Free
space path loss
Updated May 23,
2008
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Free space path loss (FSPL) Rule of Thumb, by Stefan from Fraunhofer
IZT, an institute that publishes some of the coolest microwave research.
Free space loss can be estimated
in your head by:
22 dB for the first wavelength,
plus an additional 20xlog(number of wavelengths in the path)
According to Mike, another way
of stating the rule is: 22 dB for the first wavelength plus 6 dB
for each doubling of distance. Estimate how many doublings of the
first wavelength are needed to arrive at the required distance,
then multiply this by 6 and add 22. Got that?
Example
Frequency: 1 GHz
Distance: 1 km
Wavelength at 1 GHz is 30 cm.
Take 22 dB for the first wavelength.
There's 100 cm in a meter and
1000 meters in a kilometer, so the distance is 100,000 cm
Number of wavelengths for a distance
of 1 km:
100,000/30=3300 wavelengths
Now take "normal" dB
of 3300
10xlog(3300)=~10xlog(10,000)10xlog(1/3)=40dB5dB
(actually, 10xlog of 1/3 is
4.77 dB but this is an approximation, right?)
Because of the 20 log.... the
35 dB becomes 70 dB.
Sum the two contributions to
arrive at the estimated path loss at 1 GHz for 1km:
22 dB + 70 dB = 92 dB
With some decibel
numbers in memory this estimate can be pretty easy.
Remember:
Factor 2 = 3 dB
Factor 3 = 5 dB (4.77 dB)
Factor 10 = 10 dB
For a more accurate calculation
of FSPL, go to our download center and loof for Tim's spreadsheet
on this topic.
