This article documents the process of design, prototyping, measurement and final build of a 1:49 impedance ratio (1:7 turns ratio) EFHW transformer, exploring some alternative designs along the way, a collaboration between VK3PY and VK3TU with a little guidance.
The transformer is wound on a Jaycar LO1238 35x21x13mm toroid of L15 material (L15 appears to be a NiZn ferrite based on its very high resistivity), they sell at $7 for a pack of two.
2:14 winding
The first test was of a 2:14 turn winding terminated in a 2450Ω load. The transformer is an autotransformer of 2+12t with 91pF compensation capacitor installed in shunt with the 2t winding.
As expected, |s11| is pretty poor at the low end, corresponding to an InsertionVSWR=1.7 @ 3.5MHz.
Design rejected due to high InsertionLoss, magnetising admittance too high.
3:21 turn windings
The transformer is an autotransformer of 3+18t with 91pF compensation capacitor installed in shunt with the 3t winding.
Two winding configurations were explored, the very popular cross over style winding and the less common single layer close wound plain winding.
The winding configurations were made on two different cores from the same bag… so there is possibility of some variation… ferrites are like that.
Above is a plot of InsertionVSWR of the two configurations with a nominal 2450Ω load. The red trace is the cross over winding and the blue trace is the plain winding.
3:21 cross over style winding
Above is the prototype transformer for measurement.
The transformer was measured with the 3t winding connected to Port 1 and the top end of the winding connected to Port 2 via a 2400Ω 1% 1210 SMD resistor.
Above is a plot of the loss components calculated from the .s2p file.
Above is a plot of the ReturnLoss and InsertionVSWR. It is OK but for the very high end were above 25MHz, InsertionVSWR increases rapidly.
3:21 plain winding
Above is the prototype transformer in the test jig for measurement. The white material is 4mm thick PVC to isolate the transformer somewhat from the copper plane.
Above is a plot of the loss components calculated from the .s2p file.
Above is a plot of the ReturnLoss and InsertionVSWR. It is OK but for the very high end were above 25MHz, InsertionVSWR increases rapidly… but not as bad as the cross over configuration.
A Simsmith model was constructed and calibrated for the plain winding configuration.
Above is a screenshot of the model.
Above, the blue trace is measured InsertionVSWR and the solid magenta trace is model Insertion VSWR. After calibration (adjustment of Ll and cse), the two traces are very close.
The yellow and green traces are the model input R,X. the dashed red curve is the power in the load with 1W input. The dashed magenta curve is loss (or TransmissionLoss if you want to distinguish it), this is the quantity that gives rise to heating of the core, compensation capacitor, and wire.
Temperature rise / power rating
At 100W impressed on the 50Ω primary, peak magnetic flux is way less than saturation (~0.3T). For other than pulse applications, maximum power (and flux) will usually be limited by heating rather than magnetic saturation.
We can estimate the temperature rise due to heat dissipation in free air from the surface area of the core.
40° rise on say 30° ambient is about as much as is compatible with many plastic insulation materials… but enough to burn skin. About 3W average power will achieve that rating.
So at frequencies where to efficiency is poorest at 90%, the transformer is suited to continuous or average power input of 30W (and for example, 30W continuous RTTY, 60W continuous A1 Morse Code).
If we take the average to peak ratio for SSB telephony to be -15dB (see Average power of SSB telephony), then this transformer should be capable of about 1000W PEP of unprocessed speech (still below core saturation), perhaps more like 250W PEP of speech with 6dB processing. In practice, allowing for duty cycle and conversation style speed with pauses, these become conservative ratings.
Conclusions
The transformer as built and measured has performance that should be acceptable to most users, quite probably better efficiency than some QRP style ATUs.
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