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Publication Number: N/A
Date: November 1996 |
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 Pt 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 Pt into the antenna of 1000 W. The expected power density Pd 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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