[TowerTalk] TowerTalk Digest, Vol 100, Issue 63

[Joe Subich, W4TV]

 > NEC does properly model it when you calculate the far field pattern.

NEC may model the pattern for vertical antennas but not the magnitude.
It regularly understates losses in the near field resulting in an
overstatement of absolute signal levels in the far field - particularly
at low elevations.

73,

    ... Joe, W4TV


On 4/25/2011 9:32 AM, Jim Lux wrote:
> On 4/24/11 10:21 PM, Richard (Rick) Karlquist wrote:
>> A half wave vertical or a vertical dipole doesn't work well when
>> elevated above ground because of reflection losses due to the
>> Brewster angle.  This problem is independent of any problems
>> with feeding it.  NEC may or may not model it correctly.
>>
>> Rick N6RK
>>
>
> really more the reflectivity of an interface between dielectrics being
> very high for waves polarized parallel to the interface (i.e.
> Horizontal) independent of incidence angle, but varying a lot for waves
> polarized perpendicular (i.e. Vertical).
>
> Fresnel's equations can be used to find the reflected and transmitted
> wave intensity at an interface.
>
> The Brewster angle is a point solution to the Fresnel equation where the
> reflection magnitude is zero. It really only occurs when going from one
> lossless medium into another.
>
> The Pseudo Brewster Angle used in RF propagation is similar, and works
> with lossy media.  It's where the magnitude of the reflected wave
> (R-parallel) is minimum (as opposed to zero).  (or where the ratio of
> R-parallel to R-perpendicular is a minimum:  For RF over soil,
> R-perpendicular is pretty constant at -1, so it boils down to much the
> same thing)
>
> NEC does properly model it when you calculate the far field pattern.
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