Legally build a 60 Watt WiFi Link - 2.4 GHz and EIRP
Learn the ins and outs of EIRP, 2.4 GHz and the legal way to balance radio output with antenna gain. This episode is specific to the United States and FCC regulations and may not apply to your region.
Today we're building a 60 Watt WiFi Point-to-Point link
I've talked a lot about having fun with WiFi in the past. The ISM band and the rules for different regulatory domains. For example how in the US on 2.4GHz we're limited to channels 1-11 whereas Europe can use 12 and 13 while only Japan can use channel 14.
Today I'm continuing the conversation with a fun new mesh networking project. The mission here is to relay the Hak5 network across town using a point-to-point 2.4GHz link. Typically when we talk about 2.4 GHz we understand that your typical base station (or access point) is limited to 1 watt of power - but it's effectively 4 watts.
That's because of what's known as the EIRP - or Equivalent Isotropically Radiated Power. That's the theoretical output of a radio combined with the gain of an antenna. In the 2.4GHz realm an access point typically configured for point-to-multipoint (meaning it can handle multiple clients at once) is limited to 30 dBm output power plus a 6 dBi gain antenna.
- dBm is a power ratio of decibels to milliwatt. This is what the actual transmit power of the radio is measured in.
- dBi, or decibels isotropic, is the forward gain of the antenna.
The EIRP of the transmitter system - the radio and the antenna - is calculated by adding the output power in dBm plus the antenna gain in dBi. So for a typical base station with a 30 dBm radio and a 6 dBi gain antenna we'll have a total EIRP of 36 dBm, or 1 watt.
- dBm, Watt
- 24, 0.25W
- 27, 0.5W
- 30, 1W
- 36, 4W
- 39, 8W
- 40, 10W
- 42, 16W
- 44, 25W
- 46, 40W
- 48, 62W
- 50, 100W
- 1. Max power is 30 dBm - that's 1 watt out of the radio
- 2. Max EIRP is 36 dBm - that's 4 watts total
- 27 dBm, 15 dBi, 42 dBm, 15 Watt
- 26 dBm, 18 dBi, 44 dBm, 25 Watt
- 25 dBm, 21 dBi, 46 dBm, 40 Watt
- 24 dBm, 24 dBi, 48 dBm, 62 Watt - Sweet spot
So you might be thinking, awesome let's hook up a 9 dBi antenna to a 30 dBm radio and we'll have 8 Watts of power! Not quite.
The FCC rules, in addition to Part 15, are such:
So you could have a 30 dBm radio with a 6 dBi gain antenna to achieve the max 36 dBm EIRP, or you could have a 27 dBm radio with a 9 dBi gain antenna to achieve the 36 dBm EIRP. Either way, 36 dBm (4W) is the limit.
Then there's the 3rd rule, which is the most interesting to us. There's an exception for fixed point-to-point links. That means if we were to have a station to station backhaul we can exceed the 36 dBm limitation, and it works like this.
For every 1 dBm power the radio is reduced by from the maximum of 30, you can add 3 dBi gain to the antenna. So, for example, if we were to lower our power output from 30 to 29, instead of using the 6dBi gain antenna we can use a 9dbi for a total of 38 dBm EIRP - which is 6.3 watts.
Likewise if we drop to 28 dBm we can use 12 dBi for a total of 40 dBm EIRP or 10 Watt.
Now configuring the radio in linux is typically a matter of setting your regulatory domain and the txpower with iwconfig.
ifconfig wlan0 down iw reg set US iwconfig wlan0 txpower 24 dBm ifconfig wlan0 up
What's really nice with the WiFi Pineapple Mark V is that these commands can be put together as a boot mode using the DIP switches. What's more since the Mark V has two radios on one board we're able to have the best of both worlds. Our Radio0 can be set to 24 dBm and attached to a 24 dBi gain antenna for the point-to-point link, while our Radio1 interface can be set to 30 dBm with a 6 dBi gain antenna for point-to-multipoint.
This is something we'll be expanding on soon and of course I welcome your comments on this so email firstname.lastname@example.org.