NanoVNA-H4 – inductor challenge – part 4

NanoVNA-H4 – inductor challenge – part 3 visited the basic model of an inductor as comprising a series resistance and inductance, and its failure above perhaps SRF/5, and proposed a simple extension that may give useful prediction of impedance up to SRF and a little above.

Importantly though is that it showed that measurement of Z departed from a frequency independent inductance \(X \propto f\) and some small frequency dependent resistance \(R \propto \sqrt f\) above perhaps 15% of SRF… and so we cannot simply infer the value of the underlying inductance from Z at an arbitrary frequency.

Where \(X \propto f\) we can say that \(L=\frac{X}{2 \pi f}\) (where X is the imaginary part of measured Z).

Let’s return to the plot of L from NanoVNA-APP’s interpretation of measured Z.

At lower frequencies where the plotted value of L is independent of frequency (ie a horizontal line) we can infer that the underlying inductance of the inductor is that value, 20µH in this case (an air cored solenoid). Continue reading NanoVNA-H4 – inductor challenge – part 4

NanoVNA-H4 – inductor challenge – part 3

NanoVNA-H4 – inductor challenge – part 2 showed how to approximately undo the transmission line effects of the measurement fixture to improve accuracy of measurement of the coil end to end.

Let’s look at the impedance plot at the coil ends.

So, it is clear we have a device with multiple resonances… a resonator in broad terms and representing it as a fixed inductance in series with some small resistance is quite inadequate for the frequency range above. Continue reading NanoVNA-H4 – inductor challenge – part 3

NanoVNA-H4 – inductor challenge – part 2

Recall the fixture from NanoVNA-H4 – inductor challenge – part 1:

NanoVNA-H4 – inductor challenge – part 1 stated:

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).

Let’s explore that using Simsmith. Continue reading NanoVNA-H4 – inductor challenge – part 2

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

On testing two wire line loss with an analyser / VNA – part 6

Measuring velocity factor

This article discusses measuring velocity factor using the NanoVNA. The DUT is coax with N type connectors as it provides a better example to demonstrate and learn from. Having acquired competency, extension to two wire lines is just a matter of attending to the matters of a suitable transformer, and appropriate SOL calibration parts.

N type connectors

The ‘standard’ reference plane on N connectors is shown in the diagram above. For the purpose of this article, length measurements were made between the reference planes at both ends of the cable. Continue reading On testing two wire line loss with an analyser / VNA – part 6

On testing two wire line loss with an analyser / VNA – part 5

This article series shows how to measure matched line loss (MLL) of a section of two wire line using an analyser or VNA. The examples use the nanoVNA, a low end inexpensive VNA, but the technique is equally applicable to a good vector based antenna analyser of sufficient accuracy (and that can save s1p files).

Article On testing two wire line loss with an analyser / VNA – part 2 showed a 1:1 transformer for measuring two wire lines without encouraging significant common mode current.

Online experts suggest that the required transformer is one from 50Ω to Zo of the line being measured. It is often said that: Continue reading On testing two wire line loss with an analyser / VNA – part 5

NanoVNA-H4 – a ferrite cored test inductor impedance measurement – s11 reflection vs s21 series vs s21 pi

This article documents estimation of common mode choke impedance by three different measurement techniques.

The test uses a small test inductor, 6t on a BN43-202 binocular core and a small test board, everything designed to minimum parasitics. This inductor has quite similar common mode impedance to good antenna common mode chokes.

Above is the SDR-KITS VNWA testboard. Continue reading NanoVNA-H4 – a ferrite cored test inductor impedance measurement – s11 reflection vs s21 series vs s21 pi

On testing two wire line loss with an analyser / VNA – part 4

This article calculates and compares three models for matched line loss (MLL) based on measurement of a transmission line section with short and open termination.

This article follows on from:

Measurements

The measurements permitted calculation of MLL vs frequency over the measurement frequency range of 10-200MHz.

The measurement frequency range was chosen as appropriate to the intended application range and the available / manageable sample length. To make measurements down to 100kHz with similar measurement noise would have required a test length of hundreds of metres.

Curve fitting

The measurement data was fitted to three popular models for MLL.

Above is a plot of MLL (dB/m) calculated from the measurements saved as s1p files (raw), and fits to three models: Continue reading On testing two wire line loss with an analyser / VNA – part 4

On ferrite cored RF broadband transformers and leakage inductance

By broadband transformer, I mean a transformer intended to have nearly nominal impedance transformation over a wide frequency range. That objective might be stated as a given InsertionVSWR over a given frequency range for a stated impedance. eg InsertionVSWR<2 from 3-30MHz with 3200(+j0)Ω load.

These are used in many things, including medium to high power applications such as EFHW matching transformers.

Leakage inductance is the equivalent series inductance due to flux that cuts one winding and not the other, and vice versa. For most simple transformers, the total primary referred leakage inductance is twice the primary leakage inductance. Since the leakage inductance appears in series with the signal path, it causes degradation of nominal impedance transformation, the very simplest approximation of the frequency response is that of a LR circuit.

Above is a Simsmith model of a 1µH total leakage inductance in series with a 50+j0Ω load, the InsertionVSWR is greater than 1.5 above 3MHz.

Is this a common scenario? Continue reading On ferrite cored RF broadband transformers and leakage inductance