Alice is transmitting an electromagnetic beam to Bob.
Assume it is a well collimated beam, like a laser beam, or a maser beam.
As Bob's distance from Alice increases, his signal strength decreases according to the well known inverse square law.
Eve, the eavesdropper, is beside the beam, not in the beam.
As Eve's transverse distance from the beam increases, the strength of her signal decreases.
What is the formula for Eve's signal strength as a function of her transverse distance?
(see comment below for my speculations and subsidiary questions)