christofire wrote:
"Roy Lewallen" wrote in message
...
christofire wrote:
. . .
* HP is less effective than VP for VHF communication with mobiles because
the ever-present ground reflection has reversed polarity (i.e. it's in
antiphase with the signal propagating over a direct path).
. . .
Reflection of a CP wave results in a reversed CP wave only when the
reflection is directly normal to the reflecting surface. That's not at all
the case for typical ground reflections. What you actually get from ground
reflections is an elliptically polarized wave, likely having the same
sense as the impinging wave. In experiments I ran about 35 years ago, I
found nearly linearly polarized waves resulting from reflection of a CP
wave from a vehicle top.


No doubt, but what I wrote above was about horizontal and vertical _linear_
polarisations, HP and VP. I did wonder after I sent it if 'polarity' would
be misread as polarisation.

Chris

My mistake, I apologize for misreading it. Yes, HP does reverse
polarity, exactly as you say, when reflecting from the ground.
Furthermore, the amplitude of the reflection is nearly the same as the
incident wave (that is, the reflection coefficient is nearly -1), even
when ground conductivity is relatively poor, especially at low angles.
So the sum of the incident and reflected waves is near zero.

Unfortunately, the sum of a vertically polarized field and its
reflection from finite conductivity ground is also nearly zero at low
elevation angles, for different reasons. There is a component that
propagates via surface wave, but it attenuates to essentially zero in a
very short distance at VHF.

Roy Lewallen, W7EL