| Home |
| Search |
| Today's Posts |
|
#19
December 1st 08, 02:22 PM
posted to rec.radio.amateur.antenna
|
|||
|
|||
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 |
| Thread Tools | Search this Thread |
| Display Modes | |
|
|
Similar Threads
|
||||
| Thread | Forum | |||
| Circular Parasitic | Antenna | |||
| WTB: Heathkit Circular Dial | Equipment | |||
| Circular V.S. Vertical antenna polarization ! | Broadcasting | |||
| WANT : large QUAD hubs for 40m QUAD | Swap | |||
| circular radiation! | Antenna | |||
Threaded Mode
