coax cable balun

Hi ,everobody

I have a task to design UHF high power amp. I am using real coax cable as transformer from unbalance line to balance line. Models in ADS and AWR gave me not so good agreement with reality. How to make model of real coax line that will give good agreement with measurements and coax cable length?Model with ferrite is in this frequency range unuseful Please put some useful links,books...for this subject.
In advance thank you very much!
XTASA