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Most portable radios can easily receive signals of around 100uv, and many good radios, i.e., home stereos, go down to 25uv or so.
The discussion in this thread is about the fields of AM broadcast signals.
With more research, one might learn that the ambient r-f noise floor in the AM broadcast band existing in many urbanized areas can require a received field intensity greater than 20 mV/m for useful reception, no matter the r-f sensitivity of the AM receiver.
Such fields are much greater than suggested in the above quote.