> >It looks like the FCC rules give you extra power when opting for dual > >polarization.
No, they don't give you "extra" power. For commercial stations, horizontal polarization is the standard. You can supplement it with vertical, either as cross-polarized linear, or as elliptial/circular, but that Vpol component's ERP can't exceed the Hpol ERP. For non-commercial stations in the reserved band (i.e. below 92 MHz) within the affected area of a channel 6 station, there are many cases where they are authorized for more Vpol than Hpol to protect channel 6 (which is presumed to always be horizontally polarized). The only "extra" power you get is additional transmitter power output (TPO) due to the reduced antenna gain (assuming the number of bays remains the same, and the same bay spacing) when you go from horizontal polarizaton to mixed polarity. > That's a confusing point, I know. Every circularly-polarized FM > station I've seen (and that's a lot of them) use an antenna > design that > handles the phasing and "time-delay" to create the > circularly-polarized > signal. That's pretty much correct, but there are many stations that have a vertical component added that isn't necessarily part of a circularly-polarized array. The vertical may be added as a separate radiator, but not phased with the Hpol radiators to yield circular, so you just have two non-coherent linear polarizations. Or a single linear radiator may be tilted to give "slant" polarization, which the FCC will accept as having both an Hpol and Vpol component, with the ratio being a function of the tilt angle. > The license reference to H and V powers (regarding c-pol station) is > intended to say how much ERP should some out when the signal is V and > how much when it is H. It is possible to make the two components > different, resulting in elliptical polarization rather than circular. They can be different, and yet not be elliptical. If they aren't phased together to yield a coherent rotation at all azimuthal angles, it's just random cross-polarization, not elliptical. 99% of the current topic was covered a year or so ago on this list - might want to revisit the archives. For those thinking about building Cpol bays, I'd suggest starting out with something simple like a ring-stub. Easy to make with a tubing bender (or Armstrong method), feed with a gamma, DC-ground at the mounting bracket at the rear of the bay, decent pattern circularity (but not great axial ratio symmetry), cheap and easy way to start. For those not familiar, a ring stub bay looks like this (I don't recommend OMB, it's just a decent picture of a very basic ring stub bay): http://www.omb.com/en/index.php?option=content&task=view&id=78&Itemid=38 Ring stubs are sometimes also called "cycloids" (albeit sometimes erroneously), often built with a balanced feed. You can try Googling "cycloid", "ring stub FM antenna", etc. for more pics and design ideas or email direct. --- Jeff WN3A
