Virchanza wrote:
>     Say I tell you that a particular transmitter transmits with a power
> of 500 mW, well you can express that as decibel milliwatts as follows:
>
> 10 * log10(500 milliwatts) = 27 decibel milliwatts
>
>     What I'm curious about though, is why so many people write 27 dBm
> instead of 27 dBmW. Why does everyone leave out the underlying unit of
> "watt"?

Sorry for posting this twice, I forgot that I hadn't subscribed to [EE]
and so I was wondering what happened when my first post didn't show up.

  Gokhan SEVER responded to me as follows:

"P is defined as 10*log10(p / 1mW) with a unit of dBm. And here m means
decibels relative to a power of one milliwat (1 mW). Therefore we don't
need
to use an extra milliwatt in our final calculation. "

I don't agree with this. For instance, let's say I take the following
four sentences:

The current is 1 amp.
The current is 10 amps.
The current is 100 amps.
The current is 1000 amps.

Now let's say I convert them to milliamps, giving:

The current is 1000 milliamps.
The current is 10000 milliamps.
The current is 100000 milliamps.
The current is 1000000 milliamps.

Then I convert to bels:

The current is 3 bel milliamps.
The current is 4 bel milliamps.
The current is 5 bel milliamps.
The current is 6 bel milliamps.

Using "tenths of a bel" instead of bels, I get:

The current is 30 dB milliamps.
The current is 40 dB milliamps.
The current is 50 dB milliamps.
The current is 60 dB milliamps.

So the unit I'm left with is "decibel milliamps", or in shorthand: "db
mA". Note the presence of "A" to indicate amps.

Why one earth do people write "decibel milliwatts" as "dbm", i.e.
without the W to indicate watts? I remember when I was back in college,
we used to get docked marks if we left the units out; for instance if
you gave an answer as "5" instead of "5 amps", you'd probably only get
90% instead of 100%. So why is it OK to leave out the "watt" in "dBm"???
Where did this practise start, and why do so many people perpetuate it?

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