The FD4/Windom Antenna: A Deprecated Classic and Why the EFOC29 Is a Better Choice

For decades, the FD4 (also known as the Windom antenna) has been a familiar sight in the world of amateur radio. Initially introduced in the 1920s, the design was celebrated for its ability to cover multiple HF bands with a single wire and a simple feed system. However, in today’s world of improved understanding, better materials, and more refined impedance matching techniques, the FD4 has largely become a relic of the past. In this article, we take a closer look at the limitations of the FD4/Windom and explore why the EFOC29 antenna represents a superior and simpler alternative.

The FD4/Windom at a Glance

The FD4/Windom is an off-center fed dipole (OCFD) originally designed to be fed with a single wire connected to a tuner. The modern version typically uses a 4:1 or 6:1 balun and coaxial feedline, with the feedpoint about one-third from one end. The typical length of a FD4 for 80-10m is around 41 meters.

Known Issues with the FD4/Windom

  1. Common-Mode Currents (CMC): The asymmetrical feedpoint creates an imbalance, often leading to CMC on the coax shield, which can cause RFI issues in the shack and unpredictable radiation patterns.
  2. Requires Choke Placement Guesswork: To suppress CMC, a choke is needed. But its ideal position along the coax is frequency-dependent and must often be determined experimentally.
  3. High SWR on Some Bands: Despite its multiband design, the FD4 often exhibits poor SWR on certain bands like 15m, requiring a tuner for acceptable performance.
  4. Physical Size and Height Requirements: At 41 meters long, the FD4 demands a lot of space and height for proper performance. Many modern hams, especially those in urban or suburban areas, struggle to deploy it effectively.
  5. Unreliable Impedance Match: Feeding an off-center dipole through a fixed-ratio transformer (like 4:1) assumes consistent impedance, which is far from reality across multiple bands. This results in power losses and mismatched conditions.

Enter the EFOC29: Smarter Simplicity

The EFOC29 is an End-Fed Off-Center antenna that is just 29 meters long and designed for use with a 4:1 UNUN and a short counterpoise in the coax braid, combined with a well-placed 1:1 choke at 12.2 meters from the feedpoint.

Why It Outperforms the FD4

  • Compact Size: With a total length of only 29 meters, the EFOC29 fits into more spaces and is easier to install.
  • No Critical Center Height Needed: Being end-fed, it can be sloped or deployed in an L or inverted-L configuration with minimal compromise.
  • Lower SWR on Most Bands: The design targets a better impedance balance across HF bands, achieving SWR < 3:1 on most bands without requiring a tuner.
  • Integrated CMC Suppression: The 1:1 choke placement is part of the design, eliminating guesswork and reducing RFI risk.
  • Simple Feedline Design: Coax directly connects to the UNUN, and the system works without fine-tuned positioning or complex feed arrangements.

Conclusion

The FD4/Windom had its place in history, but today’s requirements for compact, efficient, and interference-free operation make it an outdated choice. The EFOC29 embraces the strengths of modern understanding with a minimalistic and smart approach. It offers real multiband performance, manageable size, and reliable matching without the legacy baggage.

For today’s ham, especially in limited-space environments, the EFOC29 is not just an upgrade — it’s a much-needed evolution.

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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.