[MLB-WIRELESS] Wireless Cards

[Richard Nelson]

Yeah. It's not well understood, but those who know it well can do path
calculations and tell you in advance whether a link will work well, not
at all, or overload your receiver.

Drew wrote:
> 
> it seems there is a lot of conflicting info on this subject
> 
> i swear i've seen a formula of power * gain = eirp which with a lucent
> card and 18dbi would give .03 * 18 = .54W EIRP

If you could tell us where you have seen this formula we could have a
look and see if we can work out what is going on, but I am certain that
there is something funny because AFAIK you never multiply dB.  The whole
point of using dB is to enable calculations just by adding.  Remember
this stuff was all started before the days of electronic calculators.  

> 
> then there's matthew's formula which would give .03 * 64 = 1.92W EIRP
> 
> then there's ydi's formula
> (http://www.ydi.com/decibels-radio-systems.php) which would give
> 18dbi + 15dbm = 33dbm = 2W EIRP

This is the result I agree with.
These two numbers (and formulas) are so close as to be effectively the
same.  In radio terms you generally round to the nearest dB so results
are typically accurate to +/- 10% or more.   The ratios that we deal
with are really large.  If your receiver power is -80dBm then that is a
one hundred millionth of a milliwatt or a ten billionth of a watt.  When
you have a ratio of ten billion or so then a mere 10% here or there  is
nothing really.   This is also why the difference between a 30mW card
and a 100mW card is much much less than many people realise.  It is all
in the antenna gain (and cable/connector losses).
> 
> then there's a pretty confusing one at
> http://www.personaltelco.net/index.cgi/FccRegs?action=highlight&value=eirp
> 
> aimed at the USA
> 
> ugh.
> 
Ugh is right.  That appears to be more to do with FCC regs than actual
power calculations.

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