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.

In real world (average soil and short wave communication), under very
small elevation angle, reflection coefficient is almost 1 and the
phase is 180 (so field cancellation above ground does occur). This is
valid for both H en V polarization. That is why the antenna/ground
combination cannot have its maximum of radiation at 0 degrees
elevation. The simplified "two ray" model propagation formula also
assumes RC=|1|/180degrees

Depth of reflection.
Reflection over a plane with infinite conductivity gives 0 degr or 180
degrees phase shift (even under 0 and 90 degrees elevation).

A real soil will not give 180 degrees phase shift for 90 degrees
elevation angle (in this case polarization doesn't matter). One can
convert this phase shift to a length extension, hence giving
"effective ground plane depth".

There is also a physical argument for "effective ground depth".
Because of the skin depth, the induced currents can have significant
depth in soil with low conductivity (up to tens of feet). So the
reradiation originates from below the surface. With the Phase/
amplitude approach, one can assume that reflection takes place at an
infinite thin sheet at the air/ground interface. This eases
calculation of real world radiation patterns.

When you calculate the elevation radiation pattern of, for example a
vertical dipole over ground, assuming optical 100% reflection, the
real world antenna will have some more low angle radiation then the
calculated version.

Hope this will help you a bit.

Best regards,


Wim
PA3DJS