FD4/Windom vs EFOC29: Why the Classic Falls Short
The FD4 (or Windom) has been a staple in amateur radio since the 1920s. It promised multiband coverage with a single long wire. But today’s understanding of common-mode control, impedance transformation, and real radiation efficiency shows why the FD4 is more relic than solution. Modern designs like the EFOC29 solve its long-standing problems in a shorter, smarter form.
The FD4/Windom at a Glance
The FD4 is an off-center-fed dipole (OCFD) about 41 m long, fed ~1/3 from one end. Traditionally used with 4:1 or 6:1 baluns, it covers 80–10 m with varying results. In practice, it suffers from several issues:
Limitations of the FD4/Windom
- Common-Mode Currents: Off-center feeding creates imbalance, leading to CMC on coax, RFI in the shack, and unpredictable lobes.
- Choke Placement Guesswork: A choke is essential but its ideal position is frequency-dependent — often trial and error.
- High SWR on Key Bands: Bands like 15 m are notoriously poor without a tuner.
- Size & Height Demands: At 41 m, few hams have the space or supports to deploy it correctly.
- Impedance Mismatch: A fixed 4:1 balun assumes 200 Ω; real feedpoint impedance swings wildly 80–10 m, wasting power.
The EFOC29: Modern Alternative
The EFOC29 is an end-fed off-center wire of just 29 m. It uses a 4:1 UNUN, a short coaxial counterpoise, and a 1:1 choke precisely at 12.2 m. This integrated approach tackles the very flaws that plague FD4/Windoms.
Why It Outperforms the FD4
- Compact size: 29 m vs 41 m — easier to fit in small lots.
- Flexible deployment: Works as sloper, inverted-L, or flat-top without strict center height requirements.
- Better SWR: Below 3:1 on most bands — often tuner-free for modern rigs.
- Built-in CMC suppression: The choke placement is engineered, not guesswork.
- Simpler feed: Direct coax to UNUN, no trial-and-error balun matching.
Conclusion
The FD4/Windom helped early hams, but its compromises — size, imbalance, choke dependency — make it a poor fit today. The EFOC29 is a cleaner, shorter, and smarter evolution: engineered impedance, predictable SWR, and real multiband performance without the legacy baggage.
Mini-FAQ
- Why does the FD4 cause common-mode issues? — Its off-center feed creates imbalance, driving RF currents onto the coax shield.
- Can the FD4 work well with a choke? — Only partially. A choke helps, but placement varies by band and rarely fixes all issues.
- Why is the EFOC29 better? — The EFOC29 integrates counterpoise + choke into the design, ensuring stable SWR and low CMC across bands.
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