I don't know the threshold specs involved, but you can crudely estimate
field strength if you know transmitter power. The "impedance" of free space
/ air is 377 ohms. Thus the ratio of RF field voltage and current will be
fixed and related to the power density - unless the field is distorted.

As another crude approximation, assume that (unless you know a radiation
pattern) that the unwanted signal's power is equally distributed over a
sphere or maybe a hemisphere.

The above will allow you to compute a crude approximation of field strength.

The FCC requires hams to make these calculations with transmitters above
certain power thresholds.


How close does this come to the stated "safe" field strengths? I certainly
wouldn't want to come as close as 20 dB (10 times voltage) to the stated
thresholds.


TV/FM broadcasting stations have very little field strength directly under
their tall towers. All broadcasting stations are not allowed to exceed
certain field strengths on the ground outside their fenced compounds. The
TV/FM stations in a hilltop residential area in Portland, OR, had to greatly
increase their tower heights when this rule came into effect.

de w3otc