Explain FM Power - Polarization? Maximum ERP?

When we look at the power levels at FM radio stations, most are simple. One station has an effective radiated power of 50,000 watts. Another is powered at 100,000 watts. Power + antenna height tells us the reach of a given FM station.

But some stations are more complicated. Some have different horizontal power vs. vertical power. Sometimes they're slightly different. But I notice some stations even have 0 watts vertical power. All their power is horizontal.

Also some stations, especially in Canada, have both an effective radiated power and a maximum effective radiated power. Really? Wouldn't every station broadcast at its maximum power?

Gregg. said:

But some stations are more complicated. Some have different horizontal power vs. vertical power. Sometimes they're slightly different. But I notice some stations even have 0 watts vertical power. All their power is horizontal.

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Different H and V power is usually a non-interference condition for FM allocations near TV channel 6, or a former TV 6. This is given in FCC rules at 73.525(e)(4).
There might be other conditions where different H and V powers are used.

Gregg. said:

Also some stations, especially in Canada, have both an effective radiated power and a maximum effective radiated power. Really? Wouldn't every station broadcast at its maximum power?

Click to expand...

I'm less familiar with Canada's regulations, but I believe this only applies to directional antennas on FM. The "Average ERP" is the average power in all directions, and the "Maximum ERP" is the effective power in the antenna's most favored direction.

While the FCC allows directional antennas on FM, they only deal with the maximum power. When a Class B1 station uses a directional antenna, its ERP cannot exceed 10kW in the most favored direction, and the FCC is does not regulate the average power. Instead, the FCC applies regulations that prohibit deep nulls.

FM transmission technology has evolved a lot over 75+ years.

Until the late 1960s, FM signals were transmitted only with horizontal polarization, with the expectation that a correspondingly horizontal rooftop or indoor antenna would be used for reception.

When the FCC began allowing FM stations to operate with mixed polarization in the 60s, it made it possible to achieve much better mobile reception on the typically vertical car whip antennas or portable radio whip antennas of the era. (And the stations on the 88-92 band that had to protect channel 6 TV stations by operating with only V-pol likewise had issues being received on H-pol home antennas.)

But because antenna design took some time to catch up, mixed polarization was often achieved with separate H and V antennas at the transmitter side, which is why you often saw those separate H and V power levels, especially at early master antenna sites.

It wasn't until well into the 1970s that the first true circularly polarized FM antennas went into widespread use, with equal H and V power. By the 1980s, those antennas became the industry standard, and it's only in specialized rare cases now that you'd think about running anything other than a CP antenna.

And as others have noted, for directional FMs, the Canadian "max ERP" is the same as the FCC "ERP" - the US licenses directional stations based on ERP at the maximum lobe of the DA, while Canada uses an average across all azimuths of the DA pattern for its "ERP" field and then shows the max lobe power separately.

fybush said:

FM transmission technology has evolved a lot over 75+ years.

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An unusual sidebar: when I built the first FM stations in Ecuador around 1965 to 1967,all five of them were located in the city of Quito. The terrain there is extremely mountainous. Through considerable experimentation, we found that we got the best coverage with vertical polarization. We built a variety of antennas using industrial copper tubing, and finally decided to use only vertical polarization for all of the stations in their final installations.

Gregg. said:

When we look at the power levels at FM radio stations, most are simple. One station has an effective radiated power of 50,000 watts. Another is powered at 100,000 watts. Power + antenna height tells us the reach of a given FM station.

But some stations are more complicated. Some have different horizontal power vs. vertical power. Sometimes they're slightly different. But I notice some stations even have 0 watts vertical power. All their power is horizontal.

Also some stations, especially in Canada, have both an effective radiated power and a maximum effective radiated power. Really? Wouldn't every station broadcast at its maximum power?

Click to expand...

The other category of FM stations with different horizontal and vertical power are grandfathered “super power” stations that reached their antenna height and power before the present power rules were enacted in 1962. One example is WLFP (ex-WMC-FM). The WLFP license has an authorized horizontal effective radiated power of 290,000 watts but an authorized vertical effective radiated power of 96,000 power. When WLFP added vertical polarization in the early 70’s, the FCC limited them to the maximum vertical power for their class.

Decades ago, when WBBQ-FM was using the former WJBF tower in North Augusta for the FM antennas, I tjhought their on-air promos indicated 137kW power, with 100kW on the horizontal polarization and 37kW on the vertical polarization.

I looked at the 1970 Broadcasting Yearbook and the listing has 35kW for vertical. I might have a bit of a memory lapse on the vertical polarization power.