Electromagnetic Field Theory
In case of surface waves the field strength at a point is directly proportional to

Wave frequency
Distance of the point from the antenna
Antenna height
Current of the antenna

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Electromagnetic Field Theory
Gradient of scalar field is expressed as

 Maximum rate of increase of scalar function at a point.
 Outward flux of a vector field per unit volume as the volume about the point tends to zero.
 Circulation of a vector field per unit area as the area tends to zero.
 Gradient of divergence of a vector field minus the curl of the vector field.

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