Toroid choke & balun calculator
Pick the right toroid, number of turns and coax for a 1:1 choke on your bands and power.
Quick pick: what's your power level?
QRP
5-10W
100W
Typical HF
500W
Mid Power
1 kW
High Power
1.5 kW
Legal Limit
Ferrite toroid
Tell me what you need
I have parts
Tell me what I can build
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: K9YC: duty cycles are “20-33%” for CW and SSB, “35-40% with WSJT modes,” and “can exceed 50% with RTTY.” Long transmissions heat a choke more, so give it air and use a high-impedance design.
4
Winding method?
Coax winding Recommended
- ✓ Easiest for beginners - just wind the coax through
- ✓ Built-in shielding, no exposed conductors
- ✓ Uses common coax you may already have
- ✓ Connectors attach directly to coax ends
- ⚠ Takes more space (coax is thicker)
- ⚠ Fewer turns fit on smaller cores
Bifilar wire
- ✓ More compact - fits more turns
- ✓ Slightly better high-frequency performance
- ✓ Preferred for commercial/kit designs
- ⚠ Requires careful winding technique
- ⚠ Need to add connectors or terminals
- ⚠ Must keep wires parallel (not twisted)
- ⚠ 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)
Estimated build cost
Core: ~$12-18
Coax: ~$3-8
Connectors: ~$5-10
Total: ~$20-36
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 Ampacity & Power Ratings: PowerStream AWG tables (Power Transmission column, P=I²×50Ω)
Coax Specifications: 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.
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 | 40m - 10m | Palomar: above 10 MHz, #31 is “about same as 43.” QST 2024: 31 and 43 are “the most common ferrite mixes for the HF bands.” |
| Type 61 | Not for HF chokes | Fair-Rite lists 61 for noise “above 200 MHz”; K9YC: a #61 choke “provides only inductance on the HF bands.” |
Core size
The measured designs on this page (K9YC, G3TXQ) all use 2.4-inch (FT240-size) cores. K9YC: “Power handling is directly related to the value of Rs” (the choking impedance), and 5,000 Ω is “a good starting point … at power levels below about 600W.” For more power, add chokes in series. We found no measured designs for smaller cores to recommend.
| Bands | Turns of RG-400 on one #31 2.4″ toroid | Measured choking (RS) |
|---|---|---|
| 160m | 18 turns | ~10,000 Ω |
| 80m | 16 turns | ~8,000 Ω |
| 40m | 14 turns | ~6,200 Ω |
| 30m | 14 turns | ~6,500 Ω |
| 20m | 13 turns | ~5,400 Ω |
| 15m | 11–12 turns | ~4,800 Ω |
| 10m | 10 turns | ~4,400 Ω |
| 160m–80m | 17 turns | ~6,000 Ω both |
| 80m–30m | 15 turns | ~6,500 / 5,500 Ω |
| 80m–20m | 13 turns | ~5,000 Ω all four |
| 40m–15m | 12–13 turns | ~4,600–5,000 Ω |
| 40m–10m | two 11-turn chokes in series | ~8,000 Ω (7,000 on 10m) |
| 30m–10m | two 10-turn chokes in series | ~7,000–8,500 Ω |
| 160m–40m | two 15-turn chokes in series | 6,000 / 14,000 / 11,000 Ω |
| 160m–20m | 14-turn + 17-turn in series | 8,000 → 5,000 Ω |
From K9YC, A New Choke Cookbook for the 160–10M Bands (2018), Tables 2 and 3. Chokes in series add: their combined impedance is “the sum of their RS values on each band.”
Choke balun questions
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?
If your bands include 160m or 80m, use #31 — Palomar: “Use Mix 31 below 10 MHz for higher choking impedance.” Otherwise #31 or #43 both work: QST (March 2024) says “The most common ferrite mixes for the HF bands are types 31 and 43.” Don’t use #61 for HF chokes: Fair-Rite lists 61 for noise “above 200 MHz,” and K9YC says a #61 choke “provides only inductance on the HF bands.”
How many turns do I need on my toroid choke?
It depends on your bands. K9YC’s measured designs on one 2.4-inch #31 toroid with RG-400: 160m 18 turns, 80m 16, 40m 14, 20m 13, 15m 11–12, 10m 10. More turns move the choke’s best range lower in frequency. For several bands, see the measured table above, or put chokes in series. The calculator picks from these measured designs.
What size toroid core do I need for my power level?
The measured designs we use (K9YC, G3TXQ) are on 2.4-inch (FT240-size) cores. K9YC ties power handling to choking impedance: 5,000 Ω is “a good starting point … at power levels below about 600W,” and higher power needs more — add chokes in series. We didn’t find measured designs for smaller cores to recommend.
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 but takes more care. K9YC measured #12 Teflon pairs as well as RG-400, so both have published designs. 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, within limits. K9YC: one #31 2.4-inch choke of 13 turns of RG-400 gives about 5,000 Ω on all of 80–20m. For wider coverage, put chokes in series — for 40–10m, two 11-turn chokes give about 8,000 Ω (7,000 on 10m). He notes #31 covers about an 8:1 frequency span per choke, versus 4:1 for #43.
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). Buying from ham radio specialty suppliers ensures you get the correct mix.
More questions
Do I need a balun for my dipole?
Usually, yes — and you can check. Coax-fed antennas are “inherently unbalanced,” so current can flow on the outside of the coax shield. That current “can also detune the antenna, change the standing wave ratio, and add noise,” and the coax “can cause interference to nearby electronics — and in severe cases, RF bites.” Chokes “are placed at the antenna’s feed point.” (QST, March 2024)
Measure first if you can: “If you see nothing on the meter, you probably don’t need a CM choke, but… about 50 mA or more should be dealt with.” (QST, March 2024)
Measure first if you can: “If you see nothing on the meter, you probably don’t need a CM choke, but… about 50 mA or more should be dealt with.” (QST, March 2024)
Do I need a choke for my EFHW antenna?
It’s widely recommended, and more so as power goes up. VE3IPS: “While this may not be a significant problem at QRP levels, it becomes more noticeable as power increases.” F5NPV calls the choke “mostly a safeguard system.”
The counterpoise. An end-fed wire needs something to push against — a short counterpoise wire or the coax shield. AA5TB: “The 0.05 wavelength ‘counterpoise’ length is what I also determined empirically.” (About 4 m for 80m, 2 m for 40m.)
Where the choke goes — builders disagree:
• With a short counterpoise wire: PA9X says “install the common mode choke as close as possible to the 49:1 transformer… keep it under 30 centimeters (1 foot).”
• Using the coax shield as the counterpoise: F5NPV says “the CMC shall be located about 4 meters from the 1:49 or 1:64 transformer.”
• VE3IPS warns: “It’s important NOT to install the CMC choke at the feed point, as it will choke the counterpoise and affect the SWR.”
Best advice: follow your antenna maker’s manual. If you have a choke and RF problems (tingly mic, SWR that changes when you touch the coax), try moving it.
Sources: VE3IPS · F5NPV · PA9X · AA5TB
The counterpoise. An end-fed wire needs something to push against — a short counterpoise wire or the coax shield. AA5TB: “The 0.05 wavelength ‘counterpoise’ length is what I also determined empirically.” (About 4 m for 80m, 2 m for 40m.)
Where the choke goes — builders disagree:
• With a short counterpoise wire: PA9X says “install the common mode choke as close as possible to the 49:1 transformer… keep it under 30 centimeters (1 foot).”
• Using the coax shield as the counterpoise: F5NPV says “the CMC shall be located about 4 meters from the 1:49 or 1:64 transformer.”
• VE3IPS warns: “It’s important NOT to install the CMC choke at the feed point, as it will choke the counterpoise and affect the SWR.”
Best advice: follow your antenna maker’s manual. If you have a choke and RF problems (tingly mic, SWR that changes when you touch the coax), try moving it.
Sources: VE3IPS · F5NPV · PA9X · AA5TB
What size toroid do I need? (FT140 vs FT240)
The measured designs on this page (K9YC, G3TXQ) all use 2.4-inch (FT240-size) cores. K9YC ties power handling to choking impedance: “Power handling is directly related to the value of Rs,” and 5,000 Ω is “a good starting point … at power levels below about 600W.” For more power, add chokes in series.
Digital modes: K9YC: “duty cycles of 20-33% are typical for CW and SSB and in the range of 35-40% with WSJT modes, but can exceed 50% with RTTY.” Longer transmissions heat a choke more — give it air and use a high-impedance design. (K9YC Cookbook)
Digital modes: K9YC: “duty cycles of 20-33% are typical for CW and SSB and in the range of 35-40% with WSJT modes, but can exceed 50% with RTTY.” Longer transmissions heat a choke more — give it air and use a high-impedance design. (K9YC Cookbook)
Mix 31 vs Mix 43 - Which ferrite should I use?
Both work on HF. QST: “The most common ferrite mixes for the HF bands are types 31 and 43.”
•
•
•
Sources: QST Mar 2024 · Palomar · K9YC · Fair-Rite 61
•
#31: use it if you need 160m or 80m — Palomar: “Use Mix 31 below 10 MHz for higher choking impedance.” K9YC: one #31 choke covers “about 8:1 frequency span, as compared to 4:1 for #43.”•
#43: above 10 MHz Palomar rates #31 “about same as 43.”•
#61: not for HF chokes. Fair-Rite lists it for noise “above 200 MHz.”Sources: QST Mar 2024 · Palomar · K9YC · Fair-Rite 61
How many turns do I need?
It depends on the band. K9YC’s measured designs, RG-400 on one 2.4-inch #31 toroid: 160m 18 turns, 80m 16, 40m 14, 30m 14, 20m 13, 15m 11–12, 10m 10. For 80–20m, 13 turns gives about 5,000 Ω on all four bands. More turns move the choke’s best range lower.
Each “turn” is one pass through the center hole. (K9YC Cookbook)
Each “turn” is one pass through the center hole. (K9YC Cookbook)
What coax should I use for winding?
Use coax that handles your power on its own. Published ratings are approximate — one dealer chart lists RG-58 at 400 W, RG-8X 350 W and RG-213 1,800 W at 30 MHz, “approximate and for comparison purposes only.” (Universal Radio)
K9YC’s tables use RG-400 (Teflon, RG-58 size); his recommended chokes dissipate “20W, and for most recommended chokes is 15W or less” at 1,500 W key-down. (K9YC Cookbook)
Avoid tight bends in foam coax like LMR-400: its datasheet lists a minimum bend radius of 1 inch for a one-time bend and 4 inches for repeated bends (LMR-400 datasheet), and foam can deform on small forms.
K9YC’s tables use RG-400 (Teflon, RG-58 size); his recommended chokes dissipate “20W, and for most recommended chokes is 15W or less” at 1,500 W key-down. (K9YC Cookbook)
Avoid tight bends in foam coax like LMR-400: its datasheet lists a minimum bend radius of 1 inch for a one-time bend and 4 inches for repeated bends (LMR-400 datasheet), and foam can deform on small forms.
Where should I install my choke balun?
Dipoles and other center-fed antennas: at the feedpoint — QST: chokes “are placed at the antenna’s feed point.” (QST Mar 2024)
End-fed (EFHW): builders disagree — see “Do I need a choke for my EFHW antenna?” above, and follow your antenna maker’s manual.
End-fed (EFHW): builders disagree — see “Do I need a choke for my EFHW antenna?” above, and follow your antenna maker’s manual.
Will a choke balun improve my SWR?
A choke doesn’t match your antenna — it’s 1:1. But current on the coax shield “can also detune the antenna, change the standing wave ratio, and add noise” (QST Mar 2024), so adding a choke can change your SWR reading. W8JI: “A good match does not mean good balance or lack of common mode.” (W8JI)