In the paper 'Extension of the application of conformal mapping techniques to coplanar lines with finite dimensions' the author uses the t-plane as a intermediate step for calculating the capacitance of a Coplanar Waveguide. I can't find any hint, how the mapping function is derived or what the t-plane is. I'd be very happy about a hint.
Best regards
p.s: added the first three pages of the paper. I'm sorry for the bad quality.