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?

Last update: 14th April, 2024, 1:34 PM