PD7MAA’s BN43-202 matching transformer for an EFHW – measurement of a prototype

At PD7MAA’s BN43-202 matching transformer for an EFHW I gave an estimate of the core loss in PD7MAA’s transformer.

This article reports measurement of a prototype built to his design.


Above is PD7MAA’s graphic for his transformer. It is a little confusing as an 11t wind will start and finish with ends as the blue wind, so the red winding must have and odd number of half turns which suggests the windings are actually 1t and 5.5t (pity he did not show a picture of the real transformer).

PD7MAA gives some measurements for his transformer with a 3300Ω load, but he does not give loss measurements. This experiment is to replicate his configuration, measure the loss and compare it to the estimate given at PD7MAA’s BN43-202 matching transformer for an EFHW.

The prototype uses 1t primary and 5.5t secondary. The secondary load is a 3300Ω resistor in series with the VNA 50Ω input port.

Above is a screen shot of a sweep from 6 to 8MHz. The key data is that shown for the marker at 7.1MHz.

Let us calculate the expected value of  |S21|for a lossless transformer, considering only input Mismatch Loss and loss due to division of power between the 3300Ω and 50Ω VNA input.

Using the measured input R,X, we can calculate Mismatch Loss at the input.

The loss due to input mismatch is 0.229dB.

The expected loss due to the division of power between the VNA input and 3300Ω resistor is 10*log((3300+50)/50)=18.26dB.

Total loss from forward power at the network (which includes the 3300Ω resistor) input port to forward power at the network output power is 18.26+0.229=18.49dB, therefore expected |S21| is -18.49dB

Measured |S21| was -21.46dB, so the additional loss due to the transformer (core and conductor losses),  Transmission Loss of the transformer (10*log(PowenIn/PowerOut), is -18.49–21.46=2.97dB. This will be almost entirely core loss.

This is close to the estimated core loss of 3.7dB considering that the estimate was based on matched input and considering the tolerance of ferrite components, and variation from transformer to transformer could well be 1dB.


The measurements support the estimate which remains the better figure because it is based on average core characteristics rather than measurement of a single core.

The transformer measurements confirm that the transformer is quite lossy, a feature that the designer does not disclose.