dBi Calculation Question

Guys I am working on my CWNA paper and have hit a little point that I
cannot get beyond (although you will probably think this is pretty
simple).

It is the calculation for a circuit as below.

RF Circuit

AP--connector (A) -----Cable----Connector
(B)----cable----
Connector (C)---Antenna (D)

AP is 100mW and the paper is explaining the calculation as below.

AP Point A Ponit B Point C Point D
100mW -3dB -3dB -3dB +12dBi
=100mW /2 /2 /2 (x2x2x2)
=100mW /2 /2 /2 x16
=50mW /2 /2 x16
=25mW /2 x16
=200mW

It was my understanding that all dB units (including dBi) are relative
units and can be added and subtracted from other dB units - therefore
the overall dB should be +3db = doubling the power - hey presto the
right answer!

What I cant fathom is the calc shown above - why have they suggested a
multiple of x2x2x2 ?

Can someone please explain why......only a little point but bloody
annoying when your are learning this stuff for the first time!

Thanks

"Rob D" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) om...
> Guys I am working on my CWNA paper and have hit a little point that
I
> cannot get beyond (although you will probably think this is pretty
> simple).
>
> It is the calculation for a circuit as below.
>
> RF Circuit
>
> AP--connector (A) -----Cable----Connector
> (B)----cable----
> Connector (C)---Antenna (D)
>
> AP is 100mW and the paper is explaining the calculation as below.
>
> AP Point A Ponit B Point C Point D
> 100mW -3dB -3dB -3dB +12dBi
> =100mW /2 /2 /2 (x2x2x2)
> =100mW /2 /2 /2 x16
> =50mW /2 /2 x16
> =25mW /2 x16
> =200mW


What they are showing is that 100mw/2 = 50mw, 50mw/2=25mw,
25mw/2=12.5mw which is = to -9db

the x2x2x2 means +12dbi would take the 12.5mw x 2= 25mw, 25mw x
2=50mw, 50mw x 2 = 100mw = up 9db, just where it started.
plus 3 extra dbi = 200mw

Make since.... you are right in your calculation, there is a +3db
gain, started with 100mw,,, now x2 = 200mw





> It was my understanding that all dB units (including dBi) are
relative
> units and can be added and subtracted from other dB units -
therefore
> the overall dB should be +3db = doubling the power - hey presto the
> right answer!
>
> What I cant fathom is the calc shown above - why have they suggested
a
> multiple of x2x2x2 ?
>
> Can someone please explain why......only a little point but bloody
> annoying when your are learning this stuff for the first time!
>
> Thanks

"Airhead" <(E-Mail Removed)> wrote in message
news:4208ef9c$0$22518$(E-Mail Removed) m...
>
> "Rob D" <(E-Mail Removed)> wrote in message
> news:(E-Mail Removed) om...
> > Guys I am working on my CWNA paper and have hit a little point
that
> I
> > cannot get beyond (although you will probably think this is pretty
> > simple).
> >
> > It is the calculation for a circuit as below.
> >
> > RF Circuit
> >
> > AP--connector (A) -----Cable----Connector
> > (B)----cable----
> > Connector (C)---Antenna (D)
> >
> > AP is 100mW and the paper is explaining the calculation as below.
> >
> > AP Point A Ponit B Point C Point D
> > 100mW -3dB -3dB -3dB +12dBi
> > =100mW /2 /2 /2 (x2x2x2)
> > =100mW /2 /2 /2 x16
> > =50mW /2 /2 x16
> > =25mW /2 x16
> > =200mW
>
>
> What they are showing is that 100mw/2 = 50mw, 50mw/2=25mw,
> 25mw/2=12.5mw which is = to -9db
>
> the x2x2x2 means +12dbi would take the 12.5mw x 2= 25mw, 25mw x
> 2=50mw, 50mw x 2 = 100mw = up 9db, just where it started.
> plus 3 extra dbi = 200mw
>
> Make since.... you are right in your calculation, there is a +3db
> gain, started with 100mw,,, now x2 = 200mw
>
>
>
>
>
> > It was my understanding that all dB units (including dBi) are
> relative
> > units and can be added and subtracted from other dB units -
> therefore
> > the overall dB should be +3db = doubling the power - hey presto
the
> > right answer!
> >
> > What I cant fathom is the calc shown above - why have they
suggested
> a
> > multiple of x2x2x2 ?

This should probably say x2x2x2x2. Are you getting this from the CWNA
study book?
Is so what version?




> >
> > Can someone please explain why......only a little point but bloody
> > annoying when your are learning this stuff for the first time!
> >
> > Thanks
>

"Airhead" <(E-Mail Removed)> wrote in message
news:4208f04f$0$22518$(E-Mail Removed) m...
>
> "Airhead" <(E-Mail Removed)> wrote in message
> news:4208ef9c$0$22518$(E-Mail Removed) m...
> >
> > "Rob D" <(E-Mail Removed)> wrote in message
> > news:(E-Mail Removed) om...
> > > Guys I am working on my CWNA paper and have hit a little point
> that
> > I
> > > cannot get beyond (although you will probably think this is
pretty
> > > simple).
> > >
> > > It is the calculation for a circuit as below.
> > >
> > > RF Circuit
> > >
> > > AP--connector (A) -----Cable----Connector
> > > (B)----cable----
> > > Connector (C)---Antenna (D)
> > >
> > > AP is 100mW and the paper is explaining the calculation as
below.
> > >
> > > AP Point A Ponit B Point C Point D
> > > 100mW -3dB -3dB -3dB +12dBi
> > > =100mW /2 /2 /2 (x2x2x2)
> > > =100mW /2 /2 /2 x16
> > > =50mW /2 /2 x16
> > > =25mW /2 x16
> > > =200mW
> >
> >
> > What they are showing is that 100mw/2 = 50mw, 50mw/2=25mw,
> > 25mw/2=12.5mw which is = to -9db
> >
> > the x2x2x2 means +12dbi would take the 12.5mw x 2= 25mw, 25mw x
> > 2=50mw, 50mw x 2 = 100mw = up 9db, just where it started.
> > plus 3 extra dbi = 200mw
> >
> > Make since.... you are right in your calculation, there is a +3db
> > gain, started with 100mw,,, now x2 = 200mw
> >
> >
> >
> >
> >
> > > It was my understanding that all dB units (including dBi) are
> > relative
> > > units and can be added and subtracted from other dB units -
> > therefore
> > > the overall dB should be +3db = doubling the power - hey presto
> the
> > > right answer!
> > >
> > > What I cant fathom is the calc shown above - why have they
> suggested
> > a
> > > multiple of x2x2x2 ?
>
> This should probably say x2x2x2x2. Are you getting this from the
CWNA
> study book?
> Is so what version?
>
>
>
>
> > >
> > > Can someone please explain why......only a little point but
bloody
> > > annoying when your are learning this stuff for the first time!

I just checked my CWNA second version and it says x2x2x2x2