Radiation Legalities

To start with, antennas and amplifiers are legal. The legal restrictions on the radio band we use are not about the specific hardware, but what it's capable of transmitting. We are limited to 100mW or 20dB. This means that we cannot transmit more than 20dB in any direction. Fortunately, working out what sort of power you're producing isn't too hard. Most wireless cards state on the box what power they give out. Take this number (often 15dB), and add it to the rating of the antenna you use (eg. 5db), and subtract the losses in the cable.

It's interesting to note that just because you can only transmit up to 20dB, it doesn't mean your receiver cannot be amplified. To obtain long links, it can actually be beneficial to reduce power transmitted! By using a 20dB antenna with a 1dB transmit power, and 1dB of cable loss, you can actually achieve a connection at a higher distance than using a 15dB transmit power with a 6dB antenna and 1dB cable loss. This is despite both transmitting 20dB. Sound silly? Here's why....

When connecting two locations, the distance achievable is related to the overall gain (as many of you will have guessed). Take the examples above. The signal going from one to the other leaves one card, goes down the cable, through the transmitting antenna, through the air, into the receiving antenna, down the cable and into the other card. The total gain over the link is equal to the transmit power, minus the cable loss, plus the transmitter antenna, minus the loss in the air, plus the receiving antenna, minus the cable loss. So if the loss through the air is 60dB....

1db - 1dB + 20dB - 60dB + 20dB - 1dB = -21dB

15dB - 1dB + 6dB - 60dB + 6dB - 1dB = -35dB

You'll notice the overall gain from the lower power setup is actually far more powerful! Note that the result should be higher than the receive sensitivity of the card for a connection to be possible. Calculating the losses through free space is outlined very well by Malcolm Cartledge and documented in the mailing list archives and Wiki of http://consume.net