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The Balun Paradox: Why More Cores and More Windings Don’t Guarantee Better Performance — Annotated Notes & Key Takeaways
A 1:1 current balun is a common-mode choke, not a transformer. Complexity rarely helps. Use the right toroid, enough turns, and large wire. Avoid rusty bolts, ugly baluns, counter-winding, and parallel two-toroid designs. Choking impedance is the real goal.
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- Oct 15
A **balun** is a small device that causes an outsized amount of confusion. A 1:1 current balun can be as simple as a few turns of wire on a toroid, or as complicated as two toroids with intertwined windings and a schematic that looks like a push-pull amplifier. The material examines why some 1:1 current baluns are twice as complicated as others and whether that complexity actually buys anything. The short version: often it does not. What matters is choking impedance, not visual sophistication.
The simplest design is a pair of wires wound together over a toroidal core. One side is unbalanced, the other is balanced, and the whole thing is a 1:1 current choke. In theory, this works fine. The material points out that the theory depends on the core. A website might show a particular toroid with almost no useful technical data, and what little data exists suggests it was intended for switch-mode power supplies rather than RF chokes. That kind of core is likely to saturate, or it may offer almost no choking impedance at higher frequencies. It might be justifiable on 160 meters, maybe 80 meters, but above that it is likely ineffective at best and inefficient at worst. Even on the lower bands, the choking impedance would probably be quite low.
The material also warns against a counter-winding technique that some builders use to try to eliminate saturation. This technique is needlessly complicated, and it may actually increase parasitic capacitance inside the transformer, which reduces performance rather than improving it. Several well-known experimenters—W8JI, K9YC, and VK1OD—failed to find any evidence that the supposed saturation elimination actually happens. The counter-winding likely causes a complex eddy current that may produce localized saturation in some areas of the toroid but not others. The practical advice is to ditch the counter-winding entirely. For most HF work, 14 to 18 bifilar turns of 18 to 14 AWG wire on a T240 core will do the trick. If you want specific material recommendations, Type 43 or Type 31 cores in a T106 size are fine for 40 meters and up at QRP levels, while T200 or larger is better for 100 watts, especially below 40 meters. Type 77 or Type 62 is typically recommended for 10 meters through 2 meters. The key is to use the right core for the frequency, use large enough wire, and keep the design simple.
The next design uses a pair of wires wrapped around a nail or a bolt. This is the kind of thing that looks clever in a photograph and works poorly in practice. Random steel has unpredictable and almost certainly undesirable saturation characteristics. Its impedance can vary wildly across the HF range, which means it might work on some frequencies and at low power, but it is likely to be ineffective on some bands and may turn a significant portion of your power into heat. This is especially true when the antenna is anything other than almost perfectly balanced. Use a rusty bolt balun on a dipole whose elements interact unevenly with nearby objects or terrain, or worse, on an off-center-fed dipole, and the thing will get extremely hot very quickly. The material treats this as a cautionary example, not a serious design.
The air-core solenoid balun—often called an ugly balun—is a different story. It can work. It is made by wrapping coax into a coil. The problem is that the capacitance in the coil is quite high. RG-8X, for example, has roughly 5 picofarads per foot. That capacitance turns the coil into a self-resonant tank circuit. It can present a useful impedance on a couple of bands at most, but on other bands it may be all but invisible. You can string multiple ugly baluns together along the coax, but then you end up using a lot of extra coax, which means more loss and more expense. A proper toroid costs about five dollars. The ugly balun is a compromise, not a first choice.
The most puzzling design in the material uses two toroids. It looks like a 4:1 current balun, but it is presented as a 1:1 current balun. Electrically, the two cores are in parallel, acting as two parallel bifilar chokes. If the cores and windings are identical, this actually reduces the choking impedance rather than increasing it. There is no obvious logical reason to do this unless you are building a balun to handle a very large amount of power—500 watts or more—but even then you would want the chokes in series, not parallel, with less impedance per core. The material suspects that the designer read technical discussions about why single-core 4:1 voltage baluns are not true baluns and misunderstood the lesson. Those warnings apply specifically to 4:1 baluns because of the DC short that is present. A 1:1 bifilar winding on a single core is more than capable of providing good isolation over a wide range of frequencies.
A single T240-43 core with 14 AWG magnet wire should be able to dissipate the common-mode current from even a moderately imbalanced antenna at 500 watts PEP without issue. The only reasons to use the two-toroid parallel design would be to compensate for poor component choice, to reduce the total current through each choke, or to use two different cores to cover two different frequency ranges. Even in the last case, the chokes should be in series, not parallel. The material calls this design needlessly complicated and attributes it to a failure to grasp what a choke balun is actually supposed to do.
The central concept that ties all of this together is choking impedance. A 1:1 current balun is a choke, not a transformer. It presents a high impedance to common-mode current—the current that remains when the equal and opposite currents on the two conductors cancel each other out. The differential signal, the one you actually want, passes through because the equal and opposite currents cancel and do not see the choke. The common-mode current does not cancel, so it sees the choke’s impedance and is ideally dissipated as heat. This is why the core material matters. The choke needs to present a high impedance to common-mode current across the frequencies you care about.
A choke balun does not guarantee balanced voltages at the output. It simply presents an impedance that you hope is higher than the impedance of the antenna and the feedline system. If the feedline and shack happen to be anti-resonant at some frequency, the balun might fail to choke anything, and a lot of power could be transferred as common-mode current. That is a limitation of the choke approach. When the antenna itself presents a very high impedance, common-mode current is likely to be small in magnitude, but the choke may also be ineffective. In that case, a true narrow-band transformer may be the only option that really works. The material notes that the range over which a typical balanced dipole exhibits an impedance above 3,000 ohms is quite narrow, so this is not a constant problem, but it is a boundary condition worth understanding.
The material also distinguishes between receive and transmit applications. On receive, efficiency is insignificant, but common-mode suppression can be very important. A simple bifilar winding on an unknown core might be fine for receive because saturation is hardly a concern, and the bandwidth limitation may not matter much. On transmit, efficiency is what counts, although common-mode suppression can still affect RFI in nearby equipment. For serious operators, using different antennas for transmit and receive is worth considering on bands where the sky temperature differs significantly from room temperature. That is a more advanced topic, but it reinforces the idea that the balun’s job depends on what you are asking it to do.
The material is opinionated, and a few points deserve caution. The claim that the counter-winding technique reduces performance was challenged. The response cited W8JI and admitted that the cause was flipped: the counter-winding reduces shunt capacitance rather than increasing it, but the result is a substantial increase in the frequency of peak performance. The curve also becomes sharper, meaning less useful bandwidth. There is no information to support the claim that counter-winding reduces saturation, though the claim appears in many forums. So the practical advice to avoid counter-winding stands, but the technical explanation is not fully settled.
The two-toroid design also drew disagreement. One point of view argued that the parallel cores guarantee balanced equal voltages on the outputs, which a simple choke does not. The rebuttal was that the two-toroid design is still just a choke, not a transformer, and it offers no balance advantage over a single core. The two parallel bifilar chokes do not have the cross-talk that a 4:1 two-core design has. Another observation was that two windings on two cores can make the characteristic impedance of the transmission line effectively 50 ohms instead of the 100 ohms typical of a simple bifilar choke. That might offer an advantage when feeding a low-impedance resonant dipole or Yagi driven element at 30 or 50 MHz. The material does not fully resolve this, so it remains an open question. For most HF operators, the single-core approach is simpler and well understood.
There is also the receive-only caveat. A rusty bolt or an unknown core might be acceptable for receive because saturation is not a concern and the signal levels are tiny. The material does not explore that use case in depth, but it is a reasonable exception. For transmit, the rules are stricter, and the consequences of a poor choke are real: heat, RFI, and inefficient radiation.
The overall lesson is that a balun is not magic. It is a filter for common-mode current. A 1:1 current balun should be simple: the right core, enough turns, large enough wire, and a design that keeps parasitic capacitance in mind. A single core is usually enough. Two cores in parallel are usually counterproductive. Counter-winding is usually unnecessary. A rusty bolt is a bad idea for transmit. An ugly balun is a compromise that works on a couple of bands but not across the HF range. The material’s recommended approach is to buy the right toroid, use good wire, and keep it simple. If you need impedance transformation, things get more complex, but for a 1:1 choke, the basic design is hard to beat.
The best resource cited is K9YC’s RFI document, which includes experimentally verified designs and teardowns of popular but ineffective commercial baluns. That is a good place to go if you want to build your own or understand why a particular design works. But the core message is clear: do not be impressed by complexity. A balun with two tangled toroids is not twice as good as a balun with one. It is often twice as confusing and half as effective. The balun’s job is to choke common-mode current, and the simplest design that does that reliably is usually the best one.
The Simple Bifilar Winding and Its Hidden Assumptions
The simplest design is a pair of wires wound together over a toroidal core. One side is unbalanced, the other is balanced, and the whole thing is a 1:1 current choke. In theory, this works fine. The material points out that the theory depends on the core. A website might show a particular toroid with almost no useful technical data, and what little data exists suggests it was intended for switch-mode power supplies rather than RF chokes. That kind of core is likely to saturate, or it may offer almost no choking impedance at higher frequencies. It might be justifiable on 160 meters, maybe 80 meters, but above that it is likely ineffective at best and inefficient at worst. Even on the lower bands, the choking impedance would probably be quite low.
The material also warns against a counter-winding technique that some builders use to try to eliminate saturation. This technique is needlessly complicated, and it may actually increase parasitic capacitance inside the transformer, which reduces performance rather than improving it. Several well-known experimenters—W8JI, K9YC, and VK1OD—failed to find any evidence that the supposed saturation elimination actually happens. The counter-winding likely causes a complex eddy current that may produce localized saturation in some areas of the toroid but not others. The practical advice is to ditch the counter-winding entirely. For most HF work, 14 to 18 bifilar turns of 18 to 14 AWG wire on a T240 core will do the trick. If you want specific material recommendations, Type 43 or Type 31 cores in a T106 size are fine for 40 meters and up at QRP levels, while T200 or larger is better for 100 watts, especially below 40 meters. Type 77 or Type 62 is typically recommended for 10 meters through 2 meters. The key is to use the right core for the frequency, use large enough wire, and keep the design simple.
Why a Rusty Bolt and an Ugly Balun Fall Short
The next design uses a pair of wires wrapped around a nail or a bolt. This is the kind of thing that looks clever in a photograph and works poorly in practice. Random steel has unpredictable and almost certainly undesirable saturation characteristics. Its impedance can vary wildly across the HF range, which means it might work on some frequencies and at low power, but it is likely to be ineffective on some bands and may turn a significant portion of your power into heat. This is especially true when the antenna is anything other than almost perfectly balanced. Use a rusty bolt balun on a dipole whose elements interact unevenly with nearby objects or terrain, or worse, on an off-center-fed dipole, and the thing will get extremely hot very quickly. The material treats this as a cautionary example, not a serious design.
The air-core solenoid balun—often called an ugly balun—is a different story. It can work. It is made by wrapping coax into a coil. The problem is that the capacitance in the coil is quite high. RG-8X, for example, has roughly 5 picofarads per foot. That capacitance turns the coil into a self-resonant tank circuit. It can present a useful impedance on a couple of bands at most, but on other bands it may be all but invisible. You can string multiple ugly baluns together along the coax, but then you end up using a lot of extra coax, which means more loss and more expense. A proper toroid costs about five dollars. The ugly balun is a compromise, not a first choice.
The Two-Toroid Mystery: Parallel Cores and Reduced Impedance
The most puzzling design in the material uses two toroids. It looks like a 4:1 current balun, but it is presented as a 1:1 current balun. Electrically, the two cores are in parallel, acting as two parallel bifilar chokes. If the cores and windings are identical, this actually reduces the choking impedance rather than increasing it. There is no obvious logical reason to do this unless you are building a balun to handle a very large amount of power—500 watts or more—but even then you would want the chokes in series, not parallel, with less impedance per core. The material suspects that the designer read technical discussions about why single-core 4:1 voltage baluns are not true baluns and misunderstood the lesson. Those warnings apply specifically to 4:1 baluns because of the DC short that is present. A 1:1 bifilar winding on a single core is more than capable of providing good isolation over a wide range of frequencies.
A single T240-43 core with 14 AWG magnet wire should be able to dissipate the common-mode current from even a moderately imbalanced antenna at 500 watts PEP without issue. The only reasons to use the two-toroid parallel design would be to compensate for poor component choice, to reduce the total current through each choke, or to use two different cores to cover two different frequency ranges. Even in the last case, the chokes should be in series, not parallel. The material calls this design needlessly complicated and attributes it to a failure to grasp what a choke balun is actually supposed to do.
Choking Impedance, Common Mode, and the Real Design Goal
The central concept that ties all of this together is choking impedance. A 1:1 current balun is a choke, not a transformer. It presents a high impedance to common-mode current—the current that remains when the equal and opposite currents on the two conductors cancel each other out. The differential signal, the one you actually want, passes through because the equal and opposite currents cancel and do not see the choke. The common-mode current does not cancel, so it sees the choke’s impedance and is ideally dissipated as heat. This is why the core material matters. The choke needs to present a high impedance to common-mode current across the frequencies you care about.
The Balun Paradox: Why More Cores and More Windings Don’t Guarantee Better Performance — Annotated Notes & Key Takeaways
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A choke balun does not guarantee balanced voltages at the output. It simply presents an impedance that you hope is higher than the impedance of the antenna and the feedline system. If the feedline and shack happen to be anti-resonant at some frequency, the balun might fail to choke anything, and a lot of power could be transferred as common-mode current. That is a limitation of the choke approach. When the antenna itself presents a very high impedance, common-mode current is likely to be small in magnitude, but the choke may also be ineffective. In that case, a true narrow-band transformer may be the only option that really works. The material notes that the range over which a typical balanced dipole exhibits an impedance above 3,000 ohms is quite narrow, so this is not a constant problem, but it is a boundary condition worth understanding.
The material also distinguishes between receive and transmit applications. On receive, efficiency is insignificant, but common-mode suppression can be very important. A simple bifilar winding on an unknown core might be fine for receive because saturation is hardly a concern, and the bandwidth limitation may not matter much. On transmit, efficiency is what counts, although common-mode suppression can still affect RFI in nearby equipment. For serious operators, using different antennas for transmit and receive is worth considering on bands where the sky temperature differs significantly from room temperature. That is a more advanced topic, but it reinforces the idea that the balun’s job depends on what you are asking it to do.
Where the Argument Leaves Room for Doubt
The material is opinionated, and a few points deserve caution. The claim that the counter-winding technique reduces performance was challenged. The response cited W8JI and admitted that the cause was flipped: the counter-winding reduces shunt capacitance rather than increasing it, but the result is a substantial increase in the frequency of peak performance. The curve also becomes sharper, meaning less useful bandwidth. There is no information to support the claim that counter-winding reduces saturation, though the claim appears in many forums. So the practical advice to avoid counter-winding stands, but the technical explanation is not fully settled.
The two-toroid design also drew disagreement. One point of view argued that the parallel cores guarantee balanced equal voltages on the outputs, which a simple choke does not. The rebuttal was that the two-toroid design is still just a choke, not a transformer, and it offers no balance advantage over a single core. The two parallel bifilar chokes do not have the cross-talk that a 4:1 two-core design has. Another observation was that two windings on two cores can make the characteristic impedance of the transmission line effectively 50 ohms instead of the 100 ohms typical of a simple bifilar choke. That might offer an advantage when feeding a low-impedance resonant dipole or Yagi driven element at 30 or 50 MHz. The material does not fully resolve this, so it remains an open question. For most HF operators, the single-core approach is simpler and well understood.
There is also the receive-only caveat. A rusty bolt or an unknown core might be acceptable for receive because saturation is not a concern and the signal levels are tiny. The material does not explore that use case in depth, but it is a reasonable exception. For transmit, the rules are stricter, and the consequences of a poor choke are real: heat, RFI, and inefficient radiation.
The Balun Lesson: Complexity Is Not a Virtue
The overall lesson is that a balun is not magic. It is a filter for common-mode current. A 1:1 current balun should be simple: the right core, enough turns, large enough wire, and a design that keeps parasitic capacitance in mind. A single core is usually enough. Two cores in parallel are usually counterproductive. Counter-winding is usually unnecessary. A rusty bolt is a bad idea for transmit. An ugly balun is a compromise that works on a couple of bands but not across the HF range. The material’s recommended approach is to buy the right toroid, use good wire, and keep it simple. If you need impedance transformation, things get more complex, but for a 1:1 choke, the basic design is hard to beat.
The best resource cited is K9YC’s RFI document, which includes experimentally verified designs and teardowns of popular but ineffective commercial baluns. That is a good place to go if you want to build your own or understand why a particular design works. But the core message is clear: do not be impressed by complexity. A balun with two tangled toroids is not twice as good as a balun with one. It is often twice as confusing and half as effective. The balun’s job is to choke common-mode current, and the simplest design that does that reliably is usually the best one.
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Content Disclaimer: This article is for general reference only and does not constitute professional R&D guidance, production process advice or quality certification. All material performance data has specific test premises; readers should verify parameters against actual equipment and working conditions.
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