Fold-Back Antenna Tuning – SWR & Resonance Guide
Fold-Back Antenna Tuning – SWR & Resonance Guide
When tuning an antenna by folding back the wire instead of cutting it, the fold introduces extra capacitance, inductance, and mutual coupling. Small fold-backs are fine, but long ones can alter resonance, impedance, and even radiation pattern. Here’s how to apply it correctly.
General Guidelines for Fold-Back Tuning
“% of length” explained: For a center-fed, it’s per leg. For an end-fed, it’s the total radiator length.
- 5–10% fold-back: Minimal performance impact, good for testing adjustments.
- Beyond 10–15%: Resonance shifts less predictably, impedance drifts, and lobes can distort (especially on arrays).
Antenna Type
- Simple wires (dipoles, EFHW): Tolerant of modest folds.
- Complex arrays (Yagis, loops): Long folds upset element balance and degrade gain/F/B.
Spacing Between Folded Runs
- Maintain at least 1–2 cm separation; for long folds, increase to 2–5 cm to reduce unwanted capacitive coupling.
- Avoid tight twists or overlaps; keep them parallel but apart.
Frequency Band
- HF: 5–10% manageable.
- VHF/UHF: Even 2–3% matters; trim instead of folding.
Practical Recommendations
- Keep folds short: Limit to 5–10%; trim once you exceed 15–20%.
- Measure carefully: Use an antenna analyzer and adjust in 10–20 cm steps.
- Avoid high-voltage fold points: On EFHW ends, folding raises E-field stress and may lower power handling in wet conditions. Shift fold inward or trim.
- Maintain spacing: Parallel, separated runs give cleaner behavior.
Example: 40 m Dipole
- Total length: ~20 m (10 m per leg).
- Fold-back 1–2 m per leg: Minimal effect, good for resonance adjustment.
- Fold-back >3 m per leg: Noticeable SWR/pattern shifts—better to trim.
Conclusion
Fold-back is a safe way to “sneak up” on resonance. Keep folds modest, keep runs separated, verify with an analyzer, and trim for permanence. For expert setup advice, contact RF.Guru.
Mini-FAQ – Fold-Back Tuning
- Does folding back change SWR? — Yes, especially beyond ~10–15% of length.
- Is fold-back better than trimming? — Fold-back is temporary/test; trimming is permanent/stable.
- Best tool to verify? — An antenna analyzer for SWR/resonance checks.
- EFHW caution? — Avoid large folds at the high-voltage end; can lower power handling.
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