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Why Do Some FM Stations Have Different Vertical and Horizontal Power?

E

EJ204

Guest
Most FM stations have one Effective Radiated Power. In most of the Northeast and California, the maximum is apx. 50,000 watts on a tower 500 feet above average terrain. In the rest of the country, that goes up to 150 kw on a 2000 foot tower.

But some FM stations have different vertical and horizontal power. Why? Is there an advantage to doing that?
 
FCC regs. require licensed ERP to be in horizontal polarization. Vertical radiation is optional, but almost all FM's use some
kind of CP (circular polarized- 50% vert, 50% horiz) array to optimize reception in vehicles.Only several commercial
FM's in Calif. are still horizontal, see KSIQ. A special exemption is KSDS, San Diego, which is vertical because of a cross border
agreement with Mexico protecting it's nearby channel 6.
 
The maximum under the current rules is actually 100 kW, although there are a few scattered stations with a lot more power thanks to being grandfathered in.

One reason stations may have differing H and V powers listed on their license is their proximity to TV channel 6, like the earlier mentioned KSDS in Sandy Eggo. Stations below ~90 MHz have additional protection requirements. For example in the Birmingham, AL market all three stations below 90 MHz have veritcal-only polarization or have very low horizontal outputs, like WSJL, which is licensed for 20 kW V but only 10 watts H. As Big 121 said that's to help minimize interference to TV stations on channel 6, which is a holdover from the analog days when TV stations almost always broadcast in horizontal polarization only.

I have seen other stations outside the non-commercial band that have odd power levels, like 50 kW V and 44 kW H, but I'm not totally sure why they do that. I've been told that in mountainous areas like West Virginia and East Tennessee that dropping one or the other polarization helps with multipath and mountain reflections. (Anyone's who heard FM in Knoxville or Chattanooga knows what a mess mountains can cause.)

Hopefully an engineer can come along and set me (us) straight on this once and for all, because I'd like to know, too!
 
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