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Electronics & Antennas for Ham Radio

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The Illusion of Resonance: Appearance vs. Reality

Related reading:
Hybrid Baluns vs Chokes in End-Fed and Off-Center Antennas

In antenna design, few concepts are as misunderstood as “resonance.” Many confuse a low SWR or tuner setting with resonance. But a good match is not the same as true resonance.

What Is True Resonance?

Resonance means reactance = 0 at a given frequency. The antenna looks purely resistive (≈50–72 Ω depending on design). This occurs naturally when antenna length and geometry support a standing wave at a harmonic of the frequency. Example: a half-wave dipole with current maximum and voltage minimum at the feedpoint.

Making Something Appear Resonant

Systems often made to appear resonant use tricks like:

  • Inline capacitors or inductors (e.g. EFHW transformers with “magic” capacitors)
  • Coaxial stubs
  • Loading coils or hats
  • Excess coax as impedance transformer
  • Heavy tuner intervention

These produce a “good SWR,” but currents remain reactive, efficiency is low, and common-mode often dominates.

Symptoms of Faked Resonance

  • Very narrow bandwidth despite good SWR
  • Coax radiating (shield hot with RF)
  • High common-mode pickup and RFI
  • Unexpected RF feedback in shack
  • Poor low-band efficiency

Why It Matters

A true resonant antenna:

  • Radiates efficiently with broad bandwidth
  • Needs little tuner correction
  • Is less prone to CM noise and RFI

But: resonance ≠ guaranteed efficiency, and non-resonance ≠ poor performance. Good examples: terminated loops, phased arrays, or well-designed OCFs.

Receive Antennas: Resonance Is Often a Drawback

High-Q resonant receive antennas (e.g. small loops) have gain peaks, but with narrow BW, ringing, and poor impulse response. This masks nulls and distorts patterns. Broadband non-resonant probes (active whips, resistive loops) offer:

  • Stable directionality
  • Fast impulse response
  • No detuning from hand/body

For RX work — especially mobile or in noisy QTHs — broadband non-resonant designs are superior.

Conclusion

A low SWR doesn’t prove resonance — it may just reflect matching tricks. True resonance is physical, not created by tuners or stubs.

Tuning the match doesn’t tune the antenna — it only hides the problem.

Mini-FAQ

  • Is low SWR proof of resonance? — No. It just shows a match, not whether reactance is truly cancelled.
  • Do non-resonant antennas work? — Yes. Many broadband or terminated designs outperform “fake” resonant ones.
  • Why avoid resonance in RX antennas? — Because high-Q resonance causes ringing, poor impulse response, and unstable nulls.

Interested in more technical content? Subscribe to our updates for deep-dive RF articles and lab notes.

Questions or experiences to share? Feel free to contact RF.Guru.

Joeri Van Dooren, ON6URE – RF engineer, antenna designer, and founder of RF.Guru, specializing in high-performance HF/VHF antennas and RF components.

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