Why We Do Not Use Compensation Capacitors on Our EFHW Antennas

At RF.Guru, we design and manufacture efficient, high-performance End-Fed Half-Wave (EFHW) antennas with a focus on minimal component count, optimal impedance transformation, and broadband usability. One common question we receive is: "Why don’t you use compensation capacitors on your EFHW transformers?"

The answer is simple: in most cases, they are unnecessary and can even be detrimental.

Why We Avoid Capacitors on Most Bands (20–80M)

Compensation capacitors are sometimes added across the transformer windings to cancel unwanted inductive reactance, especially at higher frequencies. However, on lower HF bands such as 20, 30, 40, 60, and 80 meters, adding a fixed capacitor (e.g., 100pF) introduces several issues:

  • It shifts the resonance and impedance transformation away from the desired operating range.
  • It can introduce unwanted peaks or dips in the SWR curve, particularly if the capacitor value is not perfectly tuned for the specific transformer and wire configuration.
  • It degrades matching on bands below 20 meters, often increasing SWR above what most tuners can handle (>3:1).

Because our EFHW designs are optimized for performance across multiple bands, adding a fixed capacitor would reduce the system's overall broadband match and stability on the lower bands.

The Exception: 20–10M and 10/12M EFHW Verticals

There is, however, one key exception in our product line: our short vertical EFHWs designed specifically for the 20–10 meter range (or just 10 and 12 meters monoband).

For these dualband or high-band focused monoband verticals, we integrate a compensation capacitor around 100pF, and here's why:

  • On shorter wavelengths, a small capacitor can help suppress residual inductive reactance at the higher end of the band (especially 10m).
  • A 100pF capacitor does not negatively affect 20, 12, or 10 meters, as its reactance is still high enough at those frequencies to not interfere with normal transformer operation.
  • It improves the match at 10 meters where impedance transformation ratios can drift, especially in vertical deployments with high radiation resistance at the feedpoint.

In our tests, including real-world deployment of 5-6 meter vertical EFHWs, a well-chosen 100pF cap helped bring down SWR on 10 meters from 1.8–2.0 down to 1.2–1.3 without degrading performance.

Summary

We refrain from using compensation capacitors on our multi-band EFHWs because:

  • They reduce performance and raise SWR on low bands like 20, 30, 40, 60, and 80 meters.
  • They complicate the design without offering consistent benefits across a wide frequency span.

However, for monoband or dual-band high-frequency verticals, a 100pF capacitor can be beneficial without compromising performance. This is why our dual band 20–10M and 10/12M monoband vertical EFHWs are the only models where this is applied.

Our design philosophy remains: keep it simple, efficient, and broad-coverage – unless there's a valid reason not to.

Interested in more technical content like this? Subscribe to our notification list — we only send updates when new articles or blogs are published: https://listmonk.rf.guru/subscription/form

Questions or experiences to share? Feel free to contact RF.Guru or join our feedback group!

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.