Last Modified: March 27, 2014

Contents: Basics; Measuring Beads; What Size;Where to Put Them; Where Not to Put Them; Stereos; Odds & Ends;


Ferrites are mixtures of iron oxide and one or more metals typically manganese, nickel, and zinc. Occasionally rare earths such as yttrium and scandium are also added (they are not always or predominately iron oxide, and may contain “soft iron” meaning magnetically soft not physically soft). They provide high magnetic permeability and high resistivity, although some formulations (known as mixes) are conductive. Combined with a variety of stabilizers and binders they can be molded to just about any desired shape, with toroids, bars and beads the most common ones encountered by amateurs. By selecting the right mixture of metals, initial permeabilities (ui) from 10 to as high as 5000 or more are easily obtained. To a lessor extent the temperature coefficient can also be adjusted to meet a specific use. There are literally thousands of uses for ferrites, and modern amateur transceivers are loaded with them. But it isn't their use as RF transformers or baluns or ununs we're concerned with in this article. It is their use as RFI suppressors, and primarily in split bead configuration.

When the right material is used, they can cure a myriad of RFI problems. If not, you're wasting your time and energy. Although this article explains how to determine which material will work, it is by no means an absolute. If you don't have a way to measure them, you're much better off buying new beads with known parameters. Remember too, some mixes work better for HF, while other work better for VHF. A few salient points need to be made however.

Just because a specific ferrite mix has a higher initial permeability (ui), does not necessarily mean it will have superior performance over one with a lessor ui. Again, it depends on the specific use, frequency of operation, and a few dozen other parameters.

The reactance of any choke, is a factor of the square of the number of turns. That is to say, 4 turns through 1 bead, is the same as 1 turn through 16 beads! Further, turns should always be as evenly spaced as possible, and not overlapped. Doing otherwise will reduce the effectiveness (reactance) of the choke.

A single turn choke, no matter the mix, is worthless for RFI suppression at HF frequencies.

Installing beads on DC power cables does absolutely nothing towards reducing RFI. If RFI issue is induced on a DC power cable (a very rare occurrence), the choke should be installed only in the positive lead. Installing one on the negative lead has the opposite affect, as it RF isolates the ground connection.

Surplus ferrite beads from unknown sources shouldn't be used. Most, like those from Radio Shack, and All Electronics, make lousy chokes at any frequency below ≈150 MHz.

Specific mixes required for baluns, are not necessarily the ones required for use as chokes. Jim Brown, W9YC, has a series of on-line articles all about chokes, baluns, common mode current problems, and other data. While some of it is esoteric in nature, the articles are well written, and easy to follow.

Split BeadSplit beads, our main interest here, are tubular chunks of ferrite split along their length and typically mounted in a plastic enclosure as shown. They're available in a variety of mixes, inside and outside diameters, and with or without enclosures. Since our focus here is high frequency RFI suppression, we'll concern ourselves primarily with just one, mix 31. Just for the record, mix 43 is still a good choice for frequencies over 15 MHz or so, and is readily available from a variety of sources.

A mix 31 split bead has an initial permeability (expressed as ui) of 1,500 and a nominal operating range of 1 to 500 MHz. When placed over a wire where there is RF energy flowing (between one and several hundred megahertz) it is equivalent to placing an inductor and resistor in series with the wire for RF currents. Depending on the frequency of the RF energy the equivalent impedance can be as high as 5 k ohms or more yet DC or audio frequencies pass through unrestricted.

The resistance to the flow of RF current is closely tied to the linear inch of material parallel to and surrounding the wire. As noted above, it goes up by the square of the number of turns. It is actually a complex impedance, with X decreasing and R increasing with increasing frequency. At some frequency X=R (loss tangent equals effective permeability) and the Q becomes unity (Q=1). It is this property which makes their use in RFI suppression so ideal, and the best part is you don't have to cut the wire! There are Smith Charts, and loss plots on Mixes 31, 43, and 44 in the Photo Gallery.

Good, high quality split beads cost from $3 to $5 each. This fact has pushed many amateurs to seek relief by purchasing surplus units from after-market sources. The problem is, you don't know what you're getting, as some of these surplus split beads are virtually worthless for HF RFI suppression. In order to be effective, the impedance of the choke must be mostly resistive within the frequency range we're trying to cover. If they're mostly inductive, applying them may actually makes matters worse, not better!

Steve Hunt, G3TXQ, compiled a chart showing the difference between mix 43, and mix 31 in several configurations, and it is located here. The black line in the charts indicate when the reactance of the choke is mostly resistive. In you're specific application, if the black line does not fall within your operating frequency range, the choke might increase the RFI, rather than suppress it.

Word Of CautionIn some cases, RFI from electronic devices, including fuel pumps, can be best cured by capacitively bypassing the leads to ground, and across one another. However, caution must be exercised! This technique should never be used to bypass speaker systems as noted below. Good wiring practices must be followed, including the use of heat shrink tubing, and the avoidance of any type of tape material, not matter what it is! Remember! Any capacitor, ceramic ones in this case, can fail short. If the circuitry is not properly fused, the results could be catastrophic! Use bypass caps as a last resort, and only if you're technically competent to install them correctly.


Measuring Beads

The section explains how to measure the reactance of a split bead using an inexpensive antenna analyzer. While effective, it isn't a panacea, and here's one reason. The reactance of most ferrite mixes are rated +Ø, -20%, thus two closely related mixes might measure the same reactance. To specifically identify a ferrite material requires a VNA (vector network analyzer) costing tens of thousands of dollars. This said, the antenna analyzer is close enough to weed out worthless surplus beads, and that's one of the things we're trying to do.

Nowadays, the MFJ 259B Antenna Analyzer has become almost ubiquitous in the modern ham shack. You can use one to check those surplus units to make sure they'll do the job. You'll need enough hookup wire, size 22 is ideal, to make 3 passes through the bead plus enough to connect the ends to the analyzer. Set the frequency to 2 MHz, and measure the reactance. If it is mix 31, the reactive value (X) will be approximately 400 to 500 ohms. Pushing the mode button three times will bring up the inductance menu which should show 40 uh or so.

Putting mix 43 under same test will require you to increase the turns to 5. The readings will then be approximately the same as mix 31. By the way, the 259B doesn't have enough range to check either mix at much more than 2.5 MHz unless you reduce the number of turns. This is because the 259B has a maximum reactive range of 650 ohms. Either test will exceed this value at approximately 2.5 MHz.

As pointed out previously, at some frequency X=R and the Q=1. For either mix this occurs near 40 MHz for a one turn core, and approximately 20 MHz for a two turn core. Attempting to measure the crossover [X=R] point is beyond the range capability of the 259B. For those who wish to get closer to the actual mix specifications, loss tangent charts are available from a variety of sources which precisely list these crossover points.


What Size

Split beads come in just about every length and diameter you can think of. However, those with snap-on plastic housings generally come in four internal sizes; .25, .375, .5, .75, and 1 inch, although the actual ID may vary slightly from these sizes. Lengths vary too with the larger ID stretching to 1.5 to 2 inches. Although you can buy split beads without the plastic covers, they're inconvenient to attach (or remove) and the difference in cost is not worth the extra effort to attach them.

Just as important as knowing where to install them is how to install them, and which ID size to use. Here are a few tips. It really doesn't matter if the bead is tight or loose when snapped over the cable in question. If it is too tight you run the risk of abrading the wire, and this condition should be avoided. Too loose and it won't stay in place, but a well-placed tyrap will keep it where it belongs.


Where to Put Them

Finished ChokeProbably the most ubiquitous use of ferrite beads is in the form of an antenna motor (and reed switch) lead choke. This requires special winding considerations if the choke is to be maximally effective. In this application, the wires should be parallel wound without twisting to minimize inter-electrode capacitance. The How To Wind A Choke article explains the details.

Coax ChokeThey're also used for common mode current suppression. As the article point out, to avoid damage to the coax, the loops should be coiled loosely as the left photo indicates.

When used for RFI suppression, beads should be installed as close to the offending or offended device as possible. While this is not always easy the results are worth the extra effort. In some cases it is impossible to get close enough for optimum results.

Again, it is very important to know which mix you have. Installing the incorrect mix is a lessen in futility, and as pointed out above, may actually increase the RFI problem you're trying to suppress! Mix 31 is the one you want. Mix 43 is a viable alternative on the VHF bands. Both mixes are available from DX Engineering, Mouser Electronics, and perhaps others.

If you own an Icom 706, and have one of the early extension cables without the factory installed beads, you’ll need one on each end as well as shown in the right photo. If you don't head this advice the radio has a tendency to lock up or shut down, and most likely erase the contents of your memories.

If your vehicle has an LED CHMSL, you should bead the lead where it enters the trunk area. Remember, an LED is a diode and they will rectify RF and cause all sorts of RFI problems, especially at VHF when the antenna is mounted atop the trunk lid.


Where Not to Put Them

Where not to place them includes low impedance DC power cables. If you adequately sized your power cabling (see my Wiring article), the use of split beads (or brute force filters) is a waste of resources. Using them in an attempt to cure alternator whine is also a waste. In most cases, alternator whine appears when there is a ground loop caused by incorrect wiring practices. It is also quite common when mag mount antennas are being used. Incidentally, real alternator whine is caused by one or more leaky diodes. Brute force filters might help, but their excessive voltage drop can cause more problems than they cure.

As mentioned above, single-turn chokes are ineffective at frequencies below about 100 MHz, depending on the mix used, and the specific application. As a result, placing them on power cables to devices which interfere with you (egress), may not be effective. Good examples are electronic fuel pumps, AC and cooling fans, COP (coil over plug) units, fuel injectors, IAC (idle air control) motors, windshield wiper motors, and ABS motors. This said, if you're an industrious type, it is possible to purchase the mating connectors to all manner of automotive electrical devices, thus allowing you to lengthen the wiring harness enough to allow multi turn chokes. However, be warned that doing can integrate the digital signals sent to these devices, making them ineffective and/or inoperable.



Stereos systems can be problematic when it comes to RFI ingress. Speaker, interconnect, and power cabling are all susceptible to RFI, and shielding them is out of the question. Beads do help if installed close to the electronics as possible. However, the power amplifiers used in most models are connected to DC power any time the ignition key is inserted even if the system is turned off! Whether it is powered or not, some designs allow rectification of the offending RF. Low level audio from the speakers is one thing, but fully amplified audio is another. GM products are particularly susceptible in this regard (think OnStar®). In some cases, it maybe necessary to add a secondary power switch to the amplifier.

Word Of CautionPlacing capacitors across speaker terminals should be avoided as this can cause the offending amplifiers to go into oscillation, and destroy themselves. There is a further caveat with respect to some mobile transceivers, and that is floating outputs. That is to say, the speaker output is not referenced to ground. Thus grounding either lead can cause the audio amplifier to fail. Yet another reason not to use built-in stereo systems as the audio output stage.


Odds & Ends

Mouser Electronics also sells mix 31 beads that are not split; they're cylindrical. However, there are times where these beads are the best choice. Since the outside is cylindrical, they're easier to wind than split beads. Their added length also increases their reactive component by about 20% over the same size ID split bead. If you decide to use cylindrical beads, buy yourself some 3M #27 glass tape from Mouser too. Use it to cover the ferrite, in and out, then wind the wire on, and then protect it, in and out, with #27 as well.

Beads work very efficiently for most types of RFI and EMI, but they are not a cure all. Sometimes the installation of multiple beads is still inadequate to cure stubborn cases. In these few cases, it might be preferable to install discrete capacitors and chokes, as a last resort. However, be very careful where and how you install them.

Modern vehicles use a lot of solid state devices to switch and control the various functions. Haphazardly installing noise suppression capacitors and chokes could spell disaster. If in doubt, leave it out. In any case, always seek professional advice before taking this route.

Here's something to keep in mind. Some otherwise identical vehicles are noisier than others. Further, some individual sub assemblies are somewhat noisier than their counterparts in other vehicles. Worse, some vehicles are literally cars from hell (most hybrids), and no amount of noise suppression will make them quiet enough for amateur radio operation. Let's hope yours isn't one of them. In any case, don't give up hope.