Month: June 2024

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 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

NanoVNA – how accurate does the LOAD need to be – part 2? answered the question in the context of an instrument that assumes the LOAD is exactly 50+j0Ω.

As mentioned in the article, there are more sophisticated models of the imperfection of nominal SHORT, OPEN and LOAD calibration parts (or ‘standards’), but the NanoVNA-H4 does not currently implement any of them.

A simple improvement is to accurately measure the DC resistance of the LOAD part, and use that resistance to calculate the expected LOAD response, and to use that in calculating the error terms that will be used in correction of raw measurements. Continue reading NanoVNA – how accurate does the LOAD need to be – part 3?

This article continues on from NanoVNA – how accurate does the LOAD need to be – part 1?

Measures such as ReturnLoss, Gamma, s11, ro |s11|, VSWR are all wrt some reference impedance, often, but not necessarily 50+j0Ω.

The process of SOL calibration of a VNA, and its subsequent correction of DUT measurements, means that raw measurements are corrected with error terms that are derived from the calibration measurements and a set of responses expected of the calibration parts. Continue reading NanoVNA – how accurate does the LOAD need to be – part 2?