When the NPRM came out about AMs on FM translators, I made mention of the fact that the FCC should create a standard for the coverage of these translators not tied to the AM station’s 2 mV/m daytime contour. The reason is that exactly the same AM facilities will vary greatly in coverage across the U.S due to differences in ground conductivity.
The example I used was a 1450 AM with 1KW in Cornelia GA gets a 2 mV/m daytime contour at 9 miles from its tower while the exact same facility at Yankton, SD enjoys a 2 mV/m contour at 34 miles from its tower.
So. an exact copy of an AM in the midwest with one is the south will get nearly 4 times the coverage. So, the translator on the AM in the south or other regions of low AM ground conductivity will be restricted greatly compared to the one in the midwest.
I suggested a simple standard to be applied across the U.S. An FM translator duplicating an AM station can’t have a 60 dBu contour extending beyond 25 miles of the AM transmitter site. The maximum power allowed by the FCC for a FM translator is 250 Watts. So, how high up would the FM translator have to be to produce a 60 dBu of 25 miles. 1000 meters or 3280 feet..assuming the translator were to meet all signal separation rules.
The tallest TV tower in the U.S. is just over 2,000 feet so you can see that in reality, there will be no FM translators producing a 60 dBu of more than 25 miles…not even close. However, why limit them to just 9 miles to equal the AM 2 mV/m contour when in fact, technically the contour might could go 12 miles if not limited by the AM 2 mV/m contour?
I’d even settle on a 20 mile limit for the translator 60 dBu.
Douglas M. "Art" Sutton, Jr., President/CEO
GA-Carolina Radiocasting, Toccoa GA