[MLB-WIRELESS] Calculating Antenna Range

[Michael Borthwick]

Ryan Abbenhuys wrote:

> Okay, someone may have posted up something about this before i'm not too
> sure.
>
> Is there a formula I can use to calculate an antenna's range based on it's
> spread, gain, input power, etc?

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.

This is all pretty cruisy:



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.

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

At the other end you are just wandering around with your laptop with inbuilt antenna so just have the gain of
the radio itself ie 15dBm

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

However you have path loss to contend with - simplifying the formula taken from Flickenger's Building
Community Wireless Networks (O'Reilly 2002) the path loss equals 20 log(distance in miles) + 104

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


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.

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).

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.


--
To unsubscribe, send mail to minordomo at melbwireless.dyndns.org with a subject of 'unsubscribe melbwireless'  
Archive at: http://melbwireless.dyndns.org/cgi-bin/minorweb.pl?A=LIST&L=melbwireless
IRC at: au.austnet.org #melb-wireless