The Open Wire Balanced Feedline: The Forgotten Ultra-Low Loss Champion

When it comes to feeding multiband HF antennas like doublets, dipoles, and loops, few solutions offer the performance and flexibility of 600-ohm open wire line. Let’s break down why this classic feedline still outperforms modern alternatives like 300-ohm twin-lead (TV twin-line) and even coaxial cable, especially in demanding HF applications.

Impedance Matching and SWR Tolerance

Multiband antennas rarely present a perfect match at all frequencies. Coaxial cable, with its fixed 50-ohm characteristic impedance, suffers significant loss when SWR increases — especially at high frequencies and long runs.

In contrast, 600-ohm open wire is extremely forgiving. Its low loss per meter and high voltage handling mean it can tolerate high SWR without turning into a dummy load. It's ideal for use with a transmatch (also called a tuner).

Low Loss Even Under Mismatch

Let’s be technical: power loss in feedlines is proportional to I²R losses and dielectric losses. Coax has a lossy dielectric and carries current on both the inner and outer conductors — this becomes problematic when mismatch forces current imbalance.

Twin-lead has a foamed dielectric that degrades in wet or humid conditions. Its higher loss per meter, smaller spacing, and plastic jacket mean it performs poorly when wet, bends too easily, and detunes when run near metal.

600-ohm open wire line, by comparison:

  • Has very wide spacing (12-14 cm)
  • Uses air as the dielectric (lowest loss possible)
  • Has negligible losses, even with high mismatch
  • Maintains impedance stability regardless of moisture or bending

RF Behavior Near Objects

Balanced lines like twin-lead and open wire are sensitive to nearby objects — but twin-lead is far worse. Its narrow conductor spacing allows common-mode pickup and coupling to nearby metal, which can cause severe RF distortion and increased losses.

600-ohm line, with wider spacing and better field cancellation, suffers less from this.

Build Quality and Power Handling

TV twin-lead was never designed for high-power RF. It was a consumer-grade product for VHF/UHF television. It cannot handle kilowatts of HF power, nor survive UV exposure, moisture, or physical stress over years.

By contrast, properly built open wire line:

  • Handles high power with ease (1–5 kW+)
  • Lasts decades outdoors
  • Can be easily repaired or custom-made

Why Coax Still Fails on Multiband Antennas

Coax works best when it sees 50 ohms — and even then, only if it’s dry, short, and shielded properly. On multiband antennas where impedance swings wildly (100 ohms to >1000 ohms), coax becomes a lossy transmission line. Even high-quality coax suffers from dielectric and skin effect losses at high SWR.

Final Thoughts

If you’re serious about efficient HF operation across multiple bands — especially with high power — then 600-ohm open wire line is the gold standard. It outlasts, outperforms, and outmatches TV twin-lead and coax in almost every critical technical aspect.

Yes, it’s a little more work to install, and you’ll need a descent transmatch (tuner) and a good 1:1 current balun — but the rewards are exceptional.

A good feedline is not just a wire — it’s part of your antenna system. And for multiband HF, open wire is simply unbeatable.

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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.