UK Cantennas

¾ view of the front In April 2008 I built my first tin can wifi antenna, or cantenna.

This type of antenna is very simple to make (much simpler than a Pringles Yagi for example).

My design was inspired by Gregory Rehm's page where Gregory describes how to create a cantenna from relatively simple components.

I've created this page to give some UK specific information on building cantennas.

Components

Construction

There are a few gotchas worth knowing about prior to construction:

Soldering

The copper wire and the brass core in the N chassis connector are two large pieces of metal to solder together! It's very hard to get them hot enough for the solder to flow, and when they did get hot enough, the brass core to the N chassis connector expanded and forced its way out of its nylon surround.

If you can get a piece of metal rod and a hammer, try pushing the brass core out of the N chassis connector, so that you can get it (and the copper wire) really hot, for a good flowing solder joint without melting the nylon core in the N chassis connector.

Mounting the probe in the tin

I cheated. My friend Marc is a jeweller with a totally sweet metal mill, with which he drilled a very neat slot in my tin.

I used Kioan's CantennaCalc to calculate my probe's size and position, and you can see how Marc created a slot for positioning the probe between about 42mm and 48mm from the back of the tin.

For a 100mm diameter tin in Europe...

Channel Probe position Probe height
#1 45.4mm 31.1mm
#13 43.1mm 30.3mm

However, in practice, I haven't discovered any advantages to tuning a 100mm-wide cantenna, so you could probably just make a hole at 44mm and do whatever tuning you can by bending the probe towards the back or the front of the tin. Also, the corrugated base of these tins possibly makes sub-millimetric probe positioning a bit of a moot point.

My probe is Blu-tacked in place, and strapped on tightly with a daisy-chain of zip ties.

Mounting the cantenna

Being able to adjust the "polarity" (ie: the axial orientation of the probe element) of the cantenna is important. I suspect this is because of bounced signals having odd polarisations: I've certainly picked up different access points based on polarity.

I have a rather nifty mini tripod that makes an excellent mount. Anything with a ball-and-socket head will work similarly like a treat:

Using the cantenna

I've only tried my cantenna so far in Windows XP, which only lists the visible access points in range, giving them a strength rating of 1 to 4 bars. I've seen around 25 to 30 access points out of my kitchen window, including that of the local pub — a "BT OpenZone".

It would be more interesting to automate and log the output of access point scans (Unix style), but right now, I don't have an Ubuntu installation which supports the rt2500 chipset in my wifi card.

¾ view of the front ¾ view of the rear The cantenna pointing out my kitchen window The side, showing calibration marks.

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