[MLB-WIRELESS] Wireless Cards

[Richard Nelson]

Decibels (dB) are 10*the log of the gain.  So to get the gain you divide
by 10 and take the inverse log (10^x).  The gain of a 24 dB antenna is
then 10^(2.4)= 251 (on my calculator).  It is a bit confusing but the
advantage is that you simply add the dBs of the elements in the system
to get the total gain rather than having to multiply each time.  By this
system, 3dB is  2* gain, 10dB is 10* gain, 18dB is 63* gain.

Note that dBi is specific to antennas in that it means relative to an
isotropic antenna (i.e. directionless).

You will often see radio power output given in dBm which is relative to
a nominal 1 milliwatt.   So a 30mW card is 15dBm, a 70mW card 18.5dBm
and 100mW is 20 dBm.

To stay below 4W EIRP which is 4000 * 1mw or 36dBm the card output and
the antenna gain need to sum to less than 36.  So with a 24dBi antenna
you should use a 12 dBm radio stage (15mW).  That does not take into
account losses and in practice you would be quite safe with a 30mW card
as you could expect at least 3dB loss at your connector.  If you had a 
long cable you could use a higher power card within the limit.  I
imagine that you would be very unlikely to be pulled up for illegal
emissions with a 24dBi antenna and a 100 mW card, but you might if you
used a 4 W amplifier as well.

Richard.

Drew wrote:
> 
> I know where you get the 256 figure, but I don't think it's applicable
> to find EIRP.
> 
> I can't find my original source for the formula, but a quick search
> finds http://www.hps.org/publicinformation/ate/q773.html which backs me
> up. According to that, 4W w/ 24dbi would be a max of .166W input, which
> is greater than the .1W I quoted.
> 
> -d
> 
> Matthew Vella wrote:
> 
> > Errm, don't wish to sound picky here, but 24dbi gain is a multiplier of
> > 256 - so 25.6W EIRP in your example!
> >
> > This assumes no loss in the cabling, of course :-)
> >
> > ----- Original Message -----
> > From: "Drew" <drew at wirelessanarchy.com>
> > To: <melbwireless at melbwireless.dyndns.org>
> > Sent: Tuesday, October 23, 2001 12:13 PM
> > Subject: Re: [MLB-WIRELESS] Wireless Cards
> >
> >
> >
> >>most pcmcia cards have isa/pci adapters you can use, although there are
> >>some straight pci cards, such as one model of the compaq's. nobody
> >>actually makes a 4W card, as hooking it up to any kind of antenna would
> >>put you over the limit. ex. cisco 350 w/ 24dbi dish would be 0.1W * 24 =
> >>2.4W EIRP. You can get amplifiers to get closer to the 4W limit though.
> >>
> 
> --
> To unsubscribe, send mail to minordomo at melbwireless.dyndns.org with a subject of 'unsubscribe melbwireless'
> Archive of the Entire mailinst list at:
> http://melbwireless.dyndns.org/cgi-bin/minorweb.pl?A=LIST&L=melbwireless

--
To unsubscribe, send mail to minordomo at melbwireless.dyndns.org with a subject of 'unsubscribe melbwireless'  
Archive of the Entire mailinst list at:
http://melbwireless.dyndns.org/cgi-bin/minorweb.pl?A=LIST&L=melbwireless