Tuning Your Antenna System Using Coax Cable Length
When tuning antennas, most hams focus on trimming elements, adding traps, or adjusting matching networks. But your coaxial cable can also be a powerful tuning tool.
The length of your coax can change the impedance your transmitter sees—sometimes enough to bring a high SWR into a tunable range or optimize power transfer without touching the antenna.
The Basics: Transmission Line Transformation
Coax isn’t “just a wire” — it’s a transmission line, and its electrical length matters when the load isn’t a perfect 50 Ω. With mismatched loads, coax transforms impedance along its length based on:
- Electrical length (in wavelengths at the frequency of interest)
- Characteristic impedance (usually 50 Ω)
- Velocity factor (VF) of the coax
This is classic transmission line theory. A Smith chart shows the math, but you can apply it without getting buried in formulas.
Practical Takeaway: Coax Can Transform Impedance
Example: Antenna feedpoint at 25 Ω. Using a quarter-wave (λ/4) length of 50 Ω coax transforms it to:
Zin = (Z₀²) / Zload
50² / 25 = 100 Ω — not perfect, but often easier for your tuner than 25 Ω directly. Any odd multiple of λ/4 (3λ/4, 5λ/4) repeats the transformation. Every λ/2 restores the original impedance at the other end.
This trick works especially well for multiband verticals and wire antennas with 4:1 or 9:1 UNUNs, where band-to-band impedance swings are common. A carefully chosen coax length can shift tricky impedances into your tuner’s “happy zone.”
Suggested Quarter-Wave Coax Lengths
Multiply free-space λ/4 by the velocity factor of your coax type:
| Band | Freq | Free-space λ/4 | EF Bury 7 (VF 0.83) | EF Bury 10 (VF 0.87) | EF Bury 13 (VF 0.86) |
|---|---|---|---|---|---|
| 160m | 1.8 MHz | 41.67 m | 34.59 m | 36.25 m | 35.83 m |
| 80m | 3.5 MHz | 21.43 m | 17.79 m | 18.85 m | 18.43 m |
| 40m | 7.0 MHz | 10.71 m | 8.89 m | 9.32 m | 9.21 m |
| 20m | 14.0 MHz | 5.36 m | 4.45 m | 4.66 m | 4.61 m |
| 15m | 21.0 MHz | 3.57 m | 2.96 m | 3.11 m | 3.07 m |
| 10m | 28.0 MHz | 2.68 m | 2.22 m | 2.33 m | 2.30 m |
| 6m | 50.0 MHz | 1.50 m | 1.25 m | 1.31 m | 1.29 m |
To fine-tune, adjust in λ/8 or λ/16 increments to “nudge” impedance into range without overshooting.
How To Apply This
1. Measure SWR at the Shack
Note problem bands and how far off they are from being tunable.
2. Add or Remove Coax in 1–2 m Steps
Observe SWR curve shifts. Even a short jumper can move a stubborn band into range.
3. Use Analyzer Tools
RigExpert’s TDR and Smith chart modes reveal how coax length shifts impedance:
- TDR: Locate mismatches and see how they shift with coax length changes.
- Smith Chart: Watch impedance move toward the 50 Ω center as you add or subtract length.
- Sweep: See where the transformed impedance lands across bands.
When This Helps
- Non-resonant wires (EFHW, EFOC, EFLW, QuadLoop)
- Multiband verticals (IronWave 6, IronWave 9)
- Delta loops (DeltaRex)
- Balun/UNUN-fed antennas with awkward impedances
- When your ATU struggles on certain bands
- Reducing RFI by improving match
When It Won’t Help
- If the antenna is far off-frequency
- When coax losses are high enough to hide SWR
- Highly reactive loads that still need a tuner
Real-World Example
We use coax length tuning on our EFOC29 (and EFOC17, EFOC8) to bring 10–80 m into a <3:1 SWR envelope, tunable by most internal ATUs—no permanent antenna changes needed.
Mini-FAQ: Coax Length Tuning
- Q: Does coax length change antenna resonance? — No, it changes impedance at the shack, not the antenna’s actual resonant point.
- Q: Will it increase my efficiency? — Only if it reduces mismatch loss or improves tuner efficiency.
- Q: Should I always cut coax to exact λ/4? — No, only when you need deliberate impedance transformation.
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