The relation between the electric field and the transmitted power
In ideal free space, the electric field strength produced by a transmitter with isotropic radiator is readily calculated.
- E is the electric field strength in volts per meter
- P is the transmitter power output in watts
- d is the distance from the radiator in meters
It is clear that electric field strength is inversely proportional to the distance between the transmitter and the receiver. However, this relation is impractical for calculating the field strength produced by terrestrial transmitters, where reflections and attenuation caused by objects around the transmitter or receiver may affect the electrical field strength considerably.
Industry Canada states that between 1.75 and 10 MHz you can operate an unlicensed transmitter as long as the levels do not exceed 100 uv / m @ 30 m. Likewise between 13.1 and 14.10 MHz its 106 uv / m @30 m.