Plain-language answer: Transformer efficiency is RF power delivered usefully to the load divided by RF power accepted at the input under specified conditions.
Why the real component differs from the ideal
Copper loss, core loss, mismatch, leakage, dielectric loss, common-mode current, frequency, power, and temperature all affect it.
What to verify
Measure insertion loss with suitable calibration and matched fixtures, then test temperature and balance at realistic power and load.
The common mistake
Inferring transformer efficiency from low SWR alone.
Component boundary
Use frequency-specific ratings and the exact topology, material, geometry, load, power, duty cycle, and cooling. Begin at low power and stop for heating, odor, arcing, distortion, or unstable measurements.
