Part of our Start Here: A New Ham’s Guide to Getting on the Air guide.
When you transmit, you intend to put out one clean signal on your chosen frequency. In reality, transmitters can also leak out unwanted signals on other frequencies. These are called spurious emissions, and keeping them under control is part of being a responsible operator. Here is what they are and why they should be on your radar.
What a spurious emission is
A spurious emission is any output from your transmitter that falls outside the signal you actually intend to send. In other words, it is unwanted radio energy on frequencies other than your operating frequency and its necessary bandwidth. If your signal is a clean note, spurious emissions are the off-key extras leaking out alongside it.
They come in several forms:
- Harmonics. These are multiples of your operating frequency. If you transmit on 7 MHz, a harmonic could show up at 14 MHz, 21 MHz, and so on. Harmonics are the most common type of spurious emission.
- Mixing products. Signals inside the radio can mix together and produce output on unexpected frequencies.
- Splatter. When a signal is overdriven or distorted (often from too much audio or too much mic gain on SSB), it spreads out and interferes with adjacent frequencies.
- Parasitic oscillations and other stray output. Amplifiers and circuits can self-oscillate and generate energy where they should not.
Why they happen
Some level of spurious output is a fact of life in any transmitter, which is why radios include filtering to suppress it. Problems arise when something pushes those emissions higher than they should be. Common culprits include overdriving a transmitter or amplifier, running too much drive or mic gain, a poorly designed or faulty amplifier, a failing component, or a homebrew project without adequate output filtering. Older or cheaper equipment can also be dirtier than well-designed modern gear.
Why you should care
There are three good reasons this matters, and they build on each other.
- It is the rules. Regulations (the FCC in the US, and equivalents elsewhere) require your transmitted signal to be clean and set limits on spurious emissions. Putting out a dirty signal can put you out of compliance with your license.
- You interfere with others. A harmonic from your transmitter can land on another ham’s frequency, on another band entirely, or worse, on services outside amateur radio like public safety, aviation, or a neighbor’s TV and electronics. Splatter makes you a nuisance to stations near your own frequency.
- It reflects on you and the hobby. A clean signal is a point of pride. A dirty one can earn you a reputation, complaints, and in the extreme, attention from regulators. Being the source of interference is exactly what nobody wants to be.
There is also a practical angle: energy going into spurious emissions is energy not going into your actual signal. A clean transmitter uses its power efficiently.
How to keep your signal clean
- Do not overdrive. Set your mic gain and drive levels properly. On SSB, watch your ALC and do not crank the audio into distortion. This alone prevents a lot of splatter.
- Use a low-pass filter if needed. A good low-pass or band-pass filter on your output helps knock down harmonics, especially with older gear or amplifiers.
- Keep your gear healthy. Maintain your equipment, and be suspicious of a rig or amplifier that suddenly starts causing interference.
- Test and monitor. If you can, check your signal, or ask a trusted local ham to tell you how you sound and whether you are clean. Some operators use a spectrum analyzer or a second receiver to watch for harmonics.
- Be careful with amplifiers and homebrew. These are the most common sources of trouble, so give them extra attention and proper filtering.
The bottom line
A spurious emission is your transmitter putting out signal where it should not. Most modern radios are pretty clean out of the box. Those “cheap” radios however are often the biggest culprits of spurious emissions and just because “the radio works” doesnt mean its “ok”.
As an operator, it’s our responsibility to understand what radio brands/models have “issues” and acknowledge that and not encourage the use of those radios. After that, as an operator, the main risk is overdriving the audio or adding a questionable amplifier. Keep your levels sensible, filter when you need to, and pay attention to your signal quality. A clean transmitter keeps you legal, keeps you from interfering with others, and reflects well on you and the hobby.