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caesarv

join:1999-08-02
Santa Rosa, CA

2 recommendations

[general] dBm vs. milliwatt confusion

Warning! This is a math/engineering lesson. Save yourself!!

While trying to get some info on the output power of various Linksys wireless systems, I have found all sorts of misinformation and/or confusion regarding dBm and milliwatts (mW). Some have incorrectly said that 0dBm=0mW. Some have said that the WRT54G outputs 84dBm, which is actually 250 Kilowatts!!! Yeah, I know, we all (incl. me) make stupid mistakes and occasionally use the wrong terminators. Here is a definitive description of the two terms.

0 dBm in the RF world is arbitrarily defined as 1 mW of power. From this, we get:

dBm = 10 * log(mW) (Note: power is in mW, not Watts!)
mW = 10 ^ (dBm/10)

Both of these can easily be calculated using the Windows Calculator in Scientific mode.

Here is a simple cross reference, including some commonly found values for wireless. Remember, for all practical purposes, every 3 dB is a doubling/halving of power, and every 10 dB is an order of magnitude difference.

-10 dBm = 0.1 mW
0 dBm = 1.0 mW
10 dBm = 10 mW
17 dBm = 50 mW
19 dBm = 79 mW
20 dBm = 100 mW
23 dBm = 200 mW
26 dBm = 400 mW
27 dBm = 500 mW
28 dBm = 631 mW
30 dBm = 1 Watt
40 dBm = 10 Watts
50 dBm = 100 Watts
60 dBm = 1000 Watts ...etc.


Some of the above numbers are approximated for clarity; 26 dBm actually equals 398.107 mW.

Use the term "dBm" when referring to some specific power level; never just "dB."   e.g. "This router has 23 dBm of power."

The phase, "This outputs 25 dB" is technically meaningless, although most will interpret this to mean 25 dBm.

The term "dB" is a relative term only; such as, "This router has 5 dB more power than that AP"

Let's keep those terminators straight!


cow116
Cisco Kid
Premium
join:2003-03-10
Indianapolis, IN
wow thats awesome caesarv! mind if i copy that down and use it next time i have to complain to my cable modem company about my speed issue?
--

michaelr7

join:2004-03-26
Tucson, AZ
reply to caesarv
A quick google found this site to do dbm mw conversions.

»www.aerialix.com/calculators/dbm···ion.html

There are probably more - this is just the first one in the Google results.


caesarv

join:1999-08-02
Santa Rosa, CA
well....fine! That link is is just too easy! Where is the challenge?....the sense of accomplishment?....the hair pulling?

There is a minor cosmetic bug in that page. It always puts a "+" sign in front of a dBm result, even if it is negative. So a "-10dBm" result will appear as "+-10dBm". Other than that, it works fine.

reply to caesarv
then logically, if my math makes sense..

a 250mw access point is about 24dbm..
if paired with a 24dbi antenna..
makes for a total output of 48dbm? or ~10watts eirp?


DaDogs
Semper Vigilantis
Premium
join:2004-02-28
Deltaville, VA

4 edits
If you have a 24 dBi gain antenna and you drive it with 24 dBm of transmit power and there are no losses in the system your EIRP will be 48 dB EIRP.

Since 36 dBm ~= 4 Watts

and since every three dB of gain equates to about twice as much power....

36 + 3 dBm ~= 8 Watts of ISOTROPICALLY RADIATED Power
39 + 3 dBm ~= 16 Watts of ISOTROPICALLY RADIATED Power
42 + 3 dBm ~= 32 Watts of ISOTROPICALLY RADIATED Power
45 + 3 dBm ~= 64 Watts of ISOTROPICALLY RADIATED Power

So NO your EIRP would be about 64 Watts rather than 10 Watts. What we need to understand here is that EIRP stands for Effective Isotropically Radiated Power or Equivalent Isotropically Radiated Power. It does not mean that you are actually transmitting that much power. It means that it LOOKS LIKE YOU ARE TRANSMITTING WITH THAT MUCH POWER BUT ONLY IN THE FAVORED DIRECTION.

The truly cool thing happening here which is so often overlooked it the idea that a good antenna can make a 250 mw transmitter LOOK LIKE a 64 Watt transmitter in the favored direction. That is a whale of a lot more interesting than power tweaks on custom Linksys firmware, don't you think?


--
The Sveasoft distribution model has been ruled compliant with the GPL by the FSF. Nobody else has a right or requirement to rule on it.


caesarv

join:1999-08-02
Santa Rosa, CA
The best analogy is a flashlight. A bare bulb (dipole antenna) radiates in all directions and lights up everything a little bit. But put that reflector on it and you can see many times further....but only within a small cone of light.

Putting a flat mirror behind a bare bulb would double the light in front of it (3dB). Putting it in within a parabolic reflector gives you a huge increase, but only in one very-narrow direction.