Not exactly. AM is bloody expensive to operate. The entire tower is the antenna, and has to work against a copper wire ground system of radials that are silver soldered together at the base. The ground system degrades over time, requiring maintenance. By contrast, an FM antenna may get hit by lightning and come down as charcoal, but there's no ground system. A great number of AM stations are directional, meaning more than one tower, sometimes up to a dozen, each with a tuning unit and ground system all driven by a custom built phasor. The antenna systems are often less than stable, require periodic measurements done in the field to confirm legal operation. There could be dozens or more monitor points, and someone has to visit each one with a field strength meter to measure them, often monthly. If someone puts a new cell tower up in the area, it can drive the directional pattern out of legal limits, and that new tower has to be de-tuned. All that for a station that doesn't command the audience of an FM stereo music station, and may not even be permitted to operate at night. Locally produced talk formats are the most expensive to do, though small AMs use networked programming. A 50Kw non-directional station slurps nearly double that power, placing it in a different class of electric utility customer. And then there's the land. Stations may occupy a dozen or more acres of land, which needs to be near to their service area, which now conflicts with residential properties, wet land restrictions, etc. You can't just stick them in a corn field. FMs can go on top of tall buildings in the middle of their service area. FM towers, if stand alone, are also simpler to share and rent out space for other tenants like two-way systems, cells, and other broadcasters. AM towers usually have "hot" bases insulated off ground by a huge porcelain insulator, so to cross that with anything requires a special isolation transformer making adding other services to the tower costly. The other slight advantage an FM may have is antenna gain. An FM can get an effective power increase by a gain antenna which can double or triple the effective power over what comes from the transmitter, where an AM is stuck with whatever efficiency factor the tower has. 5Kw for an AM is just 5Kw, but a 5Kw FM can behave like 20Kw. So an FM may consume less electricity to hit its licensed power if an gain antenna is used.
Yes, AMs can go farther than FM, particularly with high power at night, but not always. They are also impacted by ground conductivity, so in some areas of the country 1Kw covers 20 miles, in others, half that. But stations of either kind get no advertising benefit from fringe coverage. Once a station covers it's primary area, it there's no real financial advantage to covering farms and bodies of water for 100 miles.
AMs are more expensive to operate than FMs on a per-watt basis.
How's that for running the thread off the rails? Sorry, everyone. Hope you enjoyed the ride.
edit: my usual stupid typos.
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