On 29 mar, 17:17, "Dave" wrote:
Simple Question helix polarization - two stations, does one have to be LHCP
and the other RHCP *? * to keep the same 'thread' pattern if you see what I
mean !

Hello Dave,

If you have a directional antenna (take a Yagi) and you look to the
back of the antenna (so you look in the direction of wave
propagation), the E-veld vector moves in the same plane as the plane
of the antenna.

When you take two of this antenna's, one vertical and one horizontal
and feed them 90 degr out of time phase (same power to each antenna),
the E-field vector will rotate when it passes a fixed reference plane.
Depending on + or – 90degr phase difference, the vector will rotate
clockwise (RHC) or counter-clockwise (LHC).

Now take two of these circular antennas (for example both RHC). Now
make one of the antennas the receive antenna. To do this, you have to
rotate the antenna around its vertical axis over 180 degr. (so they
look towards each other). This flips the horizontal yagi (causing
180 degrees phase shift in the horizontal path), but the vertical yagi
does not flip.

This 180 degrees phase shift in the horizontal path is the reason that
total phase shift for both vertical pol. as well as horizontal pol.
paths are in phase.

So for line of sight, and you use circular polarized antennas, you
need two antennas with same sense. In case of a single reflection on
a perfect conductor, you need to change the sense of one of the
antennas. The reason for that is that the horizontal part of the
circular antenna isn't flipped and both hor. and vert. polarized
reflections experience 180 degr. phase shift.

For non-perfect materials: when the angle of incidence is far below
the (pseudo) Brewster angle, you need antennas with opposite sense.
When far above the Brewster angle (so small grazing angle), the
antennas should have same sense for best results (if you want to use
the reflection path).

At the (pseudo) Brewster angle, you lose most of the vertical
polarized component (parallel polarized component in physics) when the
media interface is in the horizontal plane. So the receive antenna
sees a mostly linear polarized (horizontal) component. In practice
this component may interfere with the direct path.

Wim
PA3DJS
www.tetech.nl
without abc, PM will reach me.