Common-mode choke calculator

What actually does the work here

A choke on the feeder kills the common-mode current — the one that flows on the outside of the braid and turns the cable into part of the antenna. What does the work is not the inductance but the resistive part of the ferrite's impedance: it turns that current into heat. Which is why one material is good on 80 metres and useless on 10, and another the other way round.

A choke does not touch the differential current in the cable — it kills the common-mode current on the braid

Fair-Rite material	Material 31Material 43Material 44Material 46Material 51Material 52Material 61Material 67Material 68Material 73Material 75Material 76Material 77Material 78Material 79Material 80Material 95Material 97Material 98
Toroid size	FT-16 — 3.9 × 2.2 × 1.3 mmFT-23 — 5.8 × 3 × 1.5 mmFT-23A — 5.8 × 3 × 3 mmFT-37 — 9.5 × 4.7 × 3.2 mmFT-50 — 12.7 × 7.1 × 4.8 mmFT-50A — 12.7 × 7.9 × 6.4 mmFT-50B — 12.7 × 7.9 × 12.7 mmFT-63A — 16 × 9.6 × 6.4 mmFT-82 — 21 × 13.2 × 6.4 mmFT-82A — 21 × 13.2 × 11.9 mmFT-87 — 22.1 × 13.7 × 6.4 mmFT-87A — 22.1 × 13.7 × 12.7 mmFT-100 — 25.4 × 15.5 × 6.4 mmFT-100A — 25.4 × 15.5 × 8.1 mmFT-100B — 25.4 × 15.5 × 12.7 mmFT-114 — 29 × 19 × 7.5 mmFT-114A — 29 × 19 × 13.8 mmFT-125 — 31.8 × 19 × 9.5 mmFT-140 — 35.6 × 22.9 × 12.7 mmFT-140A — 36 × 23 × 15 mmFT-150 — 38.1 × 19 × 6.4 mmFT-150A — 38.1 × 19 × 12.7 mmFT-193 — 49.1 × 31.8 × 15.9 mmFT-193A — 49.1 × 31.8 × 21.6 mmFT-240 — 61 × 35.6 × 12.7 mmFT-290 — 73.7 × 38.9 × 12.7 mmFT-337 — 85.7 × 55.5 × 12.7 mmFT-350 — 88.9 × 50.8 × 12.7 mm
Stacked toroids	OneTwoThree
Number of cable turns
Operating frequency (MHz)
At the operating frequency
Impedance magnitude |Z|
Resistive part R (what chokes the current)
Reactive part X
Permeability μ′ − jμ″
Cross-section / mean path length

The complex permeability comes from the Fair-Rite tables (src/utils/fairRiteData.js), the core geometry from the Amidon catalogue.

📐 Formulas

Geometry of the toroid:

le = π × (OD + ID) / 2          mean magnetic path length
Ae = h × (OD − ID) / 2          cross-section (× the number of stacked cores)

The impedance of the winding with a complex permeability μ = μ′ − jμ″:

Z = jω × N² × μ₀ × (μ′ − jμ″) × Ae/le

R  = ω × N² × μ₀ × μ″ × Ae/le      ← the resistive part; this is what kills the current
X  = ω × N² × μ₀ × μ′ × Ae/le
|Z| = ω × N² × μ₀ × √(μ′² + μ″²) × Ae/le

Rule of thumb:

|Z|	Verdict
≥ 2000 Ω	good
1000…2000 Ω	acceptable
< 1000 Ω	not enough

Limits of the calculator:

• The stray capacitance between turns is not included: in a real choke |Z| falls above a certain frequency, here it does not
• μ′ and μ″ come from the Fair-Rite tables; outside the table the calculator refuses to compute and says so
• Amidon and Fair-Rite cores of the same nominal size differ slightly in geometry
• The calculation is for coax wound through a toroid; a «sleeve» of several cores threaded on the cable behaves differently

🎯 Where it is used

• Choosing the material for a band. The band-by-band table shows at once where this choke works and where it does not.
• Checking a finished choke — your own or a bought one.
• Designing a choke for an EFHW or a random wire, where the common-mode current is especially noticeable.

Practical notes:

1. Material 31 is for 1.8–10 MHz, 43 for 10–30 MHz, 61 for VHF. There is no universal one.
2. Space the turns out rather than stacking them: that lowers the stray capacitance and the choke works higher up.
3. A choke neither balances nor transforms — it only kills the current. That is a different job from a transformer's.
4. One choke at the antenna is worth more than three at the transceiver.

© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.

How to cite this calculator

UR3PKI. «Common-mode choke calculator». CyberDev.Space. https://cyberdev.space/en/radio/calculators/coils/choke_balun (licence CC BY-NC-SA 4.0).

The licence lets you use this material freely, including in teaching materials, but only with attribution to the author and a link to the source.