Hi David,
Efficiency depends on style of winding, core dimensions and attention to details when construction the transformer. For example tight winding on the core and short wires at primary side play big role in good result. See my comparison tests on FT140-43
I never got to 98-99% range but managed 94.4% using FT82A-43 using KN5L method of wind. It also was only on 20m and 17m band, other bands were 91-92.?
Owen Duffy was initially sceptical as popular FT240-43 with 14:2 turns has efficiency of 82-84% at best. However, 3xFT240-52 got his approval as efficiency was over 90% with 14:2 turns. Still you need to wind it tight and look after construction details. I for instance use FT240-43 and FT140-43 inside to improve efficiency and at base QTH I use transformer which has two FT240-43 and two FT140-43 in them. Efficiency is in the range of 92-93% on most bands.
Owen also recommends method to measure efficiency of transformer by connecting antenna terminal through 2450ohm resistor to S21 input. Resistor and input impedance form voltage divider and you can calculate losses/attenuation of that system. In my case it was -17.1db as resistor was of slightly higher value than 2450ohm.
You can read transmission losses (blue line) of the system subtract losses of voltage divider to get transformer losses.?
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Example of measuring setup
I hope this will help you to explore and experiment with 49:1 transformers and antennas.
GL 73 Marek
