Introduction
There are several articles on this site describing EFHW transformers using the Jaycar LO1238 toroid, two particularly relevant ones are:
- End Fed Half Wave matching transformer – 80-20m – LO1238 variant; and
- Another small efficient matching transformer for an EFHW – LO1238 – VK3PY, VK3TU build and measurement.
This article presents a design workup of a EFHW transformer using a Jaycar LO1238 core, a pack of 2 for $8 at Jaycar stores (Australia). The LO1238 is a 35x21x13mm Toroid of L15 material (µi=1500). Boxed up, it is probably safely capable of about 5W continuous dissipation.
I will use the meanings explained at On insertion loss.
The design was developed in a SimNEC model which models a EFHW transformer, and can be calibrated against measurements of implementations. This helps evolve the model and develop some experience for likely values for leakage inductance etc.
The original 1:64 transformer
The original transformer was designed to follow the fashion of 1:64 nominal impedance ratio, ie 50:3200Ω. The transformer is compensated with a 100pF silver mica capacitor.
One suspects the popularity of 1:64 was driven by the high leakage reactance of most implementations which reduced the effective transformation ratio and gave the illusion that 1:64 was optimal for typical EFHW antennas.
Above is the calibrated model of the transformer. Modelled InsertionVSWR (magenta) compares with the measured InsertionVSWR (green). Modelled Loss (or transmission loss) is dotted magenta, and modelled InsertionLoss is dotted blue, the gap between them is MismatchLoss.
Above is a zoomed in view of the chart. Although originally billed as an 80-20m transformer, it is probably usable to 22MHz.
The revised 1:49 transformer
The revised transformer has turns ratio 3:21, nominal impedance transformation of 50:2450Ω, and with a low leakage inductance implementation, probably better suits the impedance of typical EFHW antennas. The transformer is compensated with a 100pF silver mica capacitor.
Above is the calibrated model of the transformer. Modelled InsertionVSWR (magenta) compares with the measured InsertionVSWR (green). Modelled Loss (or transmission loss) is dotted magenta, and modelled InsertionLoss is dotted blue, the gap between them is MismatchLoss.
Above is a zoomed in view of the chart. It has InsertionVSWR<1.5 from 3-25MHz. Reducing the turns ratio improves the VSWR bandwidth.
Pics of the transformer and further evaluation will be in a follow up article.
The Loss of the revised transformer is quite similar to the original, Loss is mostly core loss and it is determined mainly by the 3t primary and core characteristics. InsertionLoss is quite different at the high end, a result of lower load impedance in the context.