> Yes, CP does cause more multipath esp. in urban environments.
> Turns out there are a large number of buildings the
> preferentially reflect V better than H. CP gives V energy
> othwise lacking (mostly) in a strictly H xmt situation. What
> you get with strictly H pol. is quite a glorious & random
> mess of polarizations.
I refer to another post I just sent, and I tie in another concept. If the
reflecting surface is large enough, it's going to reflect both V and H
components. At VHF and UHF frequencies, any building of appreciable size is
going to reflect both. A typical urban environment comprised of
dozens/hundreds of large buildings is going to bounce around enough Vpol and
Hpol energy to cause multipath no matter what the originating station was
transmitting, H, V, or C.
Any surface that is anything other than a perfectly flat plane is going to
cause distortion of the incoming wave, distorting/skewing the polarization.
And when it comes to Cpol, the polarization sense is flipped when it is
reflected, RHCP becomes LHCP and vice-versa. So, suffice it to say, when
you're talking about reflections in an urban environment, or a mountainous
region, or in reality, just about anywhere other than in the flatlands with
no nearby obstructions (farmhouse with a nice yagi on the roof), you may as
well assume you've got mulitpath comprised of a mix of polarizations.
> Neither the BBC or the Germans use CP in FM broadcast, strictly H.
And they also don't have the FCC, but we won't get into that...
> TV tried & abandoned CP due to ghosting.
Well, I wouldn't say that. One of the local high-power UHF stations,
relatively high in the band, replaced their antenna about 10 years ago, and
went to CP. Their coverage noticibly improved. I live about 15 miles from
the transmitter and saw quite an improvement. The antenna is a typical slot
with sort of Z-shaped parasitic elements on the outside of the slots which
couple a little horizontal energy, converting it to vertical, to create the
Cpol.
One of our contracts is with one of the new nationwide 700 MHz services that
broadcasts digital mobile TV subscription services. Subscribers receive on
cell phones with extendable whip antennas. Those can be oriented in any
position, but would typically be held vertically. Guess what polarization
they're using?
Also keep in mind that 8VSB isn't the same game as NTSC/VSB, especially when
it comes to receivers and how they deal with multipath. What worked well
(or didn't work well) for analog TV doesn't hold true for DTV. For example,
lowband is no longer the place to be!
--- Jeff WN3A
