Toroid choke estimator
Tell it the toroid and coax you already have, and see how well a choke built from them will work on each band.
Results
Will it fit?
With RG-8X (0.24" OD) on FT240-43, you can fit approximately 12-14 turns single-layer around the inner diameter.
Estimated choking impedance by band
| Band | Freq (MHz) | 8 turns | 10 turns | 12 turns | 14 turns |
|---|
Recommended configurations
When to use the choke estimator
Use this estimator when you already have the parts: a toroid with a known part number, such as an FT240-43, and some coax or magnet wire. It estimates the choking impedance your winding will give on each band and suggests a turn count. If you're starting from scratch and want to know which core and wire to buy for your bands and power, use the choke and balun designer instead.
Which ferrite mix?
- Mix 31: the strongest choice for the low bands, such as 160, 80 and 40 metres.
- Mix 43: a good all-round choice across most of HF.
- Mix 61: for the higher bands, roughly 20 through 6 metres. It won't choke well on the low bands.
Reading the results
Higher impedance means less common-mode current on the outside of your coax. Aim for Good (2,000–5,000 Ω) or Excellent (over 5,000 Ω) on the bands you actually use. The figures are estimates, so see the note below. New to chokes? Read chokes and baluns explained.
Data sources and credits
- Complex Permeability Data: Fair-Rite Corporation published material datasheets (Types 31, 43, 61)
- AL Values & Core Dimensions: toroids.info toroid database
- Calculation Methodology: VK3CPU RF Toroid Calculator by Miguel VK3CPU
- Choke Design Research: K9YC's "A Ham's Guide to RFI"
- Additional Research: G3TXQ's ferrite studies, Palomar Engineers application notes
- Wire Specifications: PowerStream AWG tables
- Magnet Wire Data: MWS Wire heavy build specs (NEMA MW 1000)
- Coax Specifications: Cooner Wire and manufacturer datasheets
Note: Impedance calculations are estimates based on published complex permeability data. Actual results may vary ±20% due to manufacturing tolerances, winding technique, and measurement conditions. For critical applications, verify with a VNA.