f antenna 434mhz

Hi,
I need to realize an antenna for 434 Mhz range on PCB.
I would like the antenna as omnidirectional as possible and obviously efficient.
I've an available area of 8x6 cm. Have you any suggestion?
I'm not interested in loop antennas possibly because they have poor radiation.
I've tried to make an Inverted-F antenna resizing one I've found in an article for 2.4GHz application but it resulted in very poor S11 at resonance...
Have you any ideas or examples to give me?
If this couldn't be possible there's someone who can give me some help to define and model a coil loaded monopole for the same frequency? I'm using HFSS.
Thanks a lot!

If you are trying for a monopole-type pattern from a planar antenna, then a center-fed circular p_a_t_c_h can do this. It excites a higher-order mode that will give gain on the horizon.