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