SIM900 antenna design
I would take a look at the Taoglas PA.22a chip antenna or the Pulse W3073.

They tend to be a little pricey on low quantities, but they are purpose built quad-band (850/900/1800/1900 MHz) antennas that should work with the SIM900 if appropriately designed.

A well matched and well designed chip antenna will certainly radiate enough power based on the max-power output of the SIM900 (1W-2W).