Why Resonance Isn’t Always the SWR Sweet Spot

Many beginners assume that the lowest Standing Wave Ratio (SWR) automatically means their antenna is resonant. This is a common misconception.

Resonance Defined

An antenna is said to be resonant when its reactance (X) is zero. In other words, the impedance at that frequency is purely resistive: Z = R + j0. At this point, the antenna neither stores nor returns reactive energy (capacitive or inductive).

This is a critical concept in antenna theory, but it's not the whole story.

SWR and Impedance Matching

SWR is a measure of how well the antenna's impedance matches the transmission line (usually 50 ohms). The lowest SWR point is where this match is closest. But here's the catch: this doesn’t always occur at the resonant frequency.

For example:

  • An antenna could be resonant at 7.15 MHz with an impedance of 75 ohms.
  • The same antenna might show the lowest SWR at 7.05 MHz, where its impedance is closer to 50 ohms but includes a small reactive component.

Why the Mismatch?

The key is that SWR depends on the magnitude of the mismatch between the antenna and the feedline, not on whether the impedance is purely resistive.

Factors affecting this include:

  • Antenna height above ground
  • Nearby objects (roof, tower, trees)
  • Ground conductivity
  • Feedline length and loss

These factors shift both the resonant point and the point of minimum SWR, sometimes by tens of kilohertz.

So What Should You Aim For?

  • If you're building a narrowband antenna (e.g., monoband dipole), aim for resonance near your operating frequency but don't panic if SWR is slightly lower elsewhere.
  • If you're building for multi-band use or using a tuner, don't obsess over resonance. Focus on efficiency and matching.

Bottom Line

Resonance (X = 0) is an electrical condition.
Lowest SWR is a matching condition.

They are related, but not the same.
Understanding this distinction helps you build, tune, and deploy better antennas.

 

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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.