Use a mirror or lens. Example shape:

http://robocup.mi.fu-berlin.de/bilderGross/intro06.jpg

Equivalents exist with louvers etc, look at certain street lights/garden
lights. They are the opposite of parabolic mirrors (i.e. hyperbolic). The
louver kind is technically a Fresnel hyperbolic reflector. The same thing
can be done with lenses (acoustical or rf or light). The principles are the
same.

You can realistically expect to cover more than 180 degrees of cone with
one, takes two to cover 360 degrees.


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