[MLB-WIRELESS] Calculating Antenna Range

[Tony Langdon]

> > I'm assuming if you want a single "antenna's range" as 
> opposed to the maximum distance of the link between two
> > antennas then what you have at the other end is an antenna 
> with no gain ie the card itself.

Since this sort of analysis is a _link_ analysis, it makes no sense to have
a "single ended link analysis".  A single ended version would leave one of
the following results:

1.  EIRP of the transmitting station.
2.  Field strength at a given distance from the transmitter.
3.  Detectable field strength for the receiver (of the same station as
above).

> > 
> > You simply add up all the gains in the system and subtract 
> all the losses including the path loss as the radio
> > waves expand out from the antenna.

Provided everything is expressed in dB, or dB(something-or-other), this will
work.

> > Assuming at the AP end you have an Orinoco with 15dBm 
> output (from the spec sheet) + typical 8dbi omni antenna
> > - pigtail loss of 1dB(say) = 22 dB total gain at the A.P. end

semantic point... it's actually +22dBm EIRP at the AP (since you're adding
gain to a power source).

> > Sooooooo:  the total gain for the link is 15+22=37dB woo hoo !

No, you can't add the 15dBm at the receiving end.  it's an output power (of
the Tx), so doesn't come into the calculation for that direction.  So, we're
back to Tx EIRP of +22dBm.  Now onto the path losses.

> > so lets say your links are 8K or 5 miles apart your loss is 
> 118 dB over the link
> > 
> > Gains minus losses for the system as a whole = 37 - 118 = -81 dB

2 corrections:

1.  The original 15dBm is not a "gain".  dBm is power, dB is gain.  So the
gain at the Tx end is +7dB, and the EIRP is +22dBm.

So putting in the correct values for system loss, you have:

Path loss = 118dB (-118dB gain)
Tx antenna/feedline combined gain = +7dB.

Total gain from Tx to Rx antenna = -111dB

Now, let's look at the Rx side...  We have

Rx sensitivity = -82dBm (for 11 Mbps)
Fade margin = 20dB (the extra you note below)
Antenna/feeder gain = ???  (let's assume 0dB as an initial value).

The power at the receiver = Transmitted power +/- combined gain/loss

= 15 dBm + 8dB -1dB -118dB +0dB = 15 - 111dB = -96dBm

Hmm, we need another 14 dB just to get enough signal for 11 Mbps operation.
Time to throw in that 24 dBi gridpack - and we will lose 1dB due to feeder
loss.

So Rx power now becomes -96dBm + 24 - 1 = -73dBm.

Yes, we can get 11 Mbps, but we only have a fade margin of 9dB (or 9dB
allowance for terrain).  It is likely that we will have to either run at a
lower bit rate, or the link will be unreliable under real world conditions
(if we can't maintain even 1 Mbps).

> > The receiver sensitivity of the Orinoco silver from the 
> manual is -82dB at 11MBps however you aren't out of
> > the woods yet as it is recommended to have about 20dB 
> headroom so in this case you are screwed. You cant
> > connect at 11Mbps to an A.P. running an 8dBi omni antenna 
> from 8 K's away with a standard card - which makes
> > sense intuitively.

See my calculations above...

> > We need to find another 20dB somewhere. Stick a 24dBi 
> parabolic at the other end and you are in business.
> > Might get away with an 18dB in good conditions or if you 
> are happy to run a lower data (the Silver is 12dB
> > more sensitive at 1Mbps than 11Mbps which is still a lot 
> faster than your modem).

Even with the 24 dB parabolic, you're still pushing things a bit.  If you go
to a 12 dB omni at the AP, your fade/terrain margin increases to 13dB, which
will probably result in a significant improvement in throughput/reliability,
though still not ideal.

Of course, a point to point link of 8km will have good margins (adding
another 16 dB over the calculated signal levels above if 24dB gain antennas
are used at both ends).

Conclusion:  With the limited power that we have for WLANs, distance to the
AP should be kept as short as possible.  Every time you double the distance,
you lose 6dB on your link budget.

> > 
> > I have ignored losses from long runs of cable and heaps of 
> connectors - if your bag is playing with that stuff
> > then you probaby have the skills to factor in the losses 
> that will result.

Yeah, we can add those in later. :-)

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