Why We Don't Build Helically Wound Antennas for 40m

When it comes to compact antennas for the 40 meter band, many amateurs consider helically wound verticals due to their reduced size. However, at RF.Guru, we deliberately choose not to build or recommend helically wound designs for this band. Here's why.

Efficiency Losses

Helical loading introduces considerable loss due to the extended length of wire used to simulate electrical length. This wire, often thin and tightly wound, builds up significant I²R (resistive) loss. On 40 meters, where every ohm of loss matters, this results in poor efficiency, often below 30% in real-world builds.

Narrow Bandwidth

Shortened antennas are inherently high-Q. A helically wound 40m vertical might have an operating bandwidth of just 10 to 20 kHz before SWR climbs beyond 3:1. This makes them impractical for SSB or multi-mode use, and even FT8 operation requires careful tuning.

Radiation Pattern Degradation

The RF current distribution on a helical antenna decreases rapidly along its length. Unlike a top-loaded vertical where current remains strong near the base, helical designs have weaker radiating sections. This compromises the desired low-angle radiation, making them less effective for DX.

Mechanical and Electrical Drawbacks

Winding wire over a non-conductive support is labor-intensive and fragile. Over time, UV exposure and mechanical stress can degrade the winding or the support structure. Attempting to wind the coil over a conductive mast (like aluminum) negates the inductive effect entirely, converting the design into a poorly performing capacitive structure.

Better Alternatives

For compact 40m verticals, we advise:

  • Inverted L with a good radial system

  • Base-loaded verticals with a proper coil and top hat

  • Full size 1/4 wave vertical

All of these outperform helically loaded designs in efficiency, simplicity, and maintainability.

Our Design Philosophy

RF.Guru focuses on practical, high-performance antennas. While helical antennas may seem attractive for their compact size, the trade-offs in performance are too significant, especially on a band like 40 meters where noise is already a limiting factor.

If you're height-restricted, there are smarter ways to get on 40m with better signal quality. We're happy to help you explore them.

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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.