The Illusion of Resonance: Appearance vs. Reality
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.
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