There are many ways to get a good estimate of the characteristic impedance Zo of a transmission line.

One method is to measure the input impedances of a section of line with both a short circuit and open circuit termination. From Zsc and Zoc we can calculate the Zo, and the complex propagation constant \(\gamma=\alpha + \jmath \beta\), and from that, MLL.

Calculation of Zo is quite straightforward.

The solution for γ involves the log of a complex number \(r \angle \theta\) which is one of the many possible values \(ln(r) + j \left(\theta + 2 \pi k \right)\) for +ve integer k. Conveniently, the real part α is simply \(ln(r) \). The real part of γ is the attenuation in Np/m which can be scaled to dB/m, and the imaginary part is the phase velocity in ** ^{c}**/m. The challenge is finding k.

Let’s take an example from recent measurements of 35m of CCS RG6 coax, and extract the s11 values recorded in saved .s1p files @ 1.87MHz. The saved data in MA format, magnitude and angle (in degrees).

Calculate Zo and gamma is flexible and can accept the MA format data directly.

Above, the results. Zo is 74.73-j1.156Ω, and matched line loss MLL is 0.03281dB/m. This MLL is quite a deal higher than you might find in many line loss calculators, they often fail on CCS cables. Continue reading Measure transmission line Zo – nanoVNA – CCS RG6