Tin can aerial materials
If you don't want to dig deep into your wallet for a directional antenna, you can build all kinds of antennas yourself using household materials: instructions for building antenna using CD spindles, aluminium reflectors, polystyrene-foam Yagis and many other home-brewed devices can be found on the internet. But one of the simplest easy-to-build designs is a tin-can antenna. This consists of a tin can with a coupling pin stuck into it, and a socket for the connecting cable to the router.
In principle, any electrically conductive material is suitable for making a tin-can antenna. It's worth having a look at the household-goods department in your local supermarket, or even at your domestic waste: candidates for recycling include noodle cans, food cans, coffee cans, packaging materials of high-profile spirits, and much else. The optimal internal diameter for 2.4 GHz WiFi is between 84 and 92 millimetres - or even up to 111 millimetres with slight losses in gain and directivity. Contrary to what is claimed in many do-it-yourself guides, to our knowledge it's length that counts. Since some trial and error is required for a successful build, we give some typical dimensions in our antenna calculator, below. The can's length doesn't have to agree precisely: you'll only need to reach for your saw if it's more than two centimetres out.
Antenna calculator: Dimensions for a tin-can antenna
As you can't easily take this online calculator to the shops with you, we've put together some typical dimensions in the Table below.
|Optimal can dimensions at 2.45 GHz|
|76||231, 416, 601,…||93|
|78||194, 350, 506,…||78|
|80||172, 310, 449,…||69|
|82||157, 284, 410,…||63|
|84||146, 264, 382,…||59|
|86||138, 249, 360,…||55|
|88||132, 237, 343,…||53|
|90||126, 227, 329,…||51|
|92||122, 219, 317,…||49|
|94||118, 213, 307,…||47|
|96||115, 207, 299,…||46|
|98||112, 202, 291,…||45|
|100||109, 197, 285,…||44|
|all measurements in millimetres|
If weatherproofing and durability are required, a can made of stainless steel or aluminium makes a lot of sense. The sides of the can should be smooth, without the reinforcing ribs that many food tins have. This is particularly important for the bottom of the can, which should be completely flat. Otherwise scattering and destructive interference will occur, and this will degrade both antenna gain and the associated directional effect.
Holes and slits in the sides of the can are not a problem as long as their size is less than a tenth of the WiFi wavelength – approximately six centimetres for 2.4 GHz. If you're in any doubt, a layer of aluminium foil and adhesive tape will put things right. Reinforcing rings or other metal parts that project into the interior space are major sources of interference and should be avoided.
The stainless steel designer toilet brush holders are particularly suitable. For our projects, we shopped at the nearest DIY hypermarket, because that allowed us to check the cans on the spot. Even without cutting, the dimensions of the holder we found are virtually ideal, with an internal diameter of 91.5 mm and a length of 240 mm. Stainless steel holders are rust-proof, so they are suitable as antennas for outdoor installations as well. We found these priced around £12 - £15, putting them into the luxury bracket. Similar-looking models can be found on the internet for £10 or so.