Category: Measurement

A reader of EFHW transformer measurement – how accurate does the load need to be? asked whether the discussion applies more generally, in particular to the loads used for calibration and measurement with a VNA.

In this article, unless stated otherwise, reference to |s11| and ReturnLoss are to those quantities expressed in dB. Note that |s11|=-40dB is less than |s11|=-20dB. ReturnLoss and |s11| are related, ReturnLoss=‑|s11|.

Measurement 1

As a basis for discussion, let me offer an example measurement.

Above is a scan of a certain DUT after SOLT calibration of the NanoVNA. Continue reading NanoVNA – how accurate does the LOAD need to be – part 1?

Several articles on this site use the following technique for measurement of transformer performance, and the question arises, how accurate does the load need to be?

Let’s set some limits on the range of ReturnLoss of interest. Measured ReturnLoss is limited by the instrument, and in the case of a VNA, its noise floor and the accuracy of the calibration parts used are the most common practical limits. That said, in practical DUT like an EFHW transformer, would would typically be interested in measuring ReturnLoss between say 10 and 32dB (equivalent to VSWR=1.05) with error less than say 3dB.

There are many contributions to error, and one of the largest is often the choice of transformer load resistor. This article explores that contribution alone.

2% load error

Let’s say the load resistor used is 2% high, 2450+2%=2499Ω. To measure ReturnLoss with such a resistor is to imply that the transformer is nominally  \(\frac{Z_{pri}}{Z_{sec}}=\frac{51}{2499}\) and ReturnLoss should be measured wrt reference impedance 51Ω.

To measure ReturnLoss wrt 50Ω gives rise to error.

Above is a chart of calculated ReturnLoss wrt Zref=51 (the actual ReturnLoss) and Zref=50 (the indicated ReturnLoss) for a range of load resistances, and the error in assuming RL50 when RL51 is the relevant measure. Continue reading EFHW transformer measurement – how accurate does the load need to be?

An online expert talking about compensation capacitors and EFHW ferrite cored transformers opined:

If the evaluation is done solely by the effect on measured SWR, whether it is measured with a standard reflectometer or a VNA, then it is just as likely the capacitor is changing the losses in the transformer rather than actually adjusting the match.

“Just as likely” + gobbledygook, is this just hand waving on social media?

Let’s explore it using the calibrated model used in a series of articles starting with 1:49 EFHW transformer using a Jaycar LO1238 core – design workup.

Above is the SimNEC model as calibrated to bench measurement of a prototype transformer. The compensation capacitor Ccomp is specified as 100pF with Q=1000 (reasonable for a silver mica capacitor that is well suited to the application). Continue reading 1:49 EFHW transformer using a Jaycar LO1238 core – capacitor loss

An online expert discussing broadband RF transformers recently opined “… if you measure k, the correlation of k and performance is excellent” whatever “performance” means.

Presumably he means k as in the flux coupling coefficient of two flux coupled inductors, ie inductors with mutual inductance (meaning changing current in one inductor induces an EMF in the other inductor). k is the proportion of flux due to current in one inductor that cuts the turns of the other inductor, it is usually stated pu (per unit) but sometimes in % (per cent or per 100).

A common metric for the performance of a broadband transformer is its InsertionVSWR. Other factors might be considered, but InsertionVSWR is commonly most ranked. Note that to describe a transformer as 1:49 implicitly invokes InsertionVSWR as a measure of its performance.

One of the enemies of broadband performance is flux leakage, k less than unity. The equivalent leakage reactance is usually the main contirubutor to high frequency roll off (an increase in InsertionVSWR at high frequencies) in good designs.

Let’s explore the ‘magic’ using the calibrated model used at 1:49 EFHW transformer using a Jaycar LO1238 core – design workup.

Above is a chart from that model showing: Continue reading 1:49 EFHW transformer using a Jaycar LO1238 core – the magic k factor