Ellington 3 x FT240-52 matching transformer for an EFHW

Ellington describes in a Youtube video his high power matching transformer for an EFHW, he rates it suited to 500W CW.

Like almost all such ‘designs’, they are published without supporting measurements or simulations.

The transformer is intended to be used with a load such that the input impedance Zin is approximately 50+j0Ω, Gin=0.02S.

Analysis of a simple model of the transformer with a load such that input impedance is 50+j0Ω gives insight into likely core losses.

Let us calculate the magnetising admittance of the 2t primary at 7MHz. The core is a stack of 3 x FT240-52 ferrite toroids.

Gcore is the real part of Y, 0.00104S.

If Yin of the loaded transformer is 0.02S, we can calculate the core efficiency as 1-Gcore/Gin=1-0.00104/0.02=94.8%, core loss is 0.23dB.

Whilst this might be smaller than many similar designs, it must be considered in the context of the 500W CW rating.

The average power of A1 Morse code modulation is about 44%, so the average power of a 500W CW transmitter is about 220W, and 5% dissipation is 11W average which is probably within the capability of the cores, especially with ventilation as shown.

Losses in the matching transformer (in this case 25W of the 500W CW input) are only part of the total system loss, and overall system efficiency will be lower than estimated here for the transformer alone.

The core loss for this configuration is worse at 14MHz, nearly double and solution is left as an exercise for the reader.

The magnetising impedance is so low on the lower bands that near ideal impedance transformation is unlikely, it may nevertheless perform adequately in some specific EFHW scenarios.

In summary, it uses three relatively expensive cores, has poor magnetising impedance on the lower bands compromising near ideal impedance transformation, and moderate core loss in mid HF, but on the positive side, large surface area to assist is dissipating heat.