Ferrite common-mode choke chart

Which ferrite mix gives the most impedance on which band, per turn, on FT240- and FT140-size toroids. For chokes against RFI and common-mode current on coax, mains and LED/inverter leads.

These are calculated low-frequency estimates, not measured chokes. Every value is |Z| = 2πf · µ0 · |µ*| · N² / (Σl/A), from Fair-Rite's published complex permeability (µ′, µ″) of each mix and the core constant Σl/A on Fair-Rite's part page. Between two measured frequencies µ′ and µ″ are interpolated linearly in log f. The N² rule holds only well below the choke's self-resonance: the capacitance between turns makes a real choke peak and then fall, and more turns move that peak lower. Measure your choke if it matters.

FT240 size (61.0 × 35.55 × 12.7 mm), Σl/A = 9.2 cm⁻¹

Band (MHz used)	Mix 31 \|Z\| 1 turn	Mix 43 \|Z\| 1 turn	Mix 52 \|Z\| 1 turn	Mix 61 \|Z\| 1 turn	Highest
160 m (1.9)	20.2 Ω	15.2 Ω	4.4 Ω	2.0 Ω	Mix 31
80 m (3.6)	27.1 Ω	24.9 Ω	8.6 Ω	3.8 Ω	Mix 31 ≈ 43
40 m (7.1)	34.0 Ω	34.0 Ω	18.7 Ω	7.7 Ω	Mix 31 ≈ 43
30 m (10.1)	38.9 Ω	39.1 Ω	25.2 Ω	11.2 Ω	Mix 43 ≈ 31
20 m (14.2)	45.0 Ω	44.8 Ω	29.1 Ω	16.4 Ω	Mix 31 ≈ 43
17 m (18.1)	50.0 Ω	49.4 Ω	31.6 Ω	22.4 Ω	Mix 31 ≈ 43
15 m (21.2)	53.3 Ω	52.7 Ω	33.2 Ω	28.0 Ω	Mix 31 ≈ 43
12 m (24.9)	57.0 Ω	56.2 Ω	35.1 Ω	35.2 Ω	Mix 31 ≈ 43
10 m (28.5)	60.0 Ω	59.2 Ω	37.2 Ω	41.1 Ω	Mix 31 ≈ 43
6 m (50.5)	72.3 Ω	71.3 Ω	50.1 Ω	49.7 Ω	Mix 31 ≈ 43

Core constants from Fair-Rite part 5943003801 (43), 5961003801 (61). Both parts list the same Σl/A. Check the part page of the mix you buy: the Σl/A of a 31 or 52 part of the same size is not checked here.

FT140 size (35.55 × 23.0 × 12.7 mm), Σl/A = 11.4 cm⁻¹

Band (MHz used)	Mix 31 \|Z\| 1 turn	Mix 43 \|Z\| 1 turn	Mix 52 \|Z\| 1 turn	Mix 61 \|Z\| 1 turn	Highest
160 m (1.9)	16.3 Ω	12.3 Ω	3.5 Ω	1.6 Ω	Mix 31
80 m (3.6)	21.9 Ω	20.1 Ω	6.9 Ω	3.1 Ω	Mix 31 ≈ 43
40 m (7.1)	27.4 Ω	27.4 Ω	15.1 Ω	6.2 Ω	Mix 31 ≈ 43
30 m (10.1)	31.4 Ω	31.6 Ω	20.3 Ω	9.0 Ω	Mix 43 ≈ 31
20 m (14.2)	36.3 Ω	36.1 Ω	23.5 Ω	13.2 Ω	Mix 31 ≈ 43
17 m (18.1)	40.3 Ω	39.9 Ω	25.5 Ω	18.0 Ω	Mix 31 ≈ 43
15 m (21.2)	43.0 Ω	42.5 Ω	26.8 Ω	22.6 Ω	Mix 31 ≈ 43
12 m (24.9)	46.0 Ω	45.3 Ω	28.4 Ω	28.4 Ω	Mix 31 ≈ 43
10 m (28.5)	48.4 Ω	47.7 Ω	30.0 Ω	33.2 Ω	Mix 31 ≈ 43
6 m (50.5)	58.4 Ω	57.5 Ω	40.4 Ω	40.1 Ω	Mix 31 ≈ 43

Core constants from Fair-Rite part 5943002701 (43). Check the part page of the mix you buy: the Σl/A of a 31 or 52 part of the same size is not checked here.

How many turns: multiply by N²

Below self-resonance the impedance rises with the square of the turns. Two worked examples on the FT240 size (Σl/A = 9.2 cm⁻¹):

Turns N	Factor N²	Mix 31, 80 m (3.6 MHz)	Mix 43, 20 m (14.2 MHz)
2	× 4	108 Ω	179 Ω
4	× 16	434 Ω	717 Ω
6	× 36	976 Ω	1612 Ω
8	× 64	1735 Ω	2866 Ω
10	× 100	2711 Ω	4479 Ω
12	× 144	3903 Ω	6449 Ω
14	× 196	5313 Ω	8778 Ω

Large N values are the least reliable: with many turns the self-resonance can fall inside the band, and above it the impedance drops. The toroid calculator gives the inductance for a given number of turns from the AL value.

How the chart is made

µ′ and µ″ are Fair-Rite's own measured complex permeability data for each mix (the CSV on each material data sheet page).
One turn on a toroid has L0 = µ0 / (Σl/A). The complex impedance is Z = jωL0 (µ′ − jµ″), so |Z| = ωL0 · √(µ′² + µ″²). The resistive part ωL0µ″ is the part that turns common-mode current into heat.
“Highest” is the mix with the largest calculated |Z| for one turn on that band; when the runner-up is within 10 %, both are shown (e.g. “31 ≈ 43”), because the difference is smaller than the ±20–25 % AL tolerance of the parts. It is not a recommendation for power handling or for heating at high power.
Mix 61 data starts at 1 MHz, so 160 m (1.9 MHz) is within the data for all four mixes.

Sources (retrieved 2026-11-04):

Fair-Rite 31 material, complex permeability vs. frequency (CSV): https://fair-rite.com/wp-content/uploads/2015/03/31-Material-Fair-Rite.csv
Fair-Rite 43 material, complex permeability vs. frequency (CSV): https://fair-rite.com/wp-content/uploads/2020/11/43-Material-publish.csv
Fair-Rite 52 material, complex permeability vs. frequency (CSV): https://fair-rite.com/wp-content/uploads/2015/04/52-Material-Fair-Rite.csv
Fair-Rite 61 material, complex permeability vs. frequency (CSV): https://fair-rite.com/wp-content/uploads/2021/11/61-Material-Fair-Rite.csv
Fair-Rite part page: https://fair-rite.com/product/toroids-5943003801/
Fair-Rite part page: https://fair-rite.com/product/toroids-5961003801/
Fair-Rite part page: https://fair-rite.com/product/toroids-5943002701/

Ohmsums. Calculated from Fair-Rite permeability data; low-frequency estimate, not a measured choke.