Appendix E

Computation of Expected Electric Field Strength Based on
Transmitter Power and Antenna Characteristics.

Figure 1E. Signal radiating isotropically into a hemisphere.

For a known power into a transmitting antenna and a given antenna efficiency, the expected signal strength in dB µV/m can be determined at a specified distance from the transmitter. Assuming the signal is radiating isotropically into a hemisphere (Figure 1E), the power is evenly distributed over a surface equal to where r is the distance from the transmitter in meters.

The power density at the receiver expressed in W/m² can be determined by

where e is the efficiency of the transmitting antenna and P_t is the input power.

The electric field E in dB µV/m is determined by

where 377 is the impedance of free space measured in ohms.

Take as an example a transmitter with an antenna efficiency e between 15 and 20 percent and signal power P_t into the antenna of 1000 W. The expected power density P_d at 10 km is between 238.7e-9 and 318.3e-9 W/m² (Equation E1). The expected electric field strength E at the same distance is between 79.5 and 80.8 dB µV/m (Equation E2).

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