Category: NanoVNA

Following on from Common mode choke measurement – length matters #2 which demonstrated that the following test fixture gave invalid results due to the 20mm length of resistor pigtails, the connection length in general terms

Above is a pic of my experimental setup. The resistor on Port 1 is a 10k 1% metal film resistor. The NanoVNA has been SOL calibrated at the Port 1 jack. Continue reading Common mode choke measurement – length matters #3

Following on from Common mode choke measurement – length matters…

Lots of people have reported experiments to show gross failure of s11 reflection measurement of high impedances such as those encountered measuring common mode chokes.

Above is a chart of a “10k resistor with leads” from (G4AKE 2020), the curve of interest is the s11 curve which he describes as unsuitable. He did not publish enough information to critique his measurement… so I will conduct a similar experiment. Continue reading Common mode choke measurement – length matters #2

There must be thousands of Youtube videos of “how to measure a common mode choke” to give a picture of some sort of the test configuration… though most lack important detail… and detail IS important in this case. Likewise there are lots of web pages on the same subject, and some have pics of the test configuration, again mostly lacking important detail.

For the most part, these show test configurations or ‘fixtures’ that might be appropriate for audio frequencies, but are unsuitable at radio frequencies, even at HF.

Connecting wires at radio frequencies are rarely ideal, the introduce some impedance transformation that may or may not be significant to the measurement project at hand. Such connections can be thought of as transmission lines, often mismatched so they have standing waves (meaning the impedance of the load appears to vary along the line.

Let’s take the DUT in my recent article Baluns: you can learn by doing! as an example for discussion.

Let’s take the saved s1p file from a S11 reflection impedance measurement as the example.

Above is a plot of the common mode impedance of the choke, solid line is |Z|, dashed line is R, dotted line is X. This was measured with connecting wires <10mm, see the original article. Continue reading Common mode choke measurement – length matters

The article Baluns: you can learn by doing! presented measurements of a Guanella 1:1 Balun, a common mode choke.

Above is the prototype balun being a Fair-rite 5943003801 (FT240-43) wound with 11t of solid core twisted pair stripped from a CAT5 solid core LAN cable and wound in Reisert cross over style. Note that Amidon #43 (National Magnetics Groups H material) is significantly different to Fair-rite #43.

Above is a plot of the R and X components of Zcm taken from the .s1p file saved during measurement. Continue reading Common mode choke measurement – estimating Cse

NanoVNA examination of stacked ferrite cores of different mixes studied an example stacked core scenario, presenting measurements of a stack of BN43-202 and BN73-202, 5t wound through both.

The article stated:

They are somewhat similar (but only somewhat) to two series chokes with the same number of turns, so you might expect overlap of the responses.

Continue reading NanoVNA examination of stacked ferrite cores of different mixes – more detail

A recent series of articles discussed the question of how accurate does a calibration LOAD need to be.

Following on from that I requested a change to allow the actual resistance of LOAD to be used

These tests are not conducted in a temperature stable laboratory, so allow some latitude in results.

The NanoVNA-H running NanoVNA-D firmware v1.2.35 was SOL calibrated, but the calibration kit had a LOAD that measured 51.273Ω at DC using a high accuracy ohmmeter.

Above is an |s11| sweep after calibration. Measurement is limited by the instrument noise floor, about -80dB @ 1MHz. This says nothing about the load as it is based on a flawed calibration, but it shows us the noise floor. For reasonable accuracy, we might say here that we can measure |s11| down to about -70dB… subject to an accurate calibration LOAD. Continue reading Dislord’s NanoVNA-D firmware v1.2.35 includes a facility to apply a correction based on the DC resistance of the LOAD

A chap asked the assembled experts online:

Has anybody tried stacking say a type 43 on top of a 61 on top of a 31 for a wider bandwidth? Would the losses be any greater than using one toroid for each band?

There were some very firm assertions that this will not work well (without evidential support of course). Beware of firm assertions!

Quickly the case was compared to parallel chokes… and there is no parallel (pardon the pun), they are not the same.

They are somewhat similar (but only somewhat) to two series chokes with the same number of turns, so you might expect overlap of the responses. Two series chokes carry the same current and with the same number of turns apply the same magnetomotive force (mmf) to both magnetic cores.

But rather than hypothesise, even if from experience… lets measure.

Above is the DUT. It is a stack of BN43-202 and BN73-202, 5t wound through both. This is not two chokes in series, it is just like stacking a FT240-43 and FT240-73 in concept. Continue reading NanoVNA examination of stacked ferrite cores of different mixes

This article discusses measurement of a coaxial filter. These are often referred to as “cavity filters”, but strictly speaking, a resonant cavity is different, these are a low loss transmission line section without input and output coupling.

The DUT is a single coaxial resonator configured as a band pass filter with adjustable separate coupling loops. The inside of this nearly quarter wave tube is a coaxial rod grounded at the right had end and almost reaching the left hand end with coupling loops attached to the coax connectors. The rod length is adjustable to tune it, everything is silver plated brass. The adjustable coupling loops allow some adjustment of bandwidth, but narrow bandwidth brings higher loss. Continue reading NanoVNA measurement of a coaxial filter

I have corresponded with many people trying to make valid measurements of a Guanella 1:1 balun, also known as a common mode choke.

In common mode, the device looks like a ferrite cored inductor. That might sound simple, but it is anything but, and it can be a challenge to measure.

There are lots of measures quoted, most of them IMHO are bogus or incomplete, usually specious… which accounts for their popularity.

This article focusses on making a valid measurement of the R and X components of the choke’s common mode impedance by the simplest means that is likely to give direct results. Continue reading Common mode choke measurement – for beginners