Any real transmitter produces some output at frequencies other than the desired carrier. The two most common unwanted products are harmonics (integer multiples of the carrier) and intermodulation distortion (IMD), produced when two or more signals mix in a non-linear stage.
A transmitter at fundamental frequency F radiates some energy at 2F, 3F, 4F and so on. Each successive harmonic is normally weaker than the last because the amplifier non-linearity that creates them falls off with frequency separation.
Real-world example:
293.040 MHz (between TV channels) and a 3rd harmonic at 439.560 MHz, which sits in the US 70cm amateur band.
207.000 MHz (between TV channels) and a 3rd harmonic at 310.500 MHz (aeronautical band).
When two signals at F1 and F2 enter the same non-linear stage (an overdriven amplifier, an oxidized antenna connection, a corroded tower joint), they produce mixing products at frequencies that are sums and differences of multiples of the originals:
2*F1 ± F2, 2*F2 ± F1. Closest tothe originals and hardest to filter.
3*F1 ± 2*F2. Further out, lower amplitude.practice.
The 3rd-order products are why ham repeaters at a shared mountain- top site have to coordinate frequencies carefully. Two transmitters at 146.760 and 146.940 produce 3rd-order IMD at 146.580 and 147.120, both inside the 2m band.
If you tune across the band and hear a steady carrier where nothing should be, the source is often a harmonic or IMD product from a nearby high-power transmitter (broadcast FM/AM, TV, land-mobile dispatch, public safety). The /harmonics tool's "trace a birdie" feature reverses the math: enter the mystery frequency and your location, and it ranks the most likely licensed sources within a radius.
The FCC sets per-service spurious emission limits in 47 CFR. For amateur radio, §97.307(d) says spurious emissions from transmitters operating below 30 MHz must be attenuated at least 43 dB below the mean power of the fundamental, and at least 60 dB above 30 MHz. Broadcast and commercial services have stricter limits.
A well-designed transmitter with an output low-pass filter easily meets these limits. The interference cases that show up in birdie searches are usually:
is non-linear and is producing the IMD, not the transmitter).
/harmonics tool: forward harmonic and IMD calculator plusthe "trace a birdie" reverse lookup.
full theoretical treatment.