What antenna should I buy to broadcast a wireless signal?
August 29, 2008 2:00 PM   Subscribe

I'm looking to broadcast a wireless signal 100-200yds. How many dB/mW does my antenna need to be and what's the optimal frequency?

I'm looking to broadcast a wireless (802.11x) signal 100-200yds. How many dB/mW does my antenna need to be and what is the optimal frequency? Where can I buy an antenna to do this? A directional antenna would be best, right?
I did find this site, but I was wondering about a better place to get an antenna...

Thanks in advance.
posted by jammnrose to Computers & Internet (7 answers total) 1 user marked this as a favorite
My knee-jerk reaction is that you can probably pull it off with one of those 8dbi omnis
on each end. No need for directional over such a small distance really... and it makes installation simpler.
posted by TravellingDen at 3:29 PM on August 29, 2008

I buy from fab-corp.com. They're cheap and their parabolics have always done the job for me.
The required antenna parameters are a function of the height of the antennas above ground, the distance between then, and the net input power (which is the transmitter power minus loss due to the connecting cables.) Without the height and the net input power, I can't say what the antenna parameters should be.
posted by thalakan at 3:42 PM on August 29, 2008

What will you be broadcasting through? Open air or buildings? What is your goal- providing wireless access to users that far away, or creating a bridge between two LANs? Those answers will help narrow down choices.
posted by gjc at 5:36 PM on August 29, 2008

hey jammnrose...

What you are looking for is called a link budget. A whole bunch of factors enter into the quality of service you can expect depending on what put into your link.

Some of the variables:

power output INTO your antennas (both ends)
antenna gain
cable losses
path loss

There was a good book called "Building Community Wireless Networks" by Rob Fleckinger that had some simple home brew yagis. I think he had a few plans for "pringles can" yagis that were cheap and neat, and actually, I built one but never tested it. Mostly, it was spacers, threaded rod, washers and wire. And of course, a pringles potato chip can.

The 802.11x link is bi-directional, obviously, so both ends of the link need aimed directional antennas if you want to connect over those distances. If the path is relatively clear line of sight, you may not need much gain. 1-200 yards sounds do-able.

You have no choice of frequency for 802.11. I think it's all 2.4 GHz, nominal. How many dB gain you need from your antennas and amps is a function of the anticipated losses and the noise floor. I really suspect you can get by with modest gain antennas for this distance, but I'd have to know more about the site specifics to start stating numbers.

The advantage of using lower gain directionals is that aiming is somewhat less critical.

I usually don't buy simple directionals, and usually build my own yagis since who knows what frequency I'll be interested in next? 2.4 GHz yagis are tiny, and easy to build. You need stiff wire, a drill press, some measurement skills, a little research. If you wanna go this route, I'll be glad to send you some info on design specifics.
posted by FauxScot at 5:50 PM on August 29, 2008

Are there any speed-of-light issues?

10-base-2 ethernet can't run more than 1000 feet (IIRC) because the collision-detect timeout is a bit more than a microsecond.

Some cell phone standards can't have cells larger than a certain size because handshakes between the cell and phone will time out if the distance is too great. (That's a particular problem in TDMA, where the limiting factor is the length of the time guardbands in the multiplexing.)

Sometimes there are timeout specifications which set upper limits on how far apart networked objects can be, due to speed-of-light latency. When that is the case, and if you're outside that range, nothing you do about improving signal strength will make any difference. It still won't work.

Are there any such limits for 802.11x?
posted by Class Goat at 7:46 PM on August 29, 2008

You might try an online calculator like this one for planning a wireless link.
posted by exphysicist345 at 8:12 PM on August 29, 2008

BTW... some wireless antennas are available from Linx Technologies. 800-736-6677. I have used their stuff.

Digikey and Mouser also have antennas, though the prices may not be the lowest.
posted by FauxScot at 3:54 AM on August 31, 2008

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