Electronics

Designer Cantenna

Posted in Electronics on April 17th, 2012 by Matt – Comments Off on Designer Cantenna

Ever since I read about the a directional WIFI antenna made out of a Pringles can I wanted to make one. The only problem is that had no real use. Finally I have an excuse so I though I would finally put one together.

Although cantennas can be made out of many different shape and size cans having done some preliminary calculations I worked out that the tins used to package Jean Paul Gaultier le male would work, thus creating a designer cantenne. I never can bring myself to throw these things away so I have quite a few lying around being used for various assorted purposes, so why not a directional WiFi antenna?

The calculations
There are many website with different ways to calculate the element size and location for a given can. One thing that confused me was why the calculations from the original post and all the rest were different. To cut a long story short the original Pringles cantenna was a directional Yagi antenna whereas the simpler cantennas are cavity resonators. Google these terms and and you you will find all the details you need.

The can was 87 mm in diameter and 172 mm tall. Doing the calculations resulted in a 31 mm transmitter element needing to be mounted 55 mm from the back of the can using an N-type connector. To actually mount the element and socket didn’t take very long, the most time consuming part was calculating the location of the element, and those only took an couple of hours as I wanted to understand what I was doing.

Making it active
When attached directly to my router using a short N to RP-SMA pig-tail performance was good but a 10 cm cable is not very convenient. To get around this, and because I don’t have an external arial port on my MacBook Pro I made the antenna active by directly mounting a USB wifi adaptor with removable arial to cantenna using an N to RP-SMA adaptor, an RP-SMA angle adaptor and finally an RP-SMA gender changer. Yes that’s alot of connections but better than having the USB adaptor sticking out at ninety degrees! Overall RF losses were minimal and the USB cable can be a convenient length.

Performance
Having plugged this thing in I can confirm that this simple device has high gain and is quite directional. As I live in the city there are lots of WiFi networks about. With the build-in WiFi of my laptop I can seven or so from within my house and a couple more from my balcony. With the cantenna I can find over 40, all over 20% signal strength! It would appear every man and his dog has a WiFi network in the centre of Linz! I can even pick up the free wi-fi in the Lentos art gallery at the end of the road about 500 meters away!

More worryingly about 20% of the detected networks were still using WEP to secure their networks. Surprisingly only a few networks were 802.11n with 802.11g still the most popular.