h/v vs circular polarization

[AmpedNow]

I notice that while most FM stations use circular polarization, some use horizontal/vertical polarization. The ones that do are mostly in urban areas, like the NYC stations.

What's the difference between the two? And what are the advantages and disadvantages of each?

I barely know what polarization is, so please use lay mens terms...

[genlock]

The term "circular polarization" is often used erroneously to describe mixed polarity signals used mostly in FM radio (87.5 to 108.0 MHz), where a vertical and a horizontal component are propagated simultaneously by a single or a combined array. This has the effect of producing greater penetration into buildings and difficult reception areas than a signal with just one plane of polarization.

[genlock]

http://en.wikipedia.org/wiki/Polarizati ... etic_waves

[AmpedNow]

So, there really is no difference? It's all the same?

I just wondered why why some stations use "x" kw/h, "x" kw/v when listing their ERP while most just say "x" kw.

[Tom Taggart]

It's not all the same, when viewed from the receiver.

Circular polarization--as broadcasters know it, is used because it works best with randomly polarized receiving antennas. Think clock radios with line cord antennas.

For limited power/limited range installations, v-pol is the best compromise choice. Example: translators or boosters, for a car radio audience, where the receive antenna is vertically polarized. But a good argument can be made that a single bay cp antenna is best, despite the ERP penalty (requires twice the transmitter power for the same ERP).

Horizontal is better for long distance relay uses--television before everyone went cable--where there is a fixed antenna on the receiving end.

Most commercial stations have the same power in the vertical and horizontal plains. The total is of these radiated powers is used to determine RF exposure limits, but that is the close-in field. It would not be accurate to use this combined ERP for anything far-field, because the receive antenna is going to be either H or V pol., or some intermediate polarization between (which would theoretically collect half the voltage from either the V or H signal).

The FCC assumes FM coverage is determined by the horizontal field, and calculates coverage and interference accordingly. The exception is for reserved band FM stations near a Channel 6, where they will allow stations proposing H and V antennas where the H-pol power is severely reduced to protect the Channel 6 station's coverage as received on H-pol antennas.
One I saw discussed recently has 120 watts H and 7 kw vertical.

AM stations, of course, are almost universally V-pole, most using a form of dipole where there is quarter-wave vertical radiating element above a number of quarter-wave radials, which are horizontal because they are buried in the ground.

[genlock]

So, there really is no difference? It's all the same?

I just wondered why why some stations use "x" kw/h, "x" kw/v when listing their ERP while most just say "x" kw.

For promotional use, It sounds better to quote a higher ERP.