Another small efficient matching transformer for an EFHW – 2643251002 – #4 – G8GYW build and measurement

The article
Another small efficient matching transformer for an EFHW – 2643251002 – #2 – prototype bench measurement continued the development of a transformer design.

This article analyses measurements at 7.1MHz reported by Mike, G8GYW of his build of a similar transformer.

Above is G8GYW’s build, that is a cm grid on the bench.

Above is G8GYW’s measurements using an NanoVNA of the transformer (it seems with a 100pF compensation capacitor in shunt with the 2t winding) with a 2400Ω resistor in series with Port 2 input pin. Assuming that the resistor is accurate and connections are very short, this puts approximately 2450+j0Ω on the transformer (its nominal load) and allows direct measurement of InsertionVSWR (shown above as s11 VSWR).

We can determine InsertionLoss from -|s21dB| by adjusting for the power division of the 2400 and 50Ω resistors: \(L=10 \log \frac{2400+50}{50}=16.9 \text{dB}\). So at the marker frequency 7.1MHz, \({InsertionLoss}=-|s21dB|-L=-(-17.22)-16.9=0.32 \text{dB}\).

It is very interesting to extract the simple Loss (or TransmissionLoss if you like) which is given by \(Loss=10 \log \frac{Power_{in}}{Power_{out}} \text{dB}\), as it can be used to calculate the heat dissipated in the transformer core and windings. This can be done from s11 and s21, a bit tedious but here is a handy little calculator that makes it a little easier.

The s11 and s21 values are obtained from the info panel on the NanoVNA-App screenshot above.

So, whilst the InsertionLoss is 0.32dB, the Loss is 0.26dB (rounded) and we can say that 6% of input power is lost as heat, so that with 50W average input power, dissipation is about 3W. This leads to efficiency \(\eta=\frac{Power_{out}}{Power_{in}}=94.29 \text{%}\).

Mike’s reported measurements at 7.1MHz are quite consistent with my earlier models and estimates.