Between our Sirius satellite radio receiver, the MP3 player, breaks for public radio and talking to each other, I didn’t have much time to indulge my interest in exploring the high soil conductivities that make AM radio so anomalously advantaged in the plains states. But I did notice that KOA/850 from Denver carried halfway across Kansas by day, and WNAX was audible across all of Kansas, from one end to the other — from Colby to Kansas City and beyond that well into Missouri — with just 5000 watts on 570am from Yankton, South Dakota. (The max power on U.S. and Canadian AM stations is 50000 watts.)
Long disatance AM is no big deal at night, when stations bounce off the ionosphere. But in the day AM stations need to carry along the ground. In most places the ground conductivity is low. In the entire East, much of the midwest, and nearly all mountainous areas, ground conductivity is very low. The lowest of the low are around Atlanta and in Long Island. But in some prairie regions, parts of Texas and Oklahoma, and in flat places near San Franciso and California’s Central Valley, the ground conductivity is remarkably high. For that reason a 5000-watt station at the bottom of the dial (like WNAX/570 and KFYR/550 in Bismark) can go hundreds of miles along the ground. My mother grew up listening to both WNAX and KFYR in Napoleon, North Dakota, which was near neither station. WNAX is helped also by having a full half-wave antenna, which on 570KHz is around 900 feet high. So it’s using an unusually efficient radiator. Most stations at that end of the dial use shorter towers. Signals at those frequencies carry so well that going for the full antenna length would bring diminishing returns. (On AM, the whole tower is the antenna.) And by now they’re all grandfathered with whatever facilities they put up way back when. AM stations require a lot of real estate, so the costs are now, in most cases, prohibitive.
Still, while listening to these effectively huge stations, while driving across the plains, I realized why talk radio — especially the right wing sort — sank roots here. Though I gotta say it was great that WNAX was highly focused (at least when I listened) on “the markets” for agricultural commodities. Made me think the country’s agricultural base was somehow still intact.
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