Slotted Waveguides are a common type of antenna, for more information go to http://www.trevormarshall.com/

A variation is the Downpipe Antenna at http://members.iinet.net.au/~clark/FreeNet/Waveguide/

The above waveguides side by side -- Antenna Comparison Testing
http://martybugs.net/wireless/antennacomp.cgi