The reason FM stations transmit circular polarization is to accommodate both horizontal and vertical receive antennas. Most fixed receivers are horizontal and most cars are vertical.
You can not transmit both horizontal and vertical polarization at the same time. Feeding a horizontal antenna and a vertical in phase will give 45 degree polarization. For simultaneous vertical and horizontal the antennas must be fed as circular. They then contain both the horizontal and vertical component. They are not doing this for the sake of circular polarization but only so vertical and horizontal polarizations can be transmitted together. TV has no need to transmit anything other than horizontal polarization as most TV reception is done with a horizontal antenna. 73 Gary K4FMX _____ From: [email protected] [mailto:[email protected]] On Behalf Of John Sehring Sent: Sunday, August 23, 2009 12:51 AM To: [email protected] Subject: Fw: Re: [Repeater-Builder] Diversity FM reception I turn out that use of CP in urban & suburban areas results in somewhat more signal strength on linearly polarized antennas, e.g. vertical whips on cars & straight rod aerials on portable FM radios. Due to preferential scattering of vertically polarized sigs from typical urban structures, there tends to be more of that available, esp. good for auto FM reception. The Germans for example are more concerned with signal quality than quantity & so don't use CP. However, there is a drawback: there's more multipath. So the tradeoff was made--more signal strength but at lesser quality (due to multipath distortion). Well designed FM radios reduce separation intelligently in the presence of multipath: first they gradually blend the stereo channels into mono, high audio frequencies L-R info first, then all audio (L+R) is gradually lowpass filtered. This happens dynamically, on the fly. Works well IMO when done properly. TV broadcasters tried CP as well but couldn't live the extra multipath: it was easily visible as more ghosting. See for example: http://www.ham-radio.com/k6sti/ for more on this. --John --- On Fri, 8/21/09, larynl2 <[email protected]> wrote: From: larynl2 <[email protected]> Subject: Fw: Re: [Repeater-Builder] Diversity FM reception To: [email protected] Date: Friday, August 21, 2009, 9:08 PM In reference to below, what would be the real advantage to using CP antennas in addition to the V and H you'd have already? Any signal that arrives will excite a V and/or H antenna according to it's arriving polarization, and I don't see where CP would be a help. Most FM broadcasters use CP. Those that don't are licensed for only V or H or choose to use a less-expensive single-polarization antenna. And many of them look like rototillers, and other shapes. Laryn K8TVZ --- In Repeater-Builder@ yahoogroups.. com, John Sehring <wb...@...> wrote: > > There's more to be done with polarization as well: Circular, both RH & LH. It is possibile to make omnidirectional CP antennas. FM broadcasters use a lot of them. They look like a bunch of arrows. >
