transmit power converted to dBm (1 dBm == 1 mW) minus the attenuator
loss = output power in dBm.
100 mW -> 20dBm
20dBm - 15 db att = 5 dBm
5 dBm -> 3.2 mW
Actually, I think 0 dBm = 1 mW.
dB’s are a royal pain in the butt. They eluded me for years because
they required a lot of rote memorization and made no sense. For those
of us not pickled in radio-speak from an early age, but who know basic
algebra, there’s a simple way to deal. Ignore deciBels. Use Bels.
Bels are easy and obvious. They’re a straight logarithmic scale in Base
100 mW is 2 Bm. 10 mW is 1 Bm. 1 mW is 0 Bm. 0.1 mW is -1 Bm.
DeciBels are just tenths of a bel. So if you shift the decimal point
one place, you’re suddenly calculating in an easy to use notation.
Here’s the above calculation in Bels:
100 mW -> 2 Bm
2 Bm - 1.5 B att = 0.5 Bm
0.5 Bm -> 10 to the 0.5 power -> the square root of 10 -> about 3.2 mW
See, now you not only know the answer, but you know WHY “5dBm” is 3.2
Why the EE universe settled on doing everything in tenths of a
logarithmic unit is way beyond me. It’s as if every carpenter figured
every length in deciInches or decimeters, even if inches, kilometers
or meters would be the more straightforward unit. How often do you
calculate in decivolts, deciwatts, or decimeters per second per
The rumor is that decibels were invented because somebody at Bell Labs
couldn’t cope with decimal points or negative numbers, in the days when
equipment wasn’t capable of dealing with large orders of magnitude
(e.g. the painful-to-someone 0.3 Bel became the friendly-to-someone 3
deciBel). Of course, now that people regularly see 5 to 10 orders of
magnitude (5 to 10 Bels) (50 to 100 deciBels) (factors of 10000 to 10
billion) in ratios, such as in radar, digital signal processing, or
fiber optics, the “deci” has just become a hindrance.
You can do your part to clear up this idiocy by using Bels in most
places where the lemmings use deciBels. You may actually get them to
PS: Don’t even get me started about why dBm’s aren’t referenced to
watts rather than milliwatts! Since a “milli” is 1/1000th and that’s
just 3 orders of magnitude, referencing to ordinary watts would merely
involve subtracting 3 or 30 from the number, e.g. 40 dBm = 4 Bm = 1 BW
= 10 dBW. It reminds me of how we’re still calculating speeds in
5280-foot units per 3600-second units rather than in some sane system
using basic decimal units. Actually using BW notation in your
thinking and writing may overload lemming brains, though.