Turns count for chokes, baluns, and loading coils — toroids by AL value, air-core by Wheeler's formula.
Rule of thumb: a choke needs reactance ≥ ~1000 Ω at the lowest band — XL = 2πfL.
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Toroids: ferrite turns come from N = 1000·√(LmH/AL); iron powder from N = 100·√(LµH/AL). AL values here are typical published numbers — ±20% is normal, so wind, measure, and remove a turn if needed.
Mix guide: #43 for HF chokes/baluns (1–50 MHz), #31 for low-band chokes (160–40m), #61 for high-Q transformers above 10 MHz, #2/#6 iron powder for tuned circuits.
A turn counts every time the wire passes through the center of the core.
Air core: Wheeler's approximation L = d²n² / (18d + 40ℓ) — accurate to a few percent for single-layer solenoids longer than 0.4× their diameter.
Common chokes: RG-8X on FT-240-43 → about 12 turns is the classic 1:1 feedline choke for HF.