Doublet Antenna Guide – Smart Feeding for Multiband HF
A center-fed doublet antenna can be nearly any length you can install. When paired with balanced line and a tuner, it becomes one of the most efficient and versatile HF multiband solutions. The focus isn’t resonance but pattern control and feeding technique.
Length vs. Wavelength (λ)
The doublet’s total span shapes its lobes:
- < 0.5λ: High-angle, broad NVIS coverage.
- ≈ 0.5λ: Classic dipole figure-8 broadside lobes.
- > 1λ: Multi-lobed with lower-angle DX potential.
- > 2λ: Complex multi-lobes, sharper directional gain.
Height vs. Wavelength (λ)
Height above ground dictates takeoff angle more than wire length:
- < 0.25λ: Dominated by high-angle NVIS.
- ≈ 0.5λ: Balanced angles, good compromise for DX + regional.
- > 1λ: Lower angles and multiple lobes favoring DX paths.
Radiation Pattern Evolution
Longer wires create more lobes. That unpredictability in azimuth can be useful—on higher bands, extra lobes at lower angles can help chase DX in multiple directions.
Feeding the Doublet
The doublet shines when fed with low-loss balanced line:
- 450 Ω ladder line or open wire line preferred.
- Use a balanced tuner, or a 1:1 current balun into an unbalanced tuner.
- Avoid coax if SWR is high at the feedpoint; loss multiplies under mismatch.
Feedline Efficiency Comparison
- 600 Ω open wire: Gold standard; extremely low loss even with mismatch.
- 450 Ω ladder line: Weather-resistant, very low loss—practical for most.
- 300–450 Ω twinlead: Acceptable but higher loss in wet conditions.
- Coax: Convenient, but poor with high SWR—avoid for doublets unless SWR is modest.
Typical Lengths
Install the longest symmetrical span your site allows. Examples:
- 2 × 20 m: Covers 80–10 m with tuner, good DX/NVIS balance.
- 2 × 13 m: Excellent 40–10 m compromise.
- 2 × 10 m: Compact, effective 20–10 m option.
Final Takeaways
- Don’t chase perfect resonance—feed it smart and let the tuner do the work.
- Maximize length and height within your property.
- Balanced feedlines matter—keep them symmetrical and away from metal.
- Use a balun if your tuner is unbalanced.
The doublet’s real strength is adaptability. With ladder line and a tuner, one wire can cover nearly the entire HF spectrum efficiently.
Mini-FAQ – Doublet Antenna Essentials
- What’s the best doublet length? — The longest symmetrical span possible. Pattern improves with both length and height.
- How high should I hang it? — ~0.5 λ on the primary band is a great all-round height.
- Do I need a balun? — Yes if using an unbalanced tuner; place a 1:1 current balun between ATU and ladder line.
- Can I use coax? — Only if feedpoint SWR is modest. With high mismatch, coax wastes power as heat.
- Does resonance matter? — Not much. Efficiency comes from low loss and good patterns, not perfect 1:1 SWR.
Interested in more technical content? Subscribe to our updates for deep-dive RF articles and lab notes.
Questions or experiences to share? Contact RF.Guru.