At FT82-43 matching transformer for an EFHW I wrote about the likely losses at 3.6MHz of a common design using a FT82-43 ferrite core with a 3t primary. In that case, expected efficiency (meaning PowerOut/PowerIn) of the transformer was less than 65% at 3.6MHz.

I have been offered input VSWR curves for such a configuration, and they are impressive… but VSWR curves do not address the question of loss / efficiency.
Note that building loss into antenna system components is a legitimate and common method of taming VSWR excursions, eg TTFD, CHA250, many EFHW transformers, but in some applications, users may prioritise radiated power over VSWR.
Design context / objectives
Objectives are:
- used with a load such that the input impedance Zin is approximately 50+j0Ω, Gin=0.02S;
- broadband operation from 3.5-30MHz;
- VSWR < 2 with nominal 3200Ω load; and
- transformer efficiency > 90% at 3.6MHz.
The following describes such a transformer using a Fair-rite 2643625002 core (16.25×7.29×14.3mm #43).
I mentioned in the reference article that the metric ΣA/l captures the geometry, the larger it is, the fewer turns for same inductance / impedance. ΣA/l for the chosen core is 3.5 times that of a FT82-43 yet it is only 1.6 times the mass.

The transformer is wound as an autotransformer, 3+21 turns, ie 1:8 turns ratio. Continue reading Small efficient matching transformer for an EFHW
Last update: 31st July, 2019, 9:33 AM