An online expert recently advised:
…The spec for type 43 makes it clear that it should never be used for HF unun construction. It is specifically engineered with a complex permeability that makes the core lossy on most HF frequencies. Since an unun is not a TLT (transmission line transformer) but rather an autotransformer, a low loss core is essential for efficient operation….
Now it contains the very common FUD (fear, uncertainty and doubt) that masquerades as science in ham radio, but without being specific enough to prove it categorically wrong. To a certain extent, the discussion goes to the meaning of
At Small efficient matching transformer for an EFHW I described an ‘unun’ using #43 material, and gave design calcs and measured loss over HF.
I will concede that making loss measurements by that technique becomes less accurate at the high end of HF where the distributed inductance and capacitance of the combined load become significant… but good figures can be obtained below say 10MHz. In most cases, the core losses are greatest at the lowest operating frequencies, so that works well.
Back to the transformer, designs are typically a compromise of a lot of factors such as size, mass, loss, bandwidth etc. In the example case, it is a transformer intended for low power portable operation (eg SOTA) and efficiency is traded for size and mass to name a couple.
Nevertheless, the core efficiency is 90% at the lowest design frequency, 3.6MHz, and is higher at higher frequencies.
This example questions the impression that the online expert tries to leave in readers minds that
#43… should never be used for HF unun construction.
Read widely, and analyse critically what you read.