# Coaxial cable test with VNA to estimate Wave Velocity and Attenuation

I'm doing a lab course concerning microwave measurements, during which I'm supposed to test an open, short and 50 Ohms terminated coaxial cable. The cable is connected to a VNA at the S_11 port and for each cable type, the phase as well as the magnitude (in linear and log scale) were being measured in the frequency domain. The task is to estimate the wave velocity in the cable (and from that the cable length) as well as the attenuation. I assume this is a really basic problem but I'm completely new to the field of electrical engineering, so I'm stuck at how to start. All I found is that the velocity can be estimated using the phase/ the "period length" of the phase since obviously it has a periodic behavior.

With the far_end shorted, the VNA will show a notch (or peak) at some frequency proportional to the length.

The depth of the notch, or height of the peak, indicates attenuation.

From what I recall of such measurements (discussed in the literature or the equipment manuals), an OPEN is easier to achieve than a SHORT. The SHORT needs 0 inductance, which is impossible given the finite length of wire needed to short the center pin to the shield.

Thus your instructor probably wants you to learn about that (small distance, but not zero) SHORT as an error source.

Have fun.

• Short is easier than open. Just slap a metal surface across the reference plane. Opens have fringing capacitance that needs to be characterized and corrected for. At least in coax, this is much worse than the inductance of a short standard. In other geometries (that OP didn't ask about) the parasitic of a short might be comparably problematic to those of an open. Jul 7, 2020 at 14:34