Wimpie wrote:
On 7 jun, 20:20, ve2pid wrote:

Hi ot all

It is well known that the real ground seems to ''reflects'' a radio
wave. But I think that the term ''reflects'' is a bit confusing. My
understanding of the phenomenon is that the ground absorbs the
incident wave and, with that energy it re-radiates a new wave with a
different phase/amplitude value.


That new wave modifies the TO angle as a real optical-type reflection
would do. Then, it seems that it is not a ''bending'' of the wave, but
the production of a new one. With the value of the modification of the
TO angle, one can deduces a ''reflected'' wave's angle, even if it not
a real reflection.. Am I right?

Also, I read in a older version of the ARRL's Handbook that ''The
effective ground plane, that is the plane from which ground
reflections can be considered to take place, seldom is the actual
surface of the ground, but a few feet below it, depending upon the
characteristics of the soil.''

Considering what I said about re-routing with phase/amplitude
modifications, how to interpret the text form the Handbook? How to
determine the depth of that 'effective gorund plane'? Or is there any
depth at all? As is, it could be interpreted as a optical reflection
like occuring somewhere deep in the real ground..

Thanks..

Pierre


Hello Pierre,

You are right, reflection is reradiation. The driving field causes
charges to oscillate and oscillating charges radiate. When the change
in direction of propagation changes over a volume far more then a
wavelength, most people call it "bending" (as happens in the
ionosphere).

The amplitude and phase of the reflected wave depends heavily on angle
of incidence (AoI), frequency, soil properties and polarization. In
case of vertical polarization, there is AoI where the reflection is
minimal ([pseudo] Brewster angle). For vertical polarization, the
phase of the reflected wave varies strongly with AoI. You might look
for the "Fresnel Equations". These equations covers reflection on all
type of surfaces.

To avoid confusion, physicists define the Angle of Incidence with
respect to the normal (so 0 degr elevation angle is 90 degr AoI). What
radio engineers call "vertical polarization" is called "parallel
polarization" in physics.

If you look up the wikipedia entry
http://en.wikipedia.org/wiki/Fresnel_equations

parallel = parallel to the plane of incidence = p subscript in the
Wikipedia entry