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#1
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Quad and circular polarization
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. RF |
#2
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Quad and circular polarization
On Dec 1, 8:15*am, 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 has 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. RF |
#3
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Quad and circular polarization
Richard Fry wrote:
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. RF Can you show us one of these C-POL receive antennas? |
#4
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Quad and circular polarization
On Dec 1, 8:28*am, Dave wrote:
Can you show us one of these C-POL receive antennas? Here's a link to one example for VHF and UHF TV... http://www.kathrein-scala.com/catalog/HDCA-5CP.pdf Of course the Lindenblad, or even 1/2 of one could be used, also. Using half a Lindenblad would make it true c-pol only in two directions, but if you can aim it in the right direction that may be OK. RF |
#5
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Quad and circular polarization
On Dec 1, 9:07*am, Richard Fry wrote:
Here's a link to one example for VHF and UHF TV... Oops. That one is VHF only. |
#6
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Quad and circular polarization
"Richard Fry" wrote in message ... 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 RF Hi Richard The Lindenblad is Omniazimuth CP. The QHA is hemispherical CP. Some explanation of the DCA is shown in the Feb 2008 QST. Jerry |
#7
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Quad and circular polarization
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 |
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