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You wrote: What I’ve wondered with some of these cases is why there’s a tendency to measure the field strengths at around 300 meters distance.
It has been many moons since I was an invited guest in a FCC monitoring van but I do remember that the equipment was bulky and rack mounted. It would be rather difficult to drive the van into a dwelling or onto a roof to get to within 3 meters of a radiating element.
All they have to do is demonstrate that the radiation exceeds the 250 uV/m limit at a distance beyond 3 meters. If, for example, the measurement is 3000 uV/m at 100 meters then any rational person will accept that it is over 250 uV/m at 3 meters. Though the actual FS at 3 meters is not measured it is clear that this example operation exceeds the limits.
If you read the NOUOs on the FCC site, you will conclude they are not dealing with operators who are “just a few uV/m” over the limit and it would be very difficult to argue that multipath or other effects biased the measurements to an extent that a legal operaton would appear to be in violaton.