I am interested in creating a square outer-conductor with round inner-conductor air coax post for a hex beam antenna. I found the extremely useful information at:
How do I design a round or square coaxial transmission line to have a specific impedance?
However, from a practical standpoint, in these explanations it is not clear to me what is the most appropriate measurement to use for the square outer conductor's single-side diameter. For example, if I use 2" x 2" x 0.125" square tube for construction of the air-coax, for the calculations should I use the outer 2" diameter, the inner 1.85" diameter, or the 1.925" average diameter?