Transmitter Harmonics: How Crude Radios Leak Energy Outside Their Band
Every radio transmitter produces unwanted emissions at integer multiples of its operating frequency, and unrefined or homemade transmitters can leak surprising amounts of power into bands they were never designed to use.
When a radio transmitter generates a signal at a particular frequency, it never produces a perfectly pure tone. The nonlinear behavior of amplifier stages and oscillators creates additional output at integer multiples of the fundamental frequency. These are called harmonics: the second harmonic is twice the fundamental, the third is three times it, and so on. A transmitter operating at 14 MHz therefore radiates secondary energy at 28 MHz, 42 MHz, 56 MHz, and higher. The fundamental might stay safely in the HF band, but the second and third harmonics land in the lower VHF region, which propagates very differently. Where the fundamental might bounce off the ionosphere and hug the planet, the harmonics can punch straight through into space. Commercial and licensed amateur radio gear is designed to suppress these spurious emissions. Low-pass filters on the transmitter output and careful amplifier biasing typically push harmonic output 60-80 dB below the fundamental, which is enough to satisfy regulators such as the FCC and international ITU regulations. At that level a 100 W station leaks well under a milliwatt at each harmonic. Unrefined or homebuilt transmitters are a different story. Without good output filtering and with simple Class C amplifiers, second-harmonic suppression can be as poor as 20-30 dB. That same 100 W station might leak 1-10 W at the second harmonic and additional watts at higher multiples. From space those harmonics look like an entirely separate VHF transmitter, riding above the ionospheric trap. This is why dismissing HF as 'invisible from space' undersells crude transmitters. The fundamental may stay home, but the harmonic spray does not. For a fictional or historical post-collapse society cobbling together radios from first principles, the harmonic leakage would likely exceed the intended emissions in any frequency band an outside observer could actually hear.