Basic Qualification · B-008 Interference and suppression · B-008-004-010
An interfering signal from an HF transmitter is found to have a frequency of 56 MHz. What could be the source?
- A Seventh harmonic of an 80-metre transmission
- B Third harmonic of a 15-metre transmission
- C Second harmonic of a 10-metre transmission
- D Crystal oscillator operating on its fundamental
Correct answer: C — Second harmonic of a 10-metre transmission. From the official ISED question bank, verified word for word.
Why
A 10-metre band transmission operates around 28 MHz, and a harmonic is a multiple of that frequency. The full explanation — and one for every question — is in Pro.
Practise this section
When things interfere: harmonics, overload, RFI to and from your station, filters, ferrites and how to fix the neighbour's TV.