Uni? Multi? Whip? Band? Huh? Wha?

December 30, 2006 12:58 PM

WIFI-Filter: My brother and I would like to build a wireless network between our two homes to create a secure network, but we have no idea about radio antennas, signal strength, line-of-sight, etc.

The situation: My brother lives ~1900 feet away (about 4 blocks), and about 30 feet above me in altitude. In between are the normal manner of houses, apartments, trees, etc.

We do not have direct line of sight.

What type of antenna and how strong of an amplifier would we have to pair up in order to create a strong, fast wireless link between us? Is this even feasible? Is it even nice, considering we'd have to steal away a whole channel from the spectrum?

posted by id to technology (13 comments total)

If you could hoist it high enough up to see, you could use a directional antenna (like these), that wouldn't require 'stealing' a channel. As far as amplifiers go, you'd have to build one. It's not really legal to sell them, as far as I know.

If you both already have internet access, it would be much simpler to simply setup a VPN.
posted by delmoi at 1:04 PM on December 30, 2006

If you're looking to create a secure network for sharing resources, but you're no in need of sharing the same connection to an ISP, you should consider looking into Virtual Private Networking. I use Hamachi (http://hamachi.cc) to share resources with my family, even though we're not close to each other.
posted by RossWhite at 1:04 PM on December 30, 2006

delmoi,

unfortunately, hoisting it will be tough to do to get over the tree tops for visibility- we dont think we can get directional to work.

The main reason we want to do a wireless network is for speed- a VPN over an internet connection is going through a slow cable/DSL link.
posted by id at 1:11 PM on December 30, 2006

But you pretty much need a directional antenna to get the range necessary. It focuses all the transmission power in one direction. You don't necessarily need line-of-sight, just point them at each other as accurately as you can.

An amplifier could easily push you over the legal limit for unlicensed transmissions, so I'd skip that.
posted by cillit bang at 1:47 PM on December 30, 2006

If speed weren't your primary concern, I'd suggest picking up some older 900MHz radio gear (specifically Ricochet modems), as they don't require line of sight, and 2000' is small potatoes even on the stock antennae.

As for 2.4GHz, you might be able to punch through some trees with highly directional antennae. Don't bother with omnis, you'd just be wasting signal in useless directions. Look for Yagi or dish style antennae with the highest dBi figure. If you're handy in the shop, Trevor Marshall's biquad feeding a surplus Primestar dish is a fine, cheap way to get ~28dBi of gain.

By using the most directional antennae possible, you don't steal the channel from the whole neighborhood, just people who live along the line you're painting with signal. That's in keeping with the guidelines of "most directional antenna practical" and "minimum power". Once you have a solid link, back the power down until it gets flaky, then back up just a bit. When the trees get leafy in the spring, readjust as necessary.

Getting the antennae aimed is tricky if you can't directly see the other end. Start with an omni (the stock rubber-duck) on one end, and sweep your dish back and forth, 5-10 seconds between moves, and note the signal strength on a piece of paper. When you find the yaw that gives you best results, stay there and adjust pitch. When you're dead-on, swap the rubber duck for a dish on the other end, and repeat the procedure.

Mind the polarization! Actually if you're building your own antennae, you might try some simple circular designs which reject linearly-polarized interference a little bit, and also cause less interference to linearly-polarized victims. They're hard to use as dish feeds though.

As for equipment, you'll want to locate the radios as close to the antennae as possible to minimize feedline losses. The thin pigtail cable is incredibly lossy -- look at the dB loss per foot and figure out how much you'll save on big antennae by just using short runs of decent cable.

If the antennae will be mounted near or above the roofline, consider lightning protection. This is a subject of its own.
posted by Myself at 1:57 PM on December 30, 2006 [1 favorite has favorites]

thirding or whatever Hamachi. Ultra simple and NO climbing up on rooftops, antennas and wires.
posted by stavx at 3:18 PM on December 30, 2006

You definitely want a directional antenna. This small yagi should work fine. delmoi is most certainly incorrect about it being illegal to sell amplifiers. All of these are perfectly legal and in combination with a directional antenna you can achieve a signal at distances of 5 - 10 miles with off the shelf parts. I think the magic FCC number is 1 Watt, which when combined with a directional antenna is a heck of a lot, considering that most home wireless routers put out something like 28 mW. (Of course part of that 1W will be eaten up by losses in the feed cable to get the signal to the antenna on the roof, but still.)
posted by Rhomboid at 6:42 PM on December 30, 2006

Other options....

http://www.freewave.com/products/industrial-radios.html

...if cost is no object. We get 20 miles + 900 MHz, 1W.

I have difficultly understanding how your 'slow cable/dsl connection' can't be addressed by paying slightly higher monthly costs. It's not a perfect solution, but it's damn quick.

I know radio and have much equipment and find this sort of thing 'tinkering intensive'. Are you sure you want to try it? Big headache, IMO.

Have fun, though.
posted by FauxScot at 7:51 PM on December 30, 2006

"... Is it even nice, considering we'd have to steal away a whole channel from the spectrum?"
posted by id to technology (8 comments total)

Standard WiFi gear operates under Part 15 of the FCC rules. So, it's not only not "nice" to modify WiFi gear sold for use under Part 15, it's illegal. And it is fairly stupid to use such equipment for fixed point-to-point links, because tracking such signals to a point of origin is ridiculously easy. I've done it successfully in my own neighborhood 3 times in the last 9 months, simply by triangulating the location of strong WiFi signals causing interference to my home network, and having an initial talk with the people running the interfering gear. In the one case that didn't work to get them to eliminate the interference immediately, I wound up getting a local ham radio operator to verify and co-sign my complaint letter to the FCC. The FCC sent them a Part 15 Enforcement letter, which looks something like this. If someone refused to eliminate the interference they were causing after getting a notification, they'd potentially be on the hook for fines and forfeituture of the offending equipment.

WiFi works precisely because it is a low power service, built on the proper use of unmodified Part 15 equipment. What you're talking about doing isn't even close to the intended purpose of such devices. Don't do it.
posted by paulsc at 9:34 PM on December 30, 2006

Using WiFi for your own purposes is fine, just pick the channel that interferes the least with what others are using (which around here is mostly channel 6..people don't seem to bother to switch from the default).

One of my clients uses WiFi instead of a T1 or T3 to connect two offices that are about a mile and a half distant. We get over 10Mbps, but we have roof rights on the 300 foot building housing one office (on top of a hill, no less) and a 25 foot tower at the other, so line of sight isn't really a problem.

We were lucky enough to have the Rohn tower already erected, although it was disused at the time.

You can get 15-20 foot poles that you can mount to the side of your house from Radio Shack, if you need to elevate your antennas.

Just be sure to stay within the transmit power rules and the rules for EIRP, which can be found on the FCC's website and elsewhere. Generally speaking, 12dB of gain is OK on most, but not all, stock APs. If one of you is a ham, the rules are different and you can do most anything you like.

Note that the power limits are different for point to point (which is what you'll be doing) and the normal point to multipoint usage.

Seattle Wireless has a page that explains the power limits. As an example, with an antenna with 24dB of gain (most inexpensive grids/dishes, a few beefy yagis) you can put 24dBm into the antenna, which is 250 milliwatts. Very few wireless routers can output that much cleanly, so you may need an amplifier if you actually require that hot a signal. (you probably won't) Do keep in mind that that's actual input power to the antenna, so if you have a few dB of loss in cable and connectors, you can pump that much more out of your amplifier, you just have to keep the radiated power from the antenna below the limit.

Basically, do what has already been said..get two highly directional antennas (eBay is a good source, you want a yagi or a grid, yagis will be easier to point, grids have less beamwidth, but over that distance, the side lobes will be enough if you have anything close to line of sight) with low loss cable and a couple of wrt54gl routers with some third party firmware, a couple of $30-$50 poles (they're often sold as poles to mount TV antennas on) and the associated mounting brackets, and you should be good to go, unless the foliage is dense.

That WRT54GL should be good for about 125mw without spewing noise across the rest of the 2.4GHz band. That may be enough. Personally, I'd give it a shot with that and a 24dB antenna and see if it works.

Before doing that, you might each climb on your respective roofs and see if you can see each other over the intervening "stuff."

FWIW, with good antennas, line of sight isn't strictly necessary..I've had more than one wireless internet provider (in more than one city) that I didn't have actual line of sight to. Some have been more reliable than others, so there is a bit of rolling the dice, but these were 1-2 mile links, not 4 blocks.

Lastly, Hyperlink sells amplifiers you can use..with a 24dB highly directional antenna, the 250mw would be the legal maximum, but you may well not need it, so I'd save the $200 and try it first with just decent antennas and good LMR400 cable and the two routers with third party firmware that lets you crank up the power to 100mw.
posted by wierdo at 10:53 PM on December 30, 2006

Hamachi. Hamachi. Hamachi.
posted by homodigitalis at 3:15 AM on December 31, 2006

Note: I just nosed over to Hyperlinktech this afternoon to do some window-shopping of my own, and they have all sorts of stuff on sale. I don't think you need amplifiers, but a pair of decent antennae will put you on the right track. If you don't mind the slightly odd appearance of the "backfire" antenna, it's good gain for a good price.
posted by Myself at 1:09 PM on December 31, 2006

Everyone, thanks for the help!

The main reason we're avoiding VPN over the net is the speed and cost of a reasonable uplink. 768Kbps will not cut it.

Paulsc-

What I meant by stealing a channel was that anyone along the lines of the signal will lose use of the 802.11 channel we choose to use on our wireless points. I have no intention of going over the FCC 1w maximum.

Looks like a couple of yagi's and some standard equipment should be enough for us. Thanks again!
posted by id at 2:21 PM on December 31, 2006