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The 98 foot coverage radius expectation for unlicensed AM might be related to FCC §15.209, which defines a maximum field permitted 98.4 feet (30 meters) from the transmit antenna that is hardly useful except for very good receive equipment in areas of very low radio noise.
Systems fully meeting FCC §15.219 and using resonant antenna systems can provide much higher fields at 30 meters than permitted by 15.209.
But if during an inspection the FCC observes that an unlicensed AM system doesn’t meet 15.219 in all respects (such as an elevated installation using a long, radiating “ground” conductor), then based on most AM NOUos they apparently consider that 15.209 applies to that installation, and make their analysis accordingly.
Often both 15.209 and 15.219 are referenced in unlicensed AM NOUOs.