Knowns anybody the (complex) impedance of the CC2520 chip? The datasheet only states the following:
"The RF input/output is high impedance and differential. When using an unbalanced antenna such as a monopole, a balun should be used in order to optimize performance. The balun can be implemented using low-cost discrete inductors and capacitors only or in combination with transmission lines replacing the discrete inductors."
Inductance values for transmission lines are not mentioned.
Information in the reference design (http://focus.ti.com/docs/toolsw/folders/print/cc2520em_refdes.html) only contains the layout files, no calculations, formulas or source files from the software it is designed in. This makes it very hard to develop a balun for boards with a different stackup or to choose a chip balun.