander
Received on Sat Jun 05 1999 - 02:50:17 EDT