Baluns and Common-Mode Impedance: How Much is Enough?

There is a lot of interest in baluns these days—that’s good. There is also a lot of confusion about baluns these days—that’s not so good. This article starts by defining what a balun is (and is not) along with common-mode current and impedance. Then we’ll progress to the usual uses of baluns in an antenna system and how much impedance is necessary for them to accomplish the job.

Common-Mode—A Definition

In this article, common-mode current, or CMC, means current flowing on the outside of a coaxial cable shield. Because of the skin effect, the inside and outside surfaces of the shield can carry different currents. Fields from current on the outside (the common-mode current) are not canceled by fields from an opposing current as they are for the shield and center conductor currents inside the coax. This means common-mode current will radiate like any current on a wire, and external fields picked up by the shield from the transmitted signal or from other signals or noise will create common-mode current on the shield. 

Balun—A Definition and Uses

A balun is an abbreviation of “BALanced-UNbalanced.” Its function is to transfer power between a balanced and unbalanced system while providing isolation between them. Any device, whether it is an electronic component such as windings on a ferrite core or coaxial cable in a special configuration, can perform the functions of a balun. This article assumes that ferrite cores are used. For information about coaxial cable baluns, see the ARRL Antenna Book chapter on Transmission Line System Techniques. Examples of ferrite cores can be found on the DX Engineering website—search at DXEngineering.com for “ferrite cores.”

There are four common reasons to use a balun in your antenna system:

  1. Balance currents in the terminals of a load such as a driven element

Balanced current creates symmetrical radiation from each half of the driven element, usually in a Yagi. This maintains symmetry of the antenna’s radiation pattern. This is particularly important for antennas with an unbalanced feed point connection, such as for a gamma matched antenna where one-half of the matching system is connected to the driven element and the other half to the boom which is usually grounded through the tower or mast.

There are two types of baluns used for this purpose—currentbaluns and voltage baluns. A current, or Guanella, balun ensures that identical currents flow in the terminals of the attached load, such as an antenna. A voltage, or Ruthroff, balun ensures identical voltages at the load terminals but does not ensure identical currents. (These names are of the engineers who created and described these baluns.) For that reason, the current balun is preferred because balanced currents in the antenna element on each side of the feed point are what we care about.

2. Block CMC on the coaxial feed line shield at the feed point

There are two reasons to block CMC and the balun is primarily acting as an RF choke in this application, so its common-mode impedance, or CMI, is important. Blocking CMC reduces re-radiation that distorts the radiation pattern. If blocking CMC on the shield to better balance the remaining currents in the driven element, then the choke is acting as a balun. This is usually referred to as a choke balun. This is well-described in the online article “Baluns: What They Do and How They Do It” by W7EL. The reactance of the balun’s impedance will interact with whatever reactance is present in the feed line CMI, but this effect is hard to predict. Since the impedance created by ferrite in different frequency ranges is partially resistance and partially reactance, you should use a type of ferrite that presents mostly resistance at the frequency of use—Type 31 for HF and Type 43 for higher HF and in the VHF region. These ferrite types are usually sold for suppressing EMI and not for making inductors.

The other reason to block CMC is to reduce re-radiation near equipment that can cause RFI. For example, if coaxial cable with significant CMC on the shield is routed near a home entertainment system, the resulting re-radiation from the CMC can cause RFI. A choke on the cable at or near this point, even if it is not directly connected to the equipment being interfered with, can reduce RFI by blocking CMC. The choke used this way is not a balun, even though it may be identical to a choke balun at the feed point of the antenna.

3. Block noise picked up as CMC on shield

A relatively recent use of chokes and choke baluns is to block received noise that is picked up on the shield of a feed line. CM noise currents are not a problem unless they flow to the end of the cable where, if not blocked, they will enter the cable and become a signal just like the desired signal. This usually occurs at the connection between the cable and an antenna. Adding a choke or choke balun at the end of the cable helps block that path. This is only important, however, if the noise picked up on the shield is much greater than the noise received by the antenna or if the feed line is coupling to a transmitting antenna.

4. Converting a balanced feed line to an unbalanced feed line

A good example of this use is in a G5RV antenna where a balanced feed line from the horizontal section creates a 50Ω impedance and is connected to coaxial cable at that point. If the CMC path on the outside of the coax is not blocked, current will divide between one of the parallel conductors, the inside of the shield, and the outside of the shield. The impedance will no longer be 50Ω and the function of the antenna system will be upset. In this case, a choke balun is used at the junction of the balanced and coaxial feed lines.

Note that while there may be CMC on a balanced line, it cannot be blocked by using a choke balun. The fields in the balanced line will interact with any conductive or magnetic material close to the line. This will upset the function of the balanced line. Do not wind balanced line into a coil or bundle and keep it at least one line width away from any metal surfaces. (Balanced lines include window line, open-wire or ladder line, twin-lead, and zip cord.)

Baluns and Impedance Transformation

Baluns are not necessarily impedance transformers. In the preceding example, the choke balun at the feed line junction only blocks the CMC path on the coax shield. It does not change impedance—it only ensures that the energy inside the coax is transferred to the balanced line. If the impedance presented by the balanced line at the junction is 50Ω, then all of the energy is transferred between the balanced and unbalanced parts of the system without reflections and the SWR on the coax side will be 1:1.

The section of balanced line is acting as an impedance transformer. By changing the length of the balanced line, the impedance in the line—the ratio of voltage to current, including the phase differences between them—also changes.

A choke balun never changes the impedance inside the feed line. It only blocks CMC to perform the balun function. However, there are some types of baluns that do; a 4:1 ratio is the most common. A 4:1 balun can be made from sections of coaxial cable or from parallel wires wound on a ferrite core—a transmission line transformer.

If you want some other ratio like 6:1 or 9:1 and the balun function as well, you’ll need an impedance transformer and a separate balun. This is quite common with a 1:1 current balun feeding an autotransformer or a multi-winding transformer.

How Much Impedance Do You Need?

To do the job that is required, how much CMI is necessary? It is very difficult to analyze an antenna system and come up with an exact value, so hams use “rules of thumb” based on the application. For the four cases identified earlier, here’s how I would proceed.

1. To make sure there are equal currents in each side of a driven element, or to compensate for the imbalancing effect of a gamma match, create a choke balun with about 10 times the load terminal impedance. For a 50Ω load, that’s 500 W. Since current divides according to impedance ratios, that will reduce the unwanted CMC to about 1/10th of the load current and (1/10th)2 or 1/100th of the power. That is sufficient.

2. Blocking CMC effectively at the feed point depends on power level and amount of current involved. At full power, enough CMC can flow through a balun to overheat and destroy it. A complete analysis is beyond the scope of this article, but there are two good reference articles: N6BV’s June 2015 QST article “Don’t Blow Up Your Balun” and W1VT’s Jan-Feb 2004 QEX article “Why Do Baluns Burn Up.”

If you want to try the analytical route, model the antenna system with a third wire (the outside of the shield) attached at the feed point and calculate the current in it. Add impedance then re-calculate the current and power dissipation (I2R). Keep adding impedance until the power dissipation is low enough. The cut-and-try method is to install the balun and run a modest amount of power through it continuously for a minute or so, then quickly check the temperature. If it’s hot, you need more impedance. Keep adding cores or turns or both until the balun doesn’t overheat at your expected power level and duty cycle.

3. Because of the extreme variability of noise levels, you’ll have to experiment to find the minimum amount of CMI. Start by measuring the effect of adding 100Ω or more and assess the effect. If the noise level drops, keep adding turns or cores until the noise stops dropping. Another approach is to simply add a high level of CMI, with 5kΩ being the usual value, to cables suspected of picking up noise. You’re probably wasting some ferrite by doing that, so I recommend starting with the smaller CMI value as an experiment and working your way up, if needed.

4. As in case number one, to effectively isolate the balanced and unbalanced feed lines, about 10 times the impedance at that point in the system is required. Since the usual impedance level at such a transition is 50Ω, a CMI of 500Ω would be sufficient. 

Creating the Balun or Choke

There are a lot of choices for ferrite cores, but the two that are usually used for amateur antenna systems are the clamp-on, or split, core that snaps on to a coaxial cable and the toroid core. The clamp-on core is big enough to pass an RG-213 size cable through it once, which counts as one “turn.” (For smaller cables, more turns can be wound through the core.) The toroid usually used has a 2.4-inch OD and you can usually get several turns of RG-213 through it. The following table gives the impedance of one turn through each of these cores at three frequencies and for two different types of ferrite.

Impedance of Clamp-On or Toroid Cores
1 turn on 0.514″ IDType 31Type 43
5 MHz71n/a
10 MHz10090
50 MHz208203
1 turn on 2.4″ OD coreType 31Type 43
5 MHz28n/a
10 MHz4033
50 MHz9182

Source – Fair-Rite Catalog, 17th Edition. 50 MHz values are estimated as the average of 25 MHz and 100 MHz values. Impedance values for EMI suppression cores are an approximate minimum and not tightly controlled as for inductive components. See the 2023 ARRL Handbook for more graphs of measured performance.

Additional Information

I’ve just scratched the surface of balun and choke design to give you an idea of the mechanisms involved and how to deal with CMC on feed lines. You can learn a lot more in the ARRL Antenna Book (see the chapter on Transmission Line System Techniques) and Handbook (see the Transmission Lines chapter). Even more information is available in tutorials by K9YC at k9yc.com/publish.htm: RFI, Ferrites, and Common Mode Chokes for Hams and HF Choke Cookbook. You can also find a number of OnAllBands articles about ferrite, including my earlier article, “Ferrite Chokes, What Are They Anyway?”.

As you can see, it’s not a simple subject! The term “balun” is overused to mean a lot of different things, and that’s confusing. Then, we use baluns and chokes to solve quite different problems with different requirements. However, if you’re willing to think carefully, identify what the problem is, and apply an appropriate solution, you can get the job done with a minimum of expense and effort.

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