Richard Fry wrote:
On Nov 29, 2:18 pm, Roy Lewallen wrote:
Jerry wrote:
Just for conversation, I submit that an antenna with good hemispheric
CP coverage could be made with 4 dipoles.

Yes, there's the quadrifilar helix which I believe fits that
description. Another, which I built decades ago at 450 MHz, is the "skew
planar" antenna which resembles a cloverleaf but with the "leaves"
rotated 45 degrees...

Getting back to Jerry's idea - yes, four linear dipoles can generate
nearly perfect omnidirectional c-pol. This is a design of Nils
Lindenblad many decades ago, and I've done some NEC-2 modeling of it.
The link below leads to a rendered view of that model.

http://i62.photobucket.com/albums/h8...adRendered.gif

There's still the problem of ground reflection, though. I didn't state
it very well in my last posting -- what I meant was that the sum of the
direct and ground-reflected rays tend to produce a linearly or nearly
linearly polarized wave even when you start out circular.

This isn't true at least at VHF and UHF, where the ground reflection
mostly just reverses the polarization sense of the incident wave.
This have been demonstrated by the much-improved images seen on analog
TV receivers in city centers when using c-pol transmit and receive
antennas, because multipath reflections ("ghosts") tend to be
suppressed by the receiving antenna.

The polarization reversal on reflection occurs only when the wave is
normal to a large (in terms of wavelength) flat surface. If it reflects
at a glancing angle, the sum of the direct and reflected rays end up
being nearly linear, or at least elliptical, depending on the reflection
angle and reflection coefficient. Glancing reflections from ground are
just about impossible to avoid at HF, but the also occur at VHF and above.

Roy Lewallen, W7EL