Plain-language answer: ESR represents dissipative loss in a real capacitor or other reactive component as an effective series resistance.
Why the real component differs from the ideal
RF current through ESR creates heat and reduces Q and efficiency even when the reactance value looks correct. ESR changes with frequency and temperature.
What to verify
Use frequency-relevant data, calculate expected heating, and verify component temperature at low then increasing power.
The common mistake
Treating a capacitor’s DC leakage rating as its RF-loss specification.
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.
