AN0005 Datasheet, PDF (2/2 Pages) RF Micro Devices – Converting Field Strength to Power

AN0005
To convert the power to decibels relative to a milliwatt (dBm), take the common (base 10) logarithm of the power and add
thirty as below:
P_Trans(dBm) =
10 ⋅ log
4-----⋅_---π_-----⋅_---D_----²_-
377
⋅


-1----_â‹--^E-1---0---₆-_
2
+ 30
Finally, this equation can be simplified to
P_Trans(dBm) = 20 ⋅ log (D ⋅ E ) – 104.7713
If the distance is three meters as is the case with many 47 CFR 15 specifications, the equation becomes
P_Trans(dBm) = 20 ⋅ log (E) – 95.2289
Again, D is in meters and E is in µV/m. With this set of equations, one can convert between field strength and power.
Below is a table containing some commonly encountered field strengths converted to power in dBm using the above
equations.
Field Strength
200µV/m @ 3m
500µV/m @ 3m
1250µV/m @ 3m
12500µV/m @ 3m
50mV/m @ 3m
Power
-49.20 dBm
-41.25 dBm
-33.29 dBm
-13.29 dBm
-1.25 dBm
15
15-22
Copyright 1997-2002 RF Micro Devices, Inc.