Why harmonics matter
A transmitter's output stage isn't perfectly linear, so it produces small signals at two, three and more times the operating frequency. A low-pass filter at the output keeps them tiny, and licence conditions require them to be well suppressed. Even so, a weak harmonic can interfere with a sensitive receiver nearby. The 2nd harmonic of 6m lands in the FM broadcast band, the 3rd harmonic of 4m in DAB digital radio, and HF harmonics can fall on other amateur bands and on broadcast radio.
Harmonics or overload?
Most interference to neighbours' equipment isn't caused by harmonics at all, but by the neighbour's equipment being overloaded by your strong fundamental signal, called breakthrough. The test is simple: if fitting a filter or choke on their equipment cures it, it's breakthrough. If a low-pass filter on your transmitter cures it, it's harmonics. Checking for your harmonics on an SDR is a quick way to tell.
Curing it
- Fit a low-pass filter at your transmitter's output. The low-pass filter calculator designs one.
- Choke the feedline and the neighbour's leads with ferrite. See the choke calculator.
- Check your EMF distances at the same time, since both depend on how close your antenna is to the house.
Questions
What is the 2nd harmonic of 145 MHz?
290 MHz. The 2nd harmonic of the 2m band (144 to 146 MHz) falls between 288 and 292 MHz, and 3rd harmonics land in the 432 to 438 MHz range.
Can 6m cause FM radio interference?
Its 2nd harmonic, 100 to 104 MHz, falls in the FM band, so a 6m transmitter needs good filtering near FM radios.
How do I know if it's harmonics or breakthrough?
If a filter on your transmitter fixes it, it's harmonics. If a filter or choke on the affected equipment fixes it, it's breakthrough.
Are harmonics legal?
Licences require spurious emissions to be kept very low, but not zero. A correctly working transmitter with a filter easily meets the limits.