An interfering signal from an HF transmitter is found to have a frequency of 56 MHz. What could be the source?

To find the source of an interfering 56 MHz signal, we need to look for a fundamental frequency that is an exact mathematical divisor of 56 MHz. The 10-metre amateur band spans from 28.0 MHz to 29.7 MHz. If a transmitter is operating at 28 MHz (which is the bottom edge of the 10-metre band), its second harmonic will be exactly 28 MHz times 2, which equals 56 MHz. Looking at the distractors, the third harmonic of 15 metres (21 MHz) is 63 MHz, and the seventh harmonic of 80 metres (3.5 MHz) is 24.5 MHz. Therefore, the second harmonic of a 10-metre transmission is the only mathematically correct source.