Read some application notes from frescale freescale.com In rf section about zigbee transceiver You can find lot information about design of pcb antennas for 2.4GHz
Check out the standard IEEE 802.15.4, there are useful stuff on the antenna. I built an application with it a while ago using CC2430.
A good point to know is that Chipcon has proposed an antenna for zigbee. Check the application note of CC2430. It's very good, I haven't tried it, but I am sure it works.
I can email you the AP if you have any problem downloading it.
I am for the FYP of this year, however, it is the "Antenna for Wimax". Would you mind sending me the reference received 2 years ago ? I am in urgent way at this timing ... ...
Normally those printed antennas indicated by ZigBee chip companies present very poor performance.
You should investigate other (bigger) antennas if the range of the Zigbee is too short. I guess patch antenna is Ok (though the small band it has might be a problem). Pifa is also good, but I would go for a 3D one, not printed.