Toroid Choke Calculator
Build the right choke for your antenna
𧲠Ferrite Toroid
Tell me what you need
π§ I Have Parts
Tell me what I can build
π Air Core Balun
Simple coax coil
1
What are you building?
?
Choke Balun: Goes at dipole feedpoint to prevent RF on coax shield.
Line Isolator: Placed along feedline to block common mode currents.
Common Mode Choke: General purpose RF choke for any feedline issue.
Line Isolator: Placed along feedline to block common mode currents.
Common Mode Choke: General purpose RF choke for any feedline issue.
2
What band(s) will you use?
?
Select all bands you plan to operate on. The calculator will recommend a core that works well across your selected range. For best performance, choose the narrowest range needed.
3
What's your power level?
?
Select the maximum power you'll run. Higher power needs a larger core to prevent overheating. SSB power ratings assume typical duty cycles - reduce for digital modes (50% of rating).
Digital modes (FT8, etc): 100% duty cycle - use 50% of the power rating above. A 100W choke handles ~50W digital.
4
Winding method?
π Coax Winding RECOMMENDED
- β Easiest for beginners - just wind the coax through
- β Built-in shielding, no exposed conductors
- β Uses common coax you may already have
- β Connectors attach directly to coax ends
- β Takes more space (coax is thicker)
- β Fewer turns fit on smaller cores
γ°οΈ Bifilar Wire
- β More compact - fits more turns
- β Slightly better high-frequency performance
- β Preferred for commercial/kit designs
- β Requires careful winding technique
- β Need to add connectors or terminals
- β Must keep wires parallel (not twisted)
- β PTFE/Teflon wire recommended (heat resistant)
Our recommendation: If this is your first choke, use coax. It's simpler, more forgiving, and works great. Bifilar is for when you want a more compact build or are following a specific kit design.
Your Recommended Choke
1:1 Choke Balun for 40m-10m at 100W
Toroid Core
?
FT240 = 2.4" diameter (high power)
FT140 = 1.4" diameter (100-200W)
FT82 = 0.825" diameter (QRP 25-50W)
The number after the dash is the "mix" (ferrite type).
FT140 = 1.4" diameter (100-200W)
FT82 = 0.825" diameter (QRP 25-50W)
The number after the dash is the "mix" (ferrite type).
FT240-43
Number of Turns
?
Each pass through the center hole = 1 turn. Wind evenly around the core, spacing turns for coax to prevent overheating.
10-12 turns
Wire/Coax Type
RG-8X
Approx. Coax Length
~3 ft (0.9m)
Winding Diagram
Wind coax through center hole. Each pass through = 1 turn. Space turns evenly around core.
Why This Configuration?
Type 43 ferrite provides excellent choking impedance across 80m-10m, making it the best all-around choice for general HF use. The FT240 size handles 100W+ easily with good thermal mass. 10-12 turns provides >5000 ohms of choking impedance at your target frequencies.
Shopping List
1x FT240-43 Ferrite Toroid
3 ft RG-8X Coax (or 4ft to be safe)
2x Coax connectors (SO-239 or your choice)
π° Estimated Build Cost
Core: ~$12-18
Coax: ~$3-8
Connectors: ~$5-10
Total: ~$20-36
π Data Sources & 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 Ampacity & Power Ratings: PowerStream AWG tables (Power Transmission column, P=IΒ²Γ50Ξ©)
Coax Specifications: Manufacturer datasheets
Note: Impedance calculations are estimates based on published complex permeability data. Actual results may vary Β±20% due to manufacturing tolerances, winding technique, and measurement conditions. For critical applications, verify with a VNA.
π Alternative Options
Quick Reference: Ferrite Mix Guide
| Mix (Color) | Best Bands | Notes |
|---|---|---|
| Type 31 (Gray-Blue) | 160m - 40m | Highest impedance at low HF. Best for 160/80/40m. |
| Type 43 (Black) | 80m - 10m | Best all-rounder for HF. Most popular choice. |
| Type 61 (Gray) | 20m - 6m | Lower loss at higher frequencies. Good for 6m. |
Core Size Guide
| Core Size | Diameter | Power Rating (SSB) | Best For |
|---|---|---|---|
| FT82 | 0.825" (21mm) | ~25-50W | QRP, ultraportable, SOTA/POTA |
| FT140 | 1.4" (35mm) | ~200-300W | Portable, 100W stations |
| FT240 | 2.4" (61mm) | ~1-1.5kW | 100W to legal limit |
| Band Range | Recommended Mix | Turns | Expected Impedance |
|---|---|---|---|
| 160m - 80m | Type 31 | 12-14 | >3000Ξ© |
| 80m - 10m | Type 43 | 10-12 | >5000Ξ© |
| 40m - 10m | Type 43 | 8-10 | >5000Ξ© |
| 20m - 6m | Type 61 | 8-10 | >3000Ξ© |
| 160m - 10m (All HF) | Type 31 + Type 43 stack | 10-12 each | >3000Ξ© across all |
Based on research by K9YC, PA9X, and G3TXQ. For maximum choking impedance, a stack of two different mix cores covers the widest bandwidth.
Frequently Asked Questions About Choke Baluns
What is a 1:1 choke balun and why do I need one?
A 1:1 choke balun is a device that prevents RF current from flowing on the outside of your coaxial cable (called common mode current). It's placed at the antenna feedpoint to ensure your antenna radiates properly and prevents RF from traveling back into your shack, which can cause interference and RFI issues. The "1:1" means it doesn't transform impedance: 50 ohms in, 50 ohms out. Every dipole, doublet, and many other antennas benefit from a choke balun at the feedpoint.
What ferrite mix should I use for my choke balun?
Type 43 (black) is the best all-around choice for general HF operation (80m-10m). Use Type 31 (gray-blue) for better performance on low bands like 160m, 80m, and 40m. It has higher choking impedance at lower frequencies. Use Type 61 (gray) for high bands (20m-6m) where it has lower loss. When in doubt, Type 43 works well for most ham radio applications and is the most commonly available.
How many turns do I need on my toroid choke?
The number of turns depends on the ferrite mix and your target frequencies. For most HF applications: 10-12 turns on Type 43 ferrite, 12-14 turns on Type 31 ferrite, or 8-10 turns on Type 61 ferrite. More turns increase choking impedance at lower frequencies but may reduce effectiveness at higher frequencies. Our calculator above automatically recommends the optimal turns for your selected bands.
What size toroid core do I need for my power level?
FT82 (0.825" diameter) is ideal for QRP operations up to 25-50W and ultraportable builds. FT140 (1.4" diameter) handles up to 200-300W SSB and is great for 100W stations and portable operations. FT240 (2.4" diameter) handles 1kW+ and is recommended for 100W to legal limit stations. For digital modes like FT8 with high duty cycles, size up; the larger core runs cooler. When in doubt, the FT240 gives you headroom for future power upgrades.
Should I use coax or bifilar wire for winding?
Coax winding is recommended for beginners: it's easier, more forgiving, and works great. Simply wind the coax through the toroid core and attach connectors to the ends. Bifilar winding (two parallel wires wound together) is more compact and has slightly better high-frequency performance, but requires more skill. Use bifilar when following a specific kit design or when you need a very compact choke. For your first choke, definitely start with coax.
What's the difference between a choke balun and a line isolator?
They're essentially the same thing used in different locations. A choke balun is placed at the antenna feedpoint to transition from balanced antenna to unbalanced coax. A line isolator (or common mode choke) is placed somewhere along the feedline, often near the shack entrance, to block common mode currents that made it past the antenna. Many hams use both: a choke balun at the antenna AND a line isolator before the radio for maximum RFI protection.
Is a choke a choke? Are they all built the same way?
Yes! Whether you call it a choke balun, line isolator, common mode choke, or 1:1 current balun - the construction is identical. The choking impedance depends only on three things: the toroid core (size and ferrite mix), the number of turns, and the wire type. The application name just describes where you're using it, not how to build it. A choke wound with an FT240-43 and 10 turns of RG-8X will have the same impedance whether you install it at a dipole feedpoint, along your feedline, or at your shack entrance.
Can I use a single choke for multiple bands?
Yes! That's one of the advantages of ferrite chokes. Type 43 works well across 80m-10m with a single winding. For truly wideband coverage (160m-6m), you can stack two cores of different mixes (Type 31 + Type 43) and wind through both. The calculator's "Alternative Options" section shows multi-band recommendations based on your band selections.
Where can I buy ferrite toroid cores?
Popular suppliers include DX Engineering, Palomar Engineers, Amidon, and KF7P Metalwerks. You can also find them on Amazon and eBay, but verify you're getting genuine ferrite (not counterfeit). Expect to pay $5-10 for FT140 cores and $12-20 for FT240 cores. Buying from ham radio specialty suppliers ensures you get the correct mix.