Better EFHW Transformer?
May. 5, 2024Is there a way to build a more efficient autotransformer capable of handling 100W in any data mode…
These are my working notes and summary findings gathered from several articles and videos about two toroid cores: The Fair-Rite 2643251002 for QRO and the 2643625002 for QRP applications. Over the past year, these two siblings have gained popularity for creating more efficient EFHW transformers running 100 watts on digital modes. But why these particular core?
What EFHW Core and Winding to use for HIGH POWER??
Well, many people have done tests comparing these two to other cores. And the results seem to show better efficiencies across the HF spectrum. Makes sense to use the best cores, right?
This led Scotsman, Colin Summers (MM0OPX) using the cores to make a 56:1 transformer for an end-fed half-wave transformer. Two turns on the primary, 15 turns on the secondary. The extra turn on the secondary was needed to get a good match on 40 meters.
This transformer worked great. Lots of people have now built antennas using it. I prefer winding auto-transformers over twisted primaries. They are a bit less fiddly in my experience. So I made one with two of the Fair-Rite cores and tested it on the nano VNA. The results were interesting.
The 56:1 transformer did very well, with efficiency around 90% across most bands. The auto-transformer was even a hair better, especially on the higher bands like 15m where it was almost 5% more efficient.
Now does that 5% difference really matter in the real world? Probably not. You likely wouldn’t notice either way.
But I got to wondering - what if we used this more efficient auto-transformer design but tweaked it to get acceptable matches across all the harmonic bands? Maybe better than we could get with the 56:1 transformer?
This could become my go-to 100 watt end-fed-half-wave design going forward. Although as I said, you probably can’t go too wrong with either the 56:1 or auto-transformer version in practice.
Have you tried winding one of these auto-transformers yourself? Let me know how it went when you put a wire on it. I’m curious to hear about others’ experiences.
Antennas can be a funny thing. You never know what little tweak might unlock better performance until you try it. That’s part of what makes experimenting fun. Even when the results are modest, you learn something.
Evaluating Efficiency For QRP Applications
An efficient 40-10m EFHW antenna for portable ops
Project Files on GitHubEvaluating Efficiency For QRO (100W+) Applications
