Choosing the Right Ferrite Toroid Mix for Choke Baluns

If you have ever stood in front of a parts drawer full of ferrite toroids labeled with mysterious mix numbers like 31, 43, and 73 and wondered which one to grab for a choke balun, you are not alone. Getting the mix choice wrong can mean a pretty looking choke that barely suppresses common mode current, or one that runs hot when you key up. In this article we will walk through what those mix numbers really mean and how to pick the right core for HF baluns and ununs that actually work.

Every HF antenna has two circuits hiding inside it: the differential RF current that actually does the radiating, and the unwanted common mode current that likes to flow on the outside of your coax. Choke baluns and common mode chokes are there to kill that second one by inserting a big resistive impedance in its path. The ferrite mix you choose controls how big that impedance is across the bands and how much loss turns into heat in the core.

Most of the common mixes hams use for baluns are optimized for very different frequency ranges. Mix 31 is a lossy HF and low VHF material. Mix 43 is an old standby that works best on higher HF and VHF. Mix 73 is a higher permeability mix that really likes the lower HF and MF range. Once you understand that, picking the right toroid for a 40 meter dipole choke or a 160 meter receive loop becomes much easier. If you are just getting started with ferrites, you may also want to skim the HF antenna articles on Broken Signal for more context.

What ferrite mix actually means

The mix number is basically the recipe the manufacturer used to make the ferrite material. It sets the permeability, loss curve, and where the core behaves more like an inductor or more like a resistor. For choke baluns and common mode filters we mostly care about two things: how much impedance we get per turn squared, and how much of that impedance is resistive instead of reactive across the bands we care about.

Manufacturers publish impedance vs frequency curves for each mix. If you look at those plots, mix 31 has a nice hump through the HF range. Mix 43 starts to come into its prime around the upper HF and low VHF region. Mix 73 has very high permeability so you get a lot of impedance with fewer turns, but the sweet spot leans toward 160 and 80 meters and medium frequency work. For a choke balun, the sweet spot is where the impedance is high and mostly resistive, which turns common mode current into heat instead of just shifting its phase.

One important thing to remember is that toroid cores are not magic. You still need enough turns, on a big enough core, with the right mix. Trying to do a 1.5 kW choke on a single tiny toroid, even in the right mix, is asking for trouble. That is why a lot of the better designs use stacked cores or multiple chokes in series. If you are looking for starter designs, check some of the projects under Projects on Broken Signal for inspiration.

How to choose a ferrite mix for your balun or unun

You do not need to become a ferrite scientist to pick a good core for HF work. A simple way to think about mix choice is to start with your lowest band, your power level, and whether the device is mainly a choke balun, a current balun for a specific impedance transformation, or an unun on an end fed antenna. Then match those needs against the strengths of each mix.

• Step 1: Decide which bands and power levels matter most. A 100 watt 40 meter portable vertical has very different needs than a 1 kW 160 meter transmit loop.
• Step 2: For broad HF choke baluns, default to mix 31 on reasonably large toroids (2.4 inch or bigger) and plan for 8 to 14 turns of coax depending on the design.
• Step 3: For low band receive loops, audio or control line chokes, or MF work, consider mix 73 cores with fewer turns and be realistic about duty cycle and heating.

Mix 31 vs 43 vs 73 in real HF choke baluns

Mix 31 has become the go to choice for many modern HF choke balun designs. When you wind enough turns on a big mix 31 toroid or stack several together, you can easily get 3 to 5 kΩ of resistive impedance over most of HF. That is usually enough to knock common mode current on your coax down by 20 dB or more. It is also fairly forgiving when you are running 100 to 500 watts, as long as you do not skimp on core size.

• Mix 31: Best all around HF choke material, especially from 80 through 10 meters, with good power handling when sized correctly.
• Mix 43: Works well for chokes aimed at 20 meters and above, or as part of a multi section filter where low bands are handled elsewhere.
• Mix 73: Very strong at 160 and 80 meters for receive and low duty cycle applications, but can heat quickly at higher power on upper HF.

Performance and heating considerations

On paper you might be tempted to chase the highest impedance number you can get by cramming more turns onto a toroid. In practice, once you get above roughly 5 kΩ of common mode impedance on the band of interest, you are deep into diminishing returns. At that point the real question becomes how hot the core gets on long transmissions. Mix 31 and 43 will generally handle similar flux levels, but mix 31 tends to give you that useful HF impedance with slightly fewer "gotchas" in the low HF range.

Mix 73 has such high permeability that it delivers a lot of impedance with very few turns, but those turns concentrate the flux in a smaller region of the core. That can make heating worse on transmit. For receive only devices, such as common mode chokes feeding SDRs in the shack, this is usually not a problem, and mix 73 can be a great tool. For a 1 kW end fed half wave unun, you are usually better off with multiple large mix 31 cores and conservative winding choices. If you are just getting into ferrite design, pair this knowledge with existing examples from the Getting Started section to avoid re inventing the wheel.

Practical tips for winding HF choke baluns

Once you have picked the mix, the rest of the build still matters. Coax type, winding style, and even how you mount the core can change the final result by a surprising amount. The good news is that a few simple habits go a long way toward getting repeatable performance from your mix 31, 43, or 73 builds.

1. Use good quality coax with a solid dielectric and consistent diameter so your turns lay neatly around the toroid and do not bunch up.
2. Spread the turns evenly around most of the core instead of clumping them in one spot, which helps distribute heating and flux.
3. Test your choke with a basic VNA or analyzer in common mode (through a test jig) and confirm that the impedance peak lines up with the bands you care about.

FAQ: common ferrite mix questions from HF builders

Hams ask the same handful of questions every time ferrite cores and choke baluns come up on the air or in club meetings. Here are quick answers that match the details above without sending you down a rabbit hole of datasheets right away.

• Can I mix different mixes in one choke? Generally no. Stacking toroids is common, but they should be the same mix so the impedance and heating behave predictably.
• Is a big mix 43 choke useless on 80 meters? Not useless, but often underwhelming. You may only get a kΩ or so of impedance there, which might not be enough for very noisy locations.
• Are clamp on ferrites the same as toroids? They use similar materials, but the geometry and gap change the behavior. Clamp ons are great for retrofits on cables; toroids are better when you can build the choke from scratch.