Ferrite Mix Guide: 31, 43, 61 and 77 for Ham Radio Chokes

Walk into any ham radio supplier and you will find a wall of black and grey rings, clamps and beads, each labeled with a puzzling number: 31, 43, 61, 77. Those numbers are the ferrite mix, the material recipe, and choosing the wrong one is one of the most common reasons a homemade choke fails to cure RFI or a transformer runs hot. This guide explains what the mix numbers mean, what the main types are used for, how core names are built, how to think about turns and heat, and how to match a mix to a job such as a common mode choke, a balun or an unun. The figures here are the typical ranges quoted for ham use, so always confirm with the manufacturer’s data sheet for the exact core you buy.

What a Ferrite Mix Is

A ferrite is a ceramic-like magnetic material made from iron oxide combined with other metals. The Wikipedia article on ferrite magnets explains that soft ferrites, the type used in radio work, are generally made from either manganese and zinc or nickel and zinc. Manufacturers vary the recipe to change how the material responds at different frequencies, and they label each recipe with a mix number. Two cores of the same size but different mix behave differently, and you cannot tell them apart by looking at them, so the label matters.

The two families have different personalities. Manganese-zinc (MnZn) materials have high permeability and are most effective at lower radio frequencies, but they conduct a little electricity and so are used with care in high-frequency windings. Nickel-zinc (NiZn) materials have much higher electrical resistivity and work at higher frequencies. In both cases the useful result at RF is an impedance that combines inductance and loss. For a choke, the loss is what you want: the ferrite converts unwanted RF current into a little heat. For a transformer, loss is what you do not want, because it wastes power and warms the core.

The Main Mixes and What Hams Use Them For

The table lists the mixes you will meet most often. The frequency ranges are the typical values that ham sources quote, and the data sheet for a given core is the final authority.

If you remember only one rule, make it this: use a lossy mix (31 or 43) when you want to block current, and a low-loss mix (61, or sometimes 43) when you want to transfer power through a transformer. Mixing up the two roles is the root of many failures. A choke made of the wrong mix can do almost nothing at the frequency that matters, and a transformer made of a lossy mix can overheat at power levels that a low-loss mix handles easily.

Powdered iron cores, usually identified by codes such as -2 and -6 on cores that begin with T, are a different material, not ferrite. They are used for tuned circuits, filters and matching networks where stable inductance matters more than loss, and they look like colored rings. Do not mistake them for ferrite when you buy parts for a choke.

How Core Names Are Built

Many ham cores follow a naming convention that includes the shape, the size and the mix. A name such as FT-240-31 means a ferrite toroid, FT, with an outside diameter of 2.40 inches, 240 in hundredths of an inch, made of mix 31. FT-140-43 is a smaller toroid of mix 43, and FT-114-61 a mid-size core of mix 61. Clamp-on cores, also called snap-on or split cores, are sold by dimension and mix and are designed to be added to an existing cable without cutting it. Beads and sleeves are simple tubes that slip over a wire, as described in the Wikipedia article on ferrite beads. Different makers use different naming schemes, so check the specification, not only the name.

Size matters for three reasons. A bigger core has more material, so it offers more impedance for a given number of turns. A bigger window lets you pass thicker coax, which gives you the option of a larger choke, and a larger core can handle more heat before it saturates or overheats. For common mode chokes on a typical station feedline, many hams use a large toroid or several smaller ones stacked together.

Common Mode Chokes: Choosing Mix and Turns

A common mode choke is a coil of coax, or a group of wires, passed through a ferrite core so that common mode current on the outside of the cable sees a high impedance while the wanted signal inside the coax is unaffected. Our guide to common mode chokes explains why this matters for RFI. For the HF bands, mix 31 is the usual first choice because it offers a high, resistive impedance over a wide range of frequencies. For VHF, mix 43 or 61 is common, because mix 31 loses effectiveness as frequency rises.

Turns are the second variable. More turns raise the impedance, up to a point: as you add turns, stray capacitance between them creates a resonance, and above that frequency the choke becomes less effective. A short coil with a few turns is good at high frequencies and a longer coil is better at low ones, which is why some designs use several different chokes in series. As a practical guide, a choke of a modest number of turns on one large core, or several cores along the cable, is typical on HF. The best test is to measure the choke with an antenna analyzer or a NanoVNA, or to check the actual effect on noise, because published numbers refer to ideal conditions.

• Do not pack turns tightly if you can avoid it. Spacing the turns can reduce capacitance and widen the useful range.
• Keep input and output leads apart. Leads that run close together reduce the effect of the choke.
• Use the right cable. The choke has to fit the diameter of your coax, so choose the core by the window size.
• Place the choke where the problem is. At the antenna feedpoint to keep current off the line, and near the equipment to stop noise from entering it.

Baluns and Ununs: Choosing Mix for Transformers

Baluns and ununs are transformers, and for transformers the loss in the core should be low. Our guide to baluns and ununs explains the roles. For wideband HF transformers, hams use mix 43 or mix 61 depending on the impedance ratio, the power and the frequency range. A 49:1 unun for an end-fed half-wave wire, like those in our end-fed half-wave antenna guide, usually uses a toroid of mix 43 or 61, and a 9:1 unun for a random wire, described in our random wire guide, is often wound on mix 43. The choice involves a trade-off: mix 43 gives good performance at the low end of HF, while mix 61 loses less power at the high end and handles more power without heating.

Heat is the way a poor choice shows itself. If a transformer core gets hot to the touch after a few minutes of transmission, it is turning part of your power into heat. Reduce the loss by using a lower-loss mix, a larger core or more cores, or reduce the power. If the SWR shifts as the core warms, the permeability of the material is changing with temperature, which is another reason to choose a conservative design. Do not use a high-permeability lossy mix, such as 31, in a high-power transformer: it is designed to absorb RF.

Heat, Power and Saturation

Ferrite cores have two limits that matter to hams. The first is heating. A choke that is doing its job converts some of the common mode energy to heat, which can be significant at high power or when the common mode current is large. At QRP levels this is rarely an issue, but at hundreds of watts a small bead can get very warm. The second is saturation, where the magnetic field in the core is too strong for the material and it stops behaving as an inductor. Saturation is rare in RF chokes handling only common mode current, but it can happen in transformers that carry the full power of the transmitter, especially at low frequencies.

In practice, you manage both limits by using more core material than you think you need. Stack two cores instead of one, use a bigger toroid, and keep the winding spread out so heat can escape. If a core is hot, feel for the trend after a few minutes at full power with the intended mode; a very warm core is a sign to improve the design, and a core that is too hot to touch should be fixed before further use.

Practical Uses at a Typical Station

• Feedline choke at the antenna: several turns of coax on a large mix 31 toroid, or a string of clamp-on cores, to keep current off the shield.
• Choke on computer and USB cables: a snap-on core of mix 31 or 43 near the equipment to reduce noise in and out. For tracing noise sources in your home, see our guide to finding and killing RF noise.
• Power leads: beads or a toroid on DC supply wires to stop switching noise.
• Wideband transformers: mix 43 or 61 for 9:1 and 49:1 ununs and 1:1 or 4:1 baluns.
• VHF chokes: mix 43 or 61 beads or cores on the coax near a VHF antenna.

A Simple Way to Test a Choke at Home

You do not need laboratory equipment to see whether a choke does its job. If you own a NanoVNA or an antenna analyzer, wind the choke, connect a short wire through it with the instrument leads at each end, and sweep the HF range. A good choke shows a high impedance, with a large resistive part, across the bands you care about. Compare a choke with a few turns, one with more turns and one with a different mix, and the differences become obvious: one may peak on 40 meters and fall away on 10, another may be flat but lower overall. Our guide to the NanoVNA antenna analyzer explains the calibration needed for reliable readings.

The second test is the one that counts: does it reduce the problem? If you added the choke to stop noise on receive, listen on the affected band before and after, watching the S-meter or the noise level. If you added it to stop RF in the shack on transmit, check for the original symptoms, such as audio buzz or a computer that resets, while transmitting at the usual power. Change one thing at a time. If you add three chokes at once and the problem goes away, you will not know which one mattered, and you may have spent money you did not need to spend.

Finally, keep notes. Write down the mix, the core size, the number of turns, the position on the cable and the effect. A small notebook of such results becomes a very personal guide to your own station, and it is far more reliable than general rules, because every antenna, feedline and house is different.

Buying Cores and Avoiding Fakes

Because the mix cannot be seen, the main risk is buying a core that is not what the label says. Buy from reputable suppliers, avoid unbranded bargain lots with no specification, and keep the original packaging or a note of the mix with each core. If you are unsure, an inexpensive test with an analyzer can show whether a core behaves like the mix claimed, by comparing the impedance of a known number of turns across the frequency range with the manufacturer’s data. Always label your finished chokes with the mix and the number of turns, so you can repeat or repair them later.

The Bottom Line

The mix number tells you what a ferrite core will do. Mix 31 is the lossy workhorse for HF common mode chokes, mix 43 is the general-purpose NiZn for upper HF, VHF chokes and wideband ununs, mix 61 is the low-loss choice for transformers and VHF work, and mix 77 serves low frequencies. Use lossy mixes to block current and low-loss mixes to transfer power, add more core instead of more heat, label what you build, and check the data sheet before you rely on a number.

Frequently Asked Questions

What does the number in FT-240-31 mean?

FT means ferrite toroid, 240 is the outside diameter in hundredths of an inch (2.40 inches) and 31 is the mix.

Which mix is best for HF common mode chokes?

Mix 31 is the usual choice for the HF bands because it gives a high, lossy impedance over a wide range. Use mix 43 or 61 for VHF.

Which mix should I use for a 49:1 or 9:1 unun?

Mix 43 is the common choice, and mix 61 is used where lower loss and higher power handling matter. Follow the design you are building.

Why does my transformer core get hot?

The core material is absorbing some of the power as loss. Use a lower-loss mix, a larger core or several cores, or reduce power.

Is powdered iron the same as ferrite?

No. Powdered iron cores, such as types -2 and -6, are a different material, used for tuned circuits and filters rather than for lossy chokes.

Can I tell the mix by looking at the core?

No. Different mixes look alike, so rely on the label, the supplier and the data sheet.