> 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.
If you're going to use CP on a repeater, I don't see why you would want to
also mix in linear H and/or V, nor why you would want both LHCP and RHCP.
The users are all going to be using linear polarization. As long as you are
using one rotation of CP, that's all you need.
> 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.
Most FM uses CP - true. Why some FM stations use only V or only H or some
ellipitical ratio where H <> V is for a variety of reasons, but here are the
most common:
1. Historically, FM, like TV, was predominantly horizontally polarized.
The regulations, to this day, still favor the use of horizontal
polarization, and with few exceptions (especially #2 below), you have to
have at least as much horizontal power as vertical power. Commercial
stations in the non-reserved band (i.e. above 92 MHz) that are still running
horizontal-only are doing so by choice, not by rule. It wasn't until car
radios with vertical whip antennas started to gain popularity did vertical
polarization start to become important, and CP resulted as a solution to
satisfy listeners using either horizontal or vertical antennas, while
improving multipath performance as a side-benefit. Of course this also
meant that broadcasters needed 2X the transmitter power, or 2X the number of
antenna bays, to achieve the same amount of ERP, to convert from H to CP.
2. Non-commercial stations in the reserved band (i.e. below 92 MHz) that
are within the "affected area" of a channel 6 TV station are required to
protect that channel 6 station. The rules regarding how this protection is
accomplished are the most twisted, tangled mess of lawerese engineering that
ever came out of the FCC IMHO. Anyway, in order to afford protection to
channel 6 TV, which is horizontal, non-comm FM's often end up being
restricted to less H than V (or sometimes V only) in order to get the
population within the interference area down to allowable levels. The
Channel 6 rules of 47 CFR 73.525 are the cause for probably the majority of
cases where FM stations have more V than H.
3. Directional antennas. In cases where stations are using a directional
antenna to meet protection requirements, the measured pattern of the
directional antenna may have been such that the peak horizontal power may
have been different than the peak vertical power, and, as such, the licensed
H and V ERP values reflect that difference. The station where I'm typing
from currently has a directional antenna that was designed supposedly such
that H = V, but when they put it on the antenna range, the measured pattern
came out with H <> V in the major lobe, resulting in it being licensed for
12.5 kW horizontal and 11 kW vertical.
--- Jeff WN3A
