I've been experimenting with different EFHW transformers for portable QRP ops and noticed some interesting efficiency differences. Wise men say the more cores the better the transformer. I am not entirely convinced, but there is merit to the argument that if more copper is inside the core, the magnetic linkage is better. On that basis I chose to build my EFHW transformer using 2 stacked FT140-43 cores. I also built one with a single FT80-43 "wedding ring" core for ultralight POTA. Both work, but I'm curious about real-world efficiency comparisons. It's important to use a quality 100pf cap… those cheapies you got off of Amazon 250 for $10 will not cut it… TDK from Digi-key. If you really are qrp even a 500v silver mica would work. Anyone done actual measurements?

I've tested both configurations with my nanoVNA. The stacked cores definitely show better bandwidth and slightly lower SWR across the amateur bands. Worth the extra weight for base station use but probably not for ultralight hiking.

For QRP levels, honestly I haven't noticed much practical difference between my single FT140-43 and the fancy commercial units. For QRP the answer is no regarding counterpoises too. Sometimes we overthink these things - a simple, well-built transformer beats a complex poorly-built one every time.

I made the mistake of buying a cheap Chinese 49:1 transformer and it was terrible - I cut open the sealed container to find – surprise – a small ferrite bar wound with fine wire. The kind used in old transistor radios. Learned my lesson and wound my own following the K1RF design. Quality components really do matter!