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Waveguide antenna is a type of DirectionalAntenna and is a modified waveguide. A waveguide is simply a tube, either rectangular or cylindrical in cross-section, that will carry a microwave radio signal with very low signal loss. They are sometimes used in place of co-axial antenna cable. If one end of the tube is left open to the world, the signal will exit in that direction and hey! You've got a waveguide antenna.
A WaveguideCantenna is one of the easiest DirectionalAntenna to make at home. It consists of a steel can of appropriate size with an N-connector mounted in the side. Waveguide antennas are often enhanced with funnel shaped structures on the open end to increase their gain (an example of this design is CookieCantenna). They are also frequently used to feed signals to DirectionalParabolic antennas. Combining a surplus satellite dish with a waveguide antenna can result up to 22dB gain (see DirectionalParabolic).
Not to be confused with SlottedWaveguide, another type of waveguide antenna that is flat and tall in shape, and may be either unidirectional or omnidirectional.
The very simple design of waveguide antennas (sometimes called cantennas) makes them easy to build yourself.
Detailed explanations and some theory can be found here. This page also tells how to attach a funnel to the end of the can for even better performance.
A spreadsheet to generate NEC2 models for cantennas can be found here.
Gregory Rehm had performed a Homebrew Antenna Shootout (2002.02.14), comparing Rob Flickenger's PringlesCantenna and TinCantenna designs with his own optmized verions. It appears that the TinCantenna beats PringlesCantenna everytime.
Make one yourself from the following instructions:
Waveguides can be rectangular (by example a Tetra Brik Milk Box (1L) contains enough aluminium to guide waves and has the right dimensions for 802.11b (see the bottom of MicroTVAerial). You can add a horn to get more dBs, the gain is function of the aperture (see CardboardHorn).
There is an interesting hack on Wireless-Fr about a WET11 coupled with a tin can. This allows to place the antenna on a good spot while powering the device with PoE. Ofcourse, you don't need to use all those connectors. It is in French, but there are nice pics. There are also links to pages showing the inside of the beast.