Ferrite core selection charts

A. Which mix to use (HF-focused)

Mix	Best general region	Typical “why”	Practical guidance
Mix 31	~1–30 MHz	Excellent for choking common-mode on HF; good broadband behavior	Best first choice for chokes and many broadband transformers
Mix 43	~5–50 MHz	Good general HF/VHF mix; often used in legacy designs	Works fine on HF, tends to be a bit more VHF-friendly than 31
Mix 52	Higher HF into VHF	Lower loss at higher frequencies	Less common for HF EFHW transformers; more niche

Rule of thumb:

• For chokes: start with Mix 31.
• For HF transformers (9:1, 49:1): Mix 31 or 43; for many stations Mix 31 is the safer “broad HF” bet.

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B. Core size vs power (practical guidance)

This is intentionally conservative. The exact limit depends on duty cycle (SSB vs FT8), enclosure heat, winding, and SWR.

Application	QRP (≤10W)	100W class	High duty / 400W+
4:1 current balun	1 × 2.4″ toroid often OK	1–2 × 2.4″ toroids common	2–3 larger cores, heavy wire
9:1 unun	1 × 2.4″ toroid often OK	1–2 × 2.4″ toroids recommended	2–3 cores; watch heating
49:1 EFHW	1 × 2.4″ may work (watch voltage)	2 × 2.4″ cores strongly preferred	2–4 cores + spacing/venting

Additional power/duty-cycle cautions

• Digital modes (FT8/RTTY) stress cores significantly more than SSB.
• High SWR at the transformer increases core heating sharply.

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C. Wire and winding recommendations

Build	Suggested wire	Turn count guidance	Notes
4:1 Guanella	paired insulated wires (bifilar)	8–12 turns per 1:1	Symmetry matters; keep leads short
9:1 autotransformer	enamel wire or insulated hook-up	9 turns total, tap at 3	Spread turns evenly; avoid crossing
49:1 autotransformer	enamel wire, higher voltage rating	14 total, tap at 2	Voltage stress is real; insulation matters

General guidance

• Use heavier wire as power increases (and for mechanical durability).
• Keep the transformer inside a weatherproof box; include strain relief and a drip loop outdoors.

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