Richard Seriani, Sr. wrote:
> Your disagreement seems to be based on your misunderstanding of what 
> decibels serve to represent. Where did you get the idea that you could 
> convert amps directly to dB?
>   


I can convert whatever I want to decibels. (Why wouldn't be able to?)

I have 100 dogs = I have 20 decibel dogs
I have 1 million dollars = I have 60 decibel dollars
The universe is 6 billion years old = The universe is 98 decibel years old
The diameter of Jupiter is 142,984,000 m = The diameter of Jupiter is 82
dB m



> Decibels represent the RELATIONSHIP between two values of power.


The bel is a handy way of dealing with really big numbers, nothing more.

1 bel = 10
2 bels = 100
3 bels = 1000
4 bels = 10000

Yes, people seem to use it a lot for describing a ratio, but that's
because the ratio might be a really big number. For instance, instead of
saying "The gain of this amplifier is 60 dB", there's nothing stopping
you from saying "The gain of this amplifier is 1 million".

There's nothing about the bel that forces it to be used as a ratio.


>  There are also references used by acoustitians, dB SPL 
> (referenced to the lower threshold of human hearing), and music, dBVU 
> (represented to 100% modulation).
>
> So, if you want to use amps as your reference, the calculation should be 10 
> log (I1/I2), assuming base 10 logarithm. However, dBA is already taken 
> (frequency filtered SPL measurements), making dBmA confusing (1/1000 of 
> dBA??)
>   



I had a space between "dB" and "mA".



>> Why one earth do people write "decibel milliwatts" as "dbm", i.e.
>> without the W to indicate watts?
>>     
>
> They don't. They write dBm as decibels IN REFERENCE TO 1 milliwatt and 600 
> ohms load.
>   


I've heard people say that the power output of a particular transmitter
is X dBm's.


>> 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"???
>>     
>
> Apples and oranges. If you left off the dBm your answer, you'd still lose 
> points. Actually, it you wrote it as dBmW, you'd lose points because that 
> reference is not defined. See above.
>   



"dB mW", with a space.




>> Where did this practise start, and why do so many people perpetuate it?
>>
>>     
> Does it really matter?


The reason I ask is that I'm in a position where I have to teach some of
this stuff, so I want to be able to explain why they'll hear that a
transmitter has a transmittal power of X amount of "dBm".

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