Lpc2000

Hi. Sorry for the offtopic, guys. If anybody can point me out some good 
forum/group to post this question, I'll be glad.

I need to calculate trace witdh and cleareance to achive an impedance 
of nearly 50 ohm for an RF trace surrounded by a GND polygon. My board 
is SINGLE layered, so I have no power or GND planes. There's plenty of 
online calculators for this *if* I have a GND plane, but I couldn't 
find any for surface impedance calculation (i.e., between tracks). Any 
hints?

Guille