[MLB-WIRELESS] DLink 802.11b+ and 802.11a RRP's

[Tony Langdon]

> Also while we are at it can someone explain the difference 
> between power 
> ouitput listed in mW and dB?

All means the same.  Power is usually expressed in mW (pretty obvious) or
dBm (decibels relative to 1 mW).

A decibel is merely the ratio between 2 power levels (let's ignore voltage
and current ratios for now, these confuse things too much and serve no
useful purpose in this context).  The decibel is also a logarithmic scale -
every 10 dB increase is the same as increasing the power by a factor of 10,
so an increase of 20 dB is a 100 fold power increase.

For the mathmatically inclined, you can calculate dB as follows

dB = 10 * log10 (P2/P1), where log10 is logarithm to base 10 ("common log"),
P2 and P1 are the two power levels being compared.  These may not
necessarily be electrical power, as the dB is also commonly used in audio.
One could even say that one make of car has 3dB more power than another, if
it is twice as powerful! (must be a Holden with the extra 3dB :-) ).

And going the other way..

P2/P1 = exp10 (dB/10).  i.e. P2/P1 = 10^(dB/10)

If you're not mathmatically inclined, I have written up a simple conversion
table on the wiki at http://wireless.org.au/wiki?PowerAndGainToDecibels.

dBm is decibels relative to 1 mW (i.e. P1 is set to 1 mW).  So for a 30 mW
card

dBm = 10 * log10 (30/1) or approximately 15 dBm.

Note that expressing power in dB doesn't make any sense.  power is in dBm
(or dBW, if relative to 1W).  However, amplifier gain should be measured in
just plain old dB, since the gain of an amp is literally the ratio of output
power to input power (i.e. output = P2, input = P1 in the equation above).

Antenna gain is a funny one.  It is a ratio also, but the ratio is defined
as against some sort of reference antenna.  The most common reference is the
"isotropic antenna".  An isotropic antenna is simply an (imaginary, you
can't actually physically build one) antenna which radiates equally in all
directions.  

Antenna gain is the ratio of power density in the direction of maximum
radiation to the power density that would result if the reference antenna
was used.  If the reference is an isotropic antenna (most common), then the
gain is usually expressed in dBi.  Another possibility is dBd, which uses a
1/2 wave dipole as a reference antenna.  FWIW, a dipole has around 2.14 dBi
gain, so converting from dBd to dBi means simply adding 2.14 to the figure
given.

In summary:

Power is expressed in mW or dBm
Amplifier gain is expressed as a ratio (Po/Pi) or in dB (no suffix)
Antenna gain is usually expressed in dBi (though sometimes dBd).

Converting to/from dB involves the use of logarithms, though simple lookup
tables like the one on the wiki are good enough for our purposes.

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