Hi Miro
I believe you asked the right question here.
In my opinion, not having the correct characteristic impedance (Zc) for the CRC that matches the Coax/Antenna system slightly alters the impedance seen by the coax from the antenna and also the propor resonnance frequency , CRC become a part of resonnance frequency . It also affects the impedance measurements taken at that point, especially if the NanoVNA is calibrated right at the point just below the antenna. In such a case, the measurements will be distorted by the transformation introduced by the CRC's twin-lead line.
For example, consider a CRC with a Zc of 140 Ohms used with a 50 Ohm coaxial system: the impedance measured with or without the CRC will be different, and correcting the antenna accordingly becomes much more difficult. On the other hand, if the CRC has a characteristic impedance equal to that of the Coax/Antenna system, the measured impedance will be the same before or after the CRC. This would greatly simplify both the calculations and the practical adjustments needed to optimize the antenna system.
73's Nizar
