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My outdoor wifi plan was a bad idea

For anyone who has been keeping up with the technical aspects of the Home Project, you may have noticed that I am building outdoor wifi, in addition to my indoor wifi and CAT6 Ethernet network. My plan was to place a wireless router inside my Workshop (a detached building connected to the house), connect it to a signal amplifier (PowerLink PL-2301A), and then connect that to a high gain omni-directional antenna (Alfa AOA-2415) bolted to the outside of the building.

As I stated in my previous entry, I am currently studying to get my first Amateur Radio license. In studying for this exam, I’ve also learned a lot more about radio in general. Anyway, I was reading more about Part15 devices. That’s the FCC rules for “unlicensed radio transmissions” like your wireless router, your cordless phone, garage door openers, toy walkie talkies, RC cars, etc. Without getting too far into this, basically there are restrictions on how powerful these devices can be, and you aren’t allowed to cause interference. Basically, what that means is that, if you’re talking on your cordless phone, and the phone is faulty, and it causes interference to a nearby amateur, commercial, or military radio station, you have to stop using it until it is fixed. Look on the bottom of your wireless router, and you’ll see that written on a sticker or something.

Now, here’s the part where I screwed up: how this applies to my wifi setup. There are restrictions on Effective Isotropic Radiated Power (EIRP). EIRP takes into account the power of the transmitter (the router and/or the amplifier) the loss from the coax cable, and the gain from the antenna.
EIRP = POWER – LOSS + GAIN

If you’ve ever looked at wifi antennas, they tell you what the gain is, measured in dBi. If you have less than 6 dBi, you have a “low gain” antenna. 6 dBi or more, you have a “high gain” antenna. Assuming you’re setting up a wifi access point, if you have a “low gain”, you have to have an EIRP less than 1 watt; if you have a “high gain”, you have to have an EIRP less than 4 watts. The rules are different if you have a directional antenna just connecting two devices, and I won’t go into that; we’re talking about access points.

If I had gone through with my plan of connecting my amplifier to my antenna, I would have been connecting 1 watt power to a 15 dBi antenna. Assuming a 25% signal loss, my EIRP would have been 23 watts! The math comes out to… I can send no more than 125 mW to a 15 dBi antenna, which my router can come pretty close to without any amplifier at all.

Breaking the Part15 rules can result in the FCC confiscating your equipment, and/or you could be fined. Can you be caught? Of course, especially if there’s an Amateur Radio operator in your area that might turn you in.

My new plan is to connect my router directly to my antenna, resulting in EIRP just less than 4 watts for my outdoor wireless. Another router inside my house will be connected to the amplifier, which also results in EIRP just less than 4 watts for my indoor wireless.

For reference, I used this calculator to calculate my EIRP: http://www.csgnetwork.com/antennaecalc.html

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