Characteristic Impedance of Split Coaxial Line (Correspondence)

A few years ago, the balun was studied at our laboratory and it is important to know the characteristic impedances of the line. There are several papers concerning this characteristic impedance. The cross section of the transmission lines is shown in Fig. 1 and the characteristic impedances are calculated in these papers. Last year, a paper was published in the IRE Trans. on Microwave Thoery and Technique on this problem, using similar methods described below. I wish to describe my approach and show a disparity.