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## Re: [Discuss-gnuradio] to prevent mental damages, avoid dB's.

 From: Berndt Josef Wulf Subject: Re: [Discuss-gnuradio] to prevent mental damages, avoid dB's. Date: Fri, 29 Sep 2006 08:56:40 +0930 User-agent: KMail/1.9.4

```It don't see how this makes the calculation of RF power any easier, to the
contrary it confuses the issue.

cheerio Berndt

On Thursday 28 September 2006 17:50, John Gilmore wrote:
> > transmit power converted to dBm (1 dBm == 1 mW) minus the attenuator
> > loss = output power in dBm.
> >
> > E.g.
> >   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
> 10. 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 mW.
>
> 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
> second?
>
> 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
> think (briefly).
>
>       John
>
> 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.
>
>
> _______________________________________________