Appendix C: Finding Your TCPIP Configuration Data

Power Limits

The power limits that the FCC sets on Wi-Fi gear are easy to state, but maybe not quite so easy to understand.The total equivalent isotropically radiated power (EIRP) coming off of a Wi-Fi antenna is, in most cases, limited to four watts. (And we all understand EIRP, right? Right.) If your antenna has high gain, your EIRP could be over the FCC's limits and illegal even if the microwave power measured going into the antenna were well below the legal limits. A gain antenna effectively 'boosts' signal power by focusing the radiation in certain directions at the expense of others.

EIRP is the power that you would have to apply to an isotropic antenna to get a signal equivalent to what is radiated from a real-world setup being evaluated. An isotropic antenna is one that radiates equally in all directions, including up and down. (I speak more of isotropic antennas and antenna gain in Chapter 8.) An isotropic antenna is a mathematical abstraction. It exists to provide a sort of 'base point' for calculating antenna gain. In figuring EIRP, both the power coming from the actual transmitter and the antenna doing the radiating must be taken into account. It sounds forbidding, but if you use commercial antennas it's really not that hard, because all commercial antennas come with a measured antenna gain figure.

In the fine print of Part 15, the FCC states that the transmitter output power of a piece of Wi-Fi gear is limited to 30 dBm (1 watt) when connected to an antenna with a gain of 6 dBi. (I discuss dBm and dBi in detail in Chapter 8.) If you calculate the EIRP by adding the transmitter dBm figure to the antenna gain, you get 36 dBm, which is equivalent to 3.98 watts-close enough to four watts to call it there.

If your Wi-Fi adapter puts out 1 watt (which is something I've never actually seen!) and you want to use an antenna with more than 6 dBi gain, you must reduce the transmitter power so as to keep the EIRP at 4 watts (36 dBm) or under.

In truth, most Wi-Fi access points and client adapters put out 35 to 50 milliwatts tops, and it takes a bodacious amount of antenna gain to turn 35 mw of transmitter power into 4 watts EIRP. To see just how much, convert 35 mw to 15 dBm, subtract that from 4 watts, or 36 dBm, to give you 21 dBi of necessary antenna gain. That's a big antenna! (Of course, you can buy monster 24 dBi parabolic antennas for under $100 these days, so getting in trouble is possible, and won't even cost you that much.)

If you're constructing your own gain antennas, it's a little trickier, as measuring the actual gain of an antenna in dBi requires expensive lab gear and fairly controlled conditions. However, if you're using standard access points and client adapters putting out 100 mw or less, a tin can antenna or simple 5-element shotgun Yagi will not get you anywhere near four watts EIRP. You would need 16-18 dBi gain to take 100 mw to the limit. For 35 mw (which is what most access points put out) you would need more antenna gain, to the tune of 18 to 20 dBi, and making an antenna like that in the garage out of junk takes real skill.