Re: how to measure ?

prototype_nsx <prototype_nsx.2vvv2l@no-mx.wirelessforums.org> hath
wroth:

>i have a question about wireless networking since the manufacturers
>always talk about high gain antennas they never seem to mention about
>the distance it's gonna cover like if they say as an example if u have
>this router/ap and if u attach that antenna to it u can cover upto 5
>km/miles .. like that(under normal circumstances) which will be very
>helpful for ppl like (i dont know anyone else exist like me or not..)
>anyway .. is that too much to ask... pls comment

I'll resist the temptation to answer your question with one long
sentence.

It's fairly easy to calculate the range of given system if you know
the parameters. See example at:
<http://wireless.wikia.com/wiki/Wi-Fi#Link_Calculations>

However, let's make it even easier. Let's pretent you have a generic
access point with the stock 2dBi rubber ducky antenna, and a generic
laptop, with the stock 0dBi internal antenna. You take a walk with
the laptop and find that you can surf the internet reliably to about
20 meters range.

You then think...... perhaps if I install an aftermarket antenna on
the access point. How far will I get?

Well, that's really easy. Each 6dB of gain is double the range. 12dB
is 4 times. 18dB is 8 times the range. 24dB is 16 times the range.
So, a 24dBi dish antenna will yield identical speed and thruput of:

24dBi (dish gain) - 2dBi (ducky gain) - 3dB (coax loss) = 19dB gain

Range increase = 10^(dB/20) = 10^(19/20) = 10^0.95 = 8.9 times the
range or 178 meters.

However, that might set you thinking that perhaps a bigger antenna
might be useful on the laptop. The same logic applies. I'm lazy so
I'll just attach another 24dBi dish antenna with the same lossy coax
cable. However, instead of a 2dBi rubber ducky on the access point,
we're blessed with a 0dBi ceramic antenna with a rather lossy piece of
tiny coax. I'll be generous and give it 0dBi gain.

24dBi (dish gain) - 0dBi (ceramic) -3dB (coax loss) = 21dB gain.

Range increase = 10^(dB/20) = 10^(21/20) = 10^1.05 = 11 times the
range. So, your 178 meters range is now:
178 * 11 = 1,958 meters range.
Is that far enough?

Unfortunately, this is for ideal conditions, with no interference, no
Fresnel Zone issues, perfectly aligned antennas, and perfect line of
sight. In the city, it's almost impossible to find a 2000 meter path
with such conditions. Therefore, you may not be able to go that far.


--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558