It is possible to make a homebrew omnidirectional antenna, as shown in the guides listed on Links#Antennae.
The instructions shown the websites listed on Links#Antennae were used to build some cantennae as shown in the pictures:
Left to right,Cantenna on Tripod, Conical Feed element and Cantenna with bicycle spoke element
This File:Circular waveguide optimise.xls can be used to optimise the variables when building a cantenna.
A sectoral antenna provides the best of both worlds in terms of gain as well as beam-width and can be applied to many situations in mesh networking.
Turn an Omni into a Sector
The reflector designs shown, found at Freeantennas.com can be applied to a standard omni and will turn the omni into a sectoral antenna.
Metal sheeting can be used for the reflector, however, the ideal would be some sort of mesh metal, with holes smaller than 1/4 wavelength (<3cm), so that the antenna is not affected by wind as much.
NEC 2 was used to generate the simulated field pattern for a cantenna with the dimensions used in our first test cantenna.