NanoVNA-H4 – inductor challenge -part 1

Let’s explore measurement of a test inductor with the NanoVNA.

Above is the test inductor, enamelled wire on an acrylic tube.

Let’s hook it up to the NanoVNA for an s11 reflection measurement of Z.

Above, one wire is plugged into the centre pin of the top / Port 1 connector. The other wire is clamped to the external male threads of the Port 2 connector using a plastic clothes peg. Note that this VNA is modified, it has the two coax outers bonded together.

In fact, we have the underlying inductor connected by 35mm of 570Ω two wire transmission line, so there is a small amount of impedance transformation (which could be approximately corrected in this case by setting port extension to 20ps… but that is not done for this article).

An ideal inductor of 20µH would have zero resistance and reactance proportional to frequency.

Let’s look at measured impedance from 1-5MHz. My fork of NanoVNA-App will by used.

Above, measured Z of this practical inductor looks a bit like that, very low R and X∝f.

Above, we can plot the equivalent series inductance from the measurement, and it looks like 20µH independent of frequency.

That all looks pretty good… but let’s measure Z of this practical inductor from 1-200MHz.

Above, this is nothing like zero resistance and reactance proportional to frequency.

What is going on?

Continued at NanoVNA-H4 – inductor challenge – part 2