Wireless Hacks

Calculating the Link Budget

Figure out whether a long distance link is even possible before you buy any equipment.

How far will it go? That’s a very good question. It depends on all sorts of factors, including the power output and sensitivity of your card, quality of your cable, connectors and antenna, intervening clutter and noise, and even weather patterns (on long distance links). While it’s impossible to precisely take all of these variables into account, you can make a good estimate before buying any hardware. The following describes a simple way to build an estimate, referred to as the link budget.

First, figure out how much loss the signal will incur in the space between the two sites. This is called the path loss . One common formula for estimating path loss at 2.4 GHz is:

 L = 20 log(d) + 20 log(f) + 36.6

where L is the loss in dB, d is the distance in miles, and f is the frequency in Megahertz.

So, suppose you wanted to set up a five-mile link between two points, using channel 6 (2.437 GHz):

L = 20 log(5) + 20 log(2437) + 36.6 
L = (20 * 0.69) + (20 * 3.38) + 36.6 
L = 13.8 + 67.6 + 36.6 
L = 118

At five miles, with no obstacles in between, you will lose 118 db of signal between the two points. Our link must tolerate that much loss (plus a bit extra to account for weather and miscellaneous interference) or it will be unreliable.

If you don’t want to bother calculating path loss on your own, you can use Table 6-1 to get a rough estimate. This table was computed with the above formula ...