How to Build a Cantenna
Extend Wireless Networks to 1000 Feet and Beyond
Standard commercial wireless routers are designed to create a network between a number of computers by sending and receiving messages and acting as mediator. These message packets are sent and received using omni-directional antennas which send the signal in all directions at a roughly even level. By contrast, directional antennas focus the signal in one particular direction while sacrificing signal strength in other directions.
Unfortunately for consumers, ordinary routers do not possess very powerful antennas and the range is practically limited to 50-100 feet indoors and maybe twice that outdoors if unobstructed. The cantenna is designed to address the need for a cheap antenna capable of extending the range of a wireless router. It is a directional antenna consisting of a metal can, coaxial cable with an N-type connector, and an adapter to attach the coaxial cable to the router itself.
• A router with external antennas (alternately, this works for a wireless network card)
• A can of the right proportions (we’ll get to these in a bit)
• Some means of mounting and orienting the can in the proper direction (tripod)
• A “pigtail” for connecting the router to the cantenna
• A N-type connector for mounting the signal radiator inside the can
• A 3.1cm straight segment of copper wire
• Screws and nuts for mounting the N-type connector inside the can
Router antenna connectors are unfortunately not standardized as of now. There are, however, a few more common types:
• TNC – Similar to BNC cables without the metal connectors
• MC – Smaller mini-coax (2mm diameter)
Putting it all Together
You can order the cable, pigtail and connector from this site (http://www.fab-corp.com/) among others. You can also probably find adapters for the more readily available BNC cables the department uses. If you’re feeling particularly ambitious you might be able to jury-rig a coaxial cable connector using another kind of coaxial cable (UHF,BNC,etc.) and get that to work. However you decide to do it, you will be hooking the router (or network card) to the pigtail, the pigtail to the N-type connector and mounting the connector to the can according to some equations I’ll talk about in a bit.
1. Clean out whatever can you’re using to reduce disturbance of the signal
2. Cut a ~1.23 inch segment of gauge 12 (thick) copper wire
3. Solder this segment to the end of your N-type connector
4. Measure the correct distance from the bottom of the can to mount connector
5. Drill holes to insert and mount the connector in place
6. Try to limit the amount of screws, nuts, etc. on the inside of the can
To get the best results, it is important to attach your N-type connector and copper wire in the right place. This copper wire segment acts as the radiator of outgoing wave messages and the receiver of incoming messages from computers on the network. First, calculating wire length:
• c – speed of light, m/s
• λ – wavelength
• f – wave frequency, 2.4GHz or Hz
• c = λ*f
• Lo= 12.5 cm
• We want 1/4 of this wavelength for the length of our wire
• Lo/4 = 3.123 cm or 1.23 inches
The can acts as a waveguide antenna, using the metal conductive properties of the can, and taking advantage of similar characteristics of coaxial cables to reduce the amount of signal loss. The signal from the copper wire can go both forwards and backwards. Those waves directed backwards, bounce of the bottom of the can and then combine with the original forward-oriented waves. By positioning the copper wire antenna a correct distance away from the bottom of the can, one can line it up with a node where the forward and backward-oriented waves combine for the optimal signal strength.
Meet the Cans
I experimented with the following three cans:
• Radiator Position
• Radiator Position
• Radiator Position
Distance Performance Graph:
This data was taken using an iPod Touch with the Stumbler application installed. This application reports the strength of any wireless router signals it sees. The units for this test are not included and may not actually represent anything more than a relative measurement. The largest magnitude, however, that I found for a signal was 76 and the router disappeared from those seen when it dropped below 10.
These data points were taken in the soon to be old science center in the physics department hallway. As such, there may have been disturbances along the way related to other routers or essentially any other electromagnetic devices. Even with this basic test, one can see a few points (~40m and ~70m) where signals tended to increase regardless of cantenna design.
In long-range testing, the signal strength drops off pretty quickly after 100 meters but more importantly stays above 10 before disappearing at a distance of 365 meters. This sample run was done using the pirouettes can and walking away from the router until I lost the signal. I am limited, however, in a good way by an insufficiently long distance to test things out. As it was, I was walking downhill towards the end and there was a significant amount of earth and some trees impeding the line of sight when the signal started to really die. Under better circumstances, this distance would likely improve, giving at least 1/4 mile with potential for even better results.
Recommendations for better performance or project expansion:
• Make the coaxial can attachments easily removable to switch between cans
• Perhaps try the addition of a lip at the end of the can to collect a larger amount of incoming signals
• Try a cantenna at both the wireless router and wireless network card end to see how this extends the range
• Try amplifying the signal before the cantenna itself to boost power
• Two soup cans or Pringles cans attached end to end
• Add a dielectric to the interior of the can. Perhaps the original form of the Cantenna (a dummy load consisting of a paint can filled with mineral oil and transmission fluid) would be about right.
The one issue I’d have with this is that filling the can requires a closed container, so one would need a non-conducting lid that is nevertheless solid enough to contain whatever dielectric is enclosed
• Use two cantennas, attached with coaxial cable as a repeater to get around objects (buildings, mountains, etc). Point one at the source signal you’re trying to send and the other in the direction you’d like the signal to be transmitted.
Resources that I found helpful:
Probably the most helpful from a simple how-to perspective was Turnpoint.net complete with an applet for calculating appropriate distances.
Really good theory information about waveguide antennas and microwave signals in general
A variety of sample cantenna projects using different cans and designs