Diplexers and Common Mode: The Hidden Coupling Path

Diplexers are essential tools in RF systems, enabling multiple frequency bands to share a single antenna or feedline. But what many forget is this: while diplexers separate frequencies differentially, they don’t inherently block common mode currents — and that oversight can quietly sabotage your system.

Differential vs Common Mode: A Quick Recap

In any RF transmission system:

  • Differential signals are the intended currents: equal and opposite in each conductor of a balanced line, or the forward signal and return on coaxial inner and outer.
  • Common mode currents are unintended: equal-phase signals on both conductors in the same direction, or RF flowing on the outside of the coax shield.

These common mode currents are the source of all kinds of trouble:

  • Unwanted coupling between ports,
  • RFI into equipment,
  • Increased receive noise,
  • Pattern distortion on antennas.

What a Diplexer Does — and Doesn’t Do

A diplexer is made of band-pass and/or low/high-pass filters that route different frequencies to different ports. It is a differential-mode device. It separates based on signal frequency, not signal mode.

Here’s the issue:
If a common mode current exists on the shared feedline (for example, an HF signal riding the outside of your coax shield used for a VHF antenna), the diplexer will not reject it — because to the diplexer, it isn’t a signal at all.

Worse yet, some diplexers use inductive or capacitive components referenced to ground, unintentionally offering a low-impedance return path for common mode currents at certain frequencies. This can lead to:

  • Intermodulation,
  • Cross-band leakage,
  • “Ghost” signals,
  • Desensitization on your receive path.

Typical Symptoms

Operators often report:

  • HF signals showing up on VHF receivers (and vice versa),
  • Desense on VHF/UHF when transmitting on HF,
  • Audio hum, distorted patterns, or elevated noise floor.

Many blame the diplexer itself — but the true cause is usually common mode coupling at the junction or along the coax run.

The Solution: Break the Loop

To eliminate these issues:

  1. Use a high-quality 1:1 choke (current balun or isolator) at the diplexer’s shared port.

    • Prefer ferrite-based chokes tuned to the lowest frequency in use.
    • Mix 31/43 ferrite types for broadband coverage.

  1. Install additional chokes at the feedpoints of each antenna port if the antennas themselves are unbalanced (e.g., verticals, off-center-fed wires, etc.).

  2. Bond your gear and manage your ground potential — don’t rely on the diplexer to block unwanted currents between systems.

Not Just for Transmitters

Even in receive-only applications (like using one antenna for both HF and VHF receivers), common mode can couple broadcast stations or local noise into the wrong band. Proper isolation is still critical.

In summary:
A diplexer splits frequencies, not modes. It’s blind to common mode — and if you don’t stop that current elsewhere, your whole station might suffer.

Always choke before you diplex.

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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 plat