Why Minimum Coax Length Matters for Monoband Antennas
Minimum Coax Lengths for Monoband Antennas: Why 4 m Is Not Enough
Many hams assume “shorter coax = always better.” While it’s true that short coax reduces loss when perfectly matched, at HF we are rarely matched. An antenna feedpoint is almost never exactly 50 Ω. With a very short coax — say 4 m — the tuner or balun ends up seeing the raw, extreme impedance straight from the antenna. The result: unstable SWR, hot baluns, and poor efficiency.
Coax as an Impedance Transformer
Every coax run is a transmission line with a characteristic impedance (50 Ω for RG-213, RG-8, LMR-400, etc.) and an electrical length. When terminated with anything other than 50 Ω, the line transforms the impedance depending on its length. A very short piece has no chance to smooth out or moderate those extremes — it simply delivers them unaltered to your shack.
Why a Quarter Wave Is Safer
A quarter wavelength (0.25 λ) of coax is a natural pivot point on the Smith chart. At this length, impedances are rotated far enough to avoid the “all or nothing” extremes you see with short runs. In practice, 0.25 λ of coax ensures:
- Your tuner is not exposed to the wildest possible impedances.
- Your balun operates in a more comfortable range, reducing heating and saturation.
- SWR stability improves — moving a few kHz up or down the band no longer causes violent swings at the shack.
Band by Band — Practical Minimums (0.25 λ)
Using 0.25 λ as a safe minimum guideline, here’s what that looks like on HF bands:
| Band | Wavelength | 0.25 λ (min coax length) | Practical note |
|---|---|---|---|
| 160 m (1.8 MHz) | ~166 m | ~40 m | Short runs (4–10 m) give raw extremes. 40 m stabilizes things. |
| 80 m (3.5 MHz) | ~85 m | ~20 m | At least 20 m avoids hitting the tuner with the raw antenna load. |
| 40 m (7 MHz) | ~42 m | ~10 m | Coax jumpers shorter than 10 m can cause sharp SWR swings. |
| 20 m (14 MHz) | ~21 m | ~5 m | 4 m is just shy; 5 m or more provides safer behavior. |
Coax vs Ladder Line
Ladder line has negligible loss, so it can be very short without penalty. In fact, its length is deliberately chosen to transform impedance. With coax, however, short lengths punish you — they expose the tuner to the worst conditions without useful transformation. That’s why the 0.25 λ minimum rule is the safer choice on monoband coax-fed antennas.
Mini-FAQ
- Isn’t shorter coax less loss? — Yes, but only if your antenna is a perfect 50 Ω match. Otherwise short coax delivers raw extremes.
- Why is 0.25 λ safer? — Because a quarter wave rotates impedances on the Smith chart enough to pull them into a manageable range for your tuner.
- What if I use 0.5 λ or more? — That also works, but total loss rises with length. Choose low-loss coax for longer runs.
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