Transformers and flux density

Lots of online discussions on ferrite cored transformer losses might make you think that the core material is in a path in series with the transferred power and that it acts to some extent like an attenuator.

That sort of thinking betrays a lack of understanding of how a transformer works.

If you take a good 50/60Hz 1:1 power transformer, assume no losses, no flux leakage, and ignoring distributed capacitance, you might ask: Continue reading Transformers and flux density

Selfevidently

An online expert held forth on the design of ferrite chokes and transformers, and to quote one paragraph:

Equally selfevidently we don’t want ANY real part of the reactance in a transformer and, for a practical transformer, we want the self inductance on each side (primary and secondary) to be at least j10*R(Load or Source) and the coupling to be as close to 100% from primary to secondary. It is the real part that heats up transformers a LOT and, since ALL of the current is seen by the ferrite in a transformer, not just the part that got reflected back on the outside of the coax in a choke, losses are abos-posilutely-undubiously NOT desired and the u”R needs to be as close to zero as we can get at the designed frequency for minimum loss and minimum power dissipation.

Setting aside the hyperbole and the wooly thinking, let’s drill down on u”R needs to be as close to zero as we can get at the designed frequency for minimum loss and minimum power dissipation.

It is a pretty general statement without really specific quantities, needs to be as close to zero as we can get and minimum loss and minimum power dissipation does not give useful guidance of acceptable values of µ”, and may even impart the impression that the following chart is for material that is not suitable above perhaps 200kHz, if that.

Above, µ” is greater than 10 above about 200kHz, greater than 100 from about 2 to 100MHz. Is this what the quote condemns? Continue reading Selfevidently

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

InsertionVSWR of Chinese 1-9 balun module – that load resistor

InsertionVSWR of Chinese 1-9 balun module showed a 450Ω load attached to the DUT, and I have been asked to explain further.

Above is the load resistor just visible to the left of the spring terminals on the module. The idea is that the leads need to be very short to avoid unintended / undesired impedance transformation. Continue reading InsertionVSWR of Chinese 1-9 balun module – that load resistor

InsertionVSWR of Chinese 1-9 balun module

This article documents an InsertionVSWR test of a cheap Chinese 1-9 balun purchased for less than <$5 on eBay (shipped).

Above is a 1-9 balun, it would seem to be a clone of the Noelec 1-9 balun. The balun is a compensated voltage balun with the secondary centre tap grounded for these measurements. Continue reading InsertionVSWR of Chinese 1-9 balun module

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

The Smith chart, a thing of beauty… and great utility

A recent post online provides an interesting demonstration of the value of the Smith chart in analysing a measurement problem.

I have 5.175m of “JSC 1320 300 Ohm Ladder Line 300 Ohm 20 AWG / 7 Strands Bare Copper”. … The first step is to sweep it to determine the velocity factor. Yet, when I sweep from 12-17MHz, I get the Smith chart attached. There’s no point when the impedance is close to zero.

It helps to understand the nature of what one is measuring, indeed the expected outcome if possible. Continue reading The Smith chart, a thing of beauty… and great utility

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