Reciprocity Is a Mathematical Theorem

Reciprocity in antenna theory states that the transmission and reception properties of an antenna are symmetrical. If an antenna radiates effectively at a certain angle, it should receive signals from that same angle equally well. This theorem, grounded in Maxwell's equations, is analytically sound and theoretically unbreakable — under ideal, linear, time-invariant conditions.

But reality is not a theorem.

In practice, especially on HF bands, modern stations face a wide range of non-reciprocal effects driven not by the antenna element itself, but by the environment, feed system, and noise landscape. And this is where reception-focused antennas outperform transmit antennas, despite the math insisting otherwise.

The Hidden Asymmetries

  • Common-Mode Currents (CMC): While they may barely impact your transmitted signal, they often dominate the RX chain, introducing local noise from power lines, Ethernet cables, and switching supplies.
  • Local Noise Fields: TX power punches through noise. RX is defenseless against the S5 noise floor coming from your own home. The antenna doesn't change, but the path the noise takes does.
  • Feedline Radiation and Ground Coupling: Coaxial cables become unintended antennas on RX, coupling unwanted signals directly into the receiver.
  • Impedance and Balance: An unbalanced feedline or poorly decoupled system on RX leads to pattern distortion and unpredictable performance.

Why RX Antennas Win

This is why dedicated RX antennas — with excellent CMR, low noise pickup, and narrow beamwidths — vastly outperform shared TX/RX antennas in receive performance. It's not because reciprocity is broken. It's because real systems are not ideal, and noise enters the equation asymmetrically.

The Shift to RDF

John Devoldere (ON4UN) was among the first to widely promote Reception Directivity Factor (RDF) as a key performance metric. RDF measures how well an antenna rejects noise from undesired directions. For RX performance, this is vastly more relevant than raw gain.

Others Who Address the Gap

  • N6LF (Rudy Severns): Documented how radial systems, soil conditions, and physical asymmetries lead to different TX/RX behavior.
  • W8JI (Tom Rauch): Extensively discusses how CMC, feedline issues, and grounding affect RX systems more severely.
  • Eric E. Johnson (N5FDL): Clarified that while patterns are reciprocal, SNR is not. The receive chain lives in a different noise reality.

Conclusion

Reciprocity remains valid in theory. But theory assumes a perfect world.

In the real world, it's not the antenna element but the complete RF system — including cables, grounds, devices, and local noise sources — that breaks the symmetry. That is why the best RX systems today are designed specifically to reject noise, manage common-mode current, and optimize RDF — not maximize gain.

As ON4UN implied:

When you can't change the world, design to hear around it.

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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.