Explanation: A detuned antenna coupler (also known as an antenna tuner or ATU) fails to properly match the impedance of the transmission line to the antenna's impedance. When these impedances are not matched, a portion of the radio frequency (RF) energy sent by the transmitter is reflected back down the transmission line instead of being radiated by the antenna. This mismatch and the resulting reflections create standing waves on the line, which are measured as a high Standing Wave Ratio (SWR). Excessive modulation (A) or changes in output power (B and D) relate to the signal's characteristics or strength, respectively, but do not alter the inherent electrical impedance match between the transmission line and the antenna system. High SWR is fundamentally an impedance mismatch problem.