I came across this PCB no-frills design for 4.5GHz, but all dimensions are given as wavelength fractions, so it could easily be scaled to 2.45G or anyplace else in the spectrum.
It is not fully omni directional, being a double stack panel, but a 2 way splitter/combiner with 2 of these back to back should work.
The reference is (UK) IEE Proc. Microwaves, Antennas & Propagation Vol. 146 No 4 Aug 1999.
It comprises 2 parasitic rectangular patches supported on low k foam, proximity coupled to a 50 microstrip line that has a step rise beneath the lower rectangle. The gain is about 7dBi, the FBR around 20dB, the cross-polar boresight level is around 25dB, but the most spectacular parameter for a patch antenna is the VSWR bandwidth: 58% for a VSWR of 2:1 or better. Beat that