Cantenna


Intro

The cantenna is a name given to a circular waveguide made from a metal can. A waveguide is a metal tube that allows standing EM waves to travel down its length with very little losses. They can be used as a directional antenna themselves or as an illuminator for a reflector. Some sites say that a Pringles can works for 2.4GHz operation but since the tube is so narrow it doesnít work as well as it should.

Where is/was it Used?

Rectangular waveguides were designed before circular ones. The rectangular one was realized first since it makes more sense that it would work but later on, it was found that circular ones also worked for waveguides.

How I Came Across This and why I Built it

My school has an internet filter and itís rather annoying when I need to look up some magic card and the filter blocks it because its considered games. I knew wifi routers operated at about 2.4GHz and figured if I could direct the antenna enough, I might be able to get the signal to span the distance between the roof of my house and the school. The routerís signal isnít very strong since itís not meant to go all that far so itíll be a challenge to get it to go about ĺ of a mile and still be able to communicate. I wouldnít just be able to stick an amplifier at the output of the router since it also needs to be able to receive the signal from the device, not just talk to it. There were two fairly easy to build, highly directional antennas I found. They were the cantenna and the biquad antenna. Both could be used to illuminate a parabolic dish and achieve higher directionality. The cantenna seemed easier to build so I decided to build this one first.

This biquad design has very high directionality so I may try to use something similar in the future: http://www.qsl.net/yu1aw/fid24ghz.pdf

Design and Construction

I didnít exactly understand the equations mentioned here: circular waveguide modes but will probably understand them more in the future. Till then, I took the easy way out and used a calculator to tell me what can diameter I should use.

Cantenna Dimentions Calculator

From this calculator, for channel 10, I found that my can diameters could be between about 75mm to 90mm in diameter. I went to a local store and looked for cans that fit this range. There werenít many but I did find one that had a diameter of about 82mm. The dimensions of the cantenna came from this calculator.

dim

I used N connectors since they are easy to use and have low losses at high frequencies. I got some crimp male connectors and panel mount female connectors to use. One of the pins broke while crimping it onto the central conductor of the mini 8/U cable. I soldered the pin back on trying to keep the heat to a minimum to prevent the insulation from expanding. Sadly, it had expanded so I used pliers to press it down a bit and then pushed the cable in. After I got the conductor and insulation in, it was fine and I slid the crimp rings down and crimped it. The router didnít have an N connector so I had to use a female N to RP-SMA female. Iím not sure I got the correct terminology right for the RP-SMA but it has threads on the inside and fits onto the routerís antenna connection.

can1 can2

To do a simple test, I connected the omnidirectional antenna to the router and walked up the street, to stay in line of sight, and noted the point where my droid phone said the reception was about -90dbm. Then, I connected the cantenna to the router and aimed it up the street. The phone said the reception was about -70dbm. This is highly inaccurate but it shows that the unfinished antenna is already better than the omnidirectional one.

I didnít have the proper measurement tools to see how the far field looks so I looked around for simulators and found 4nec2. Their default cantenna looks like this:

sim

And has the code:

CM COFFEE.NEC
CM Coffee can feed - open cylindrical waveguide
CM Horn 300mm long, 176mm diameter
CM by W1GHZ from PA3AEF
CE **********************************************
SP 0 3 .088 -.01394 .300 .088 0.01394 .300
SC 0 3 .088 0.01394 .260 .088 -.01394 .260
SC 0 3 .088 0.01394 .220 .088 -.01394 .220
SC 0 3 .088 0.01394 .180 .088 -.01394 .180
SC 0 3 .088 0.01394 .150 .088 -.01394 .150
SC 0 3 .088 0.01394 .120 .088 -.01394 .120
SC 0 3 .088 0.01394 .080 .088 -.01394 .080
SC 0 3 .088 0.01394 .040 .088 -.01394 .040
SC 0 3 .088 0.01394 .000 .088 -.01394 .000
SC 0 3 .035 0.00554 .000 .035 -.00554 .000
GM 0 4 .000 0.0 18.0
GM 0 0 .000 0.0 9.0
SP 0 0 .015 0.015 .000 90.0 .00000 .0009701
GX 0 110
GW 1 4 .0622 .0622 .100 .0198 .0198 .100 .002
GM 0 0 0.0 0.0 45.0 0 0 -0.300
GM 0 0 0.0 90.0 0.0 0 0 0
GE
FR 0 1 0 0 1296.0
EX 0 1 1 0 1.0 0
LD 5 0 0 0 3.72E+07
RP 0 73 73 1001 -90. 90. 5.0 5.0 10000.0
EN

I had tried making my own with a wire cylinder but it ran into problems when I tried to simulate it. Iíll model my design after this cantenna since it works. I havenít got around to doing so though.

Future

I hope I can find the time to build this antenna before the end of the year so I can actually use it to access my internet from school. Iíll also want to try the biquad design since it looks quite directional. For weatherproofing, I hope to use Tupperware containers and something to prevent water from getting into the can which would cause it to rust and would affect the performance of the antenna.