Same-Spaced RFI Across Multiple Bands? It's Probably Common Mode

When you see equally spaced RFI peaks across multiple HF bands, and they all appear with roughly the same amplitude, it's almost always a sign of common-mode pickup.

Most radio amateurs are trained to think in terms of resonance: standing waves, half-wavelengths, impedance matching, etc. And that's useful, but it's also why many don't immediately suspect common-mode issues — because this form of interference doesn’t follow resonant behaviour.

Take switch-mode power supplies, for example. If their noise gets onto your coax shield, it won't behave like a well-contained 50-ohm signal. Instead, it rides on the outside of the coax as a common-mode current, effectively turning the cable into a long, inefficient, but very broadband antenna. This results in uniform radiation across a wide frequency range.

With resonant antennas, you often see harmonic behaviour suppressed — not because the source isn’t emitting harmonics, but because your antenna system simply can’t efficiently radiate them outside of resonance. That’s the Q-factor at work. But common-mode radiation doesn’t care about resonance:

  • There's no matched transmission line,
  • No defined impedance transition to reflect or absorb power,
  • No frequency-selective filtering.

So all harmonics get radiated with little discrimination.

This is why:

  • The interference appears on every band, not just one.
  • The peaks are evenly spaced, because they follow the switching frequency of the noise source.
  • The amplitude is almost constant, because there's no resonant filter in play.

In short: if you see broad RFI across multiple bands, it’s very likely not from one specific frequency, but from wideband common-mode radiation.

And it doesn't have to enter via the coax. It can couple through speaker wires, power cables, or even just metal rails acting as antennas.

Pro tip: Common-mode issues are rarely solved by filtering your radio. They're solved by blocking the current pathat the source and choke points.

If your spectrum looks like a piano keyboard across 20 to 10 metres: suspect common mode.

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Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.